Download the module 1 on Plant and Animal Physiology

xfCOPPERBELT UNIVERSITY COLLEGE
BIO 241: Plant and Animal Physiology
MODULE 1
Copperbelt University College
Kitwe-Zambia
Science Department
BIO 241: Plant and Animal Physiology
Copyright
© Copperbelt University College 2009.
No part of this module may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording or by any information storage
and retrieval system, without permission in writing from the publisher.
Copperbelt
University College
Science Department
P.0 Box, 20382
Kitwe – Zambia
Phone:
+260212239003
E-mail: cosetco@ zamtel.zn
BIO 241: Plant and Animal Physiology
Acknowledgements
The Copperbelt University College, Science Department wishes to thank the following for
their contribution to this module:
EDITORS
Directorate of Distance Education (DODE)Lusaka
AUTHORS
Muma E : Lecturer CBUC
Muzeya J.M.: Former Lecturer CBUC
BIO 241: Plant and Animal Physiology
About this Module
1
How this module is structured .......................................................................................... 1
Welcome to the module 1 on Plant and Animal Physiology ............................................ 1
Module outcomes .............................................................................................................. 3
Timeframe ......................................................................................................................... 3
Study skills ........................................................................................................................ 3
Need help? ........................................................................................................................ 4
Assignments ...................................................................................................................... 5
Assessments ...................................................................................................................... 5
Getting around this Module
6
Margin icons ..................................................................................................................... 6
Unit 1
7
Movement of Materials into and out of the Cells in a Plant.................................... 7
1.0 Introduction ....................................................................................................... 7
1.1 Principal Methods of Movement of Substances within Cells ........................... 8
1.2. Osmotic Pressure ............................................................................................ 16
1.3. Potential Osmotic Pressure ............................................................................. 18
1.3. Plasmolysis ..................................................................................................... 22
1.4. Concept of Water Potential and Osmotic Relations of Plant Cells ................ 24
Unit summary ................................................................................................................. 36
Assessment...................................................................................................................... 37
Unit 2
38
Transport in plants ................................................................................................. 38
2.0 Introduction ..................................................................................................... 38
2.1. Water Potential re-visited ............................................................................... 39
2.2. Absorption of Water by Roots........................................................................ 40
2.3. Movement of Water through Cells: Two routes, the Symplast and the
Apoplast................................................................................................................. 42
2.4. Water Movement in Xylem through TACT Mechanism ............................... 45
2.5. Phloem Transport: Flow from Source to Sink in Angiosperms ..................... 50
Unit 3
56
Transpiration ......................................................................................................... 56
3.0 Introduction ..................................................................................................... 56
3.1 What is Transpiration ...................................................................................... 59
3.2. Kinds of Transpiration.................................................................................... 60
3.3 Transpiration as a necessary evil ..................................................................... 61
3.4 Supposed advantages of Transpiration ............................................................ 62
3.5 Mechanism of Stomatal Transpiration ........................................................... 63
3.6 Measurement of Transpiration ........................................................................ 71
3.7 Leaves: Transpiration and Pulling of Water.................................................... 72
BIO 241: Plant and Animal Physiology
3.8. Factors affecting the rate of Transpiration ..................................................... 73
3.9. The Control of Transpiration .......................................................................... 77
Unit summary ................................................................................................................. 82
Assessment...................................................................................................................... 84
Unit 4 ..................................................................................................................... 85
Photosynthesis ....................................................................................................... 85
4.0 Introduction ..................................................................................................... 85
4.0 Introduction ..................................................................................................... 86
4.1 Organisms that are able to photosynthesize .................................................... 86
4.2. Significance of Photosynthesis ....................................................................... 87
4.3. Photosynthetic Pigments ................................................................................ 87
4.4. Action and Absorption Spectra ...................................................................... 91
4.5. Light-Harvesting Complexes in Green Plants ................................................ 95
4.6. Photosystems .................................................................................................. 99
4.7. Photosynthetic Electron Transport Chains in Chloroplasts .......................... 107
4.8. Stages of Photosynthesis .............................................................................. 109
4.9. Photoassimilate ............................................................................................. 119
Unit summary ............................................................................................................... 120
BIO 241: Plant and Animal Physiology
About this Module
This module is structured as outlined below.
How this module is structured
The module overview
Welcome to the module 1 on Plant and Animal
Physiology
This module gives you the foundation to ‘Plant and Animal
Physiology’ in the Bachelor of Science- Natural Science
programme. The module exposes you to fundamental concepts in
both plant physiology and animal physiology. These concepts will
help you to understand subsequent topics in physiology as you
pursue your Bachelor of Education in Natural Sciences.
To complete this module successfully, you will need to spend three
(3) hours per week studying the module, and make sure you work
out all the activities in each unit. Don’t move to another unit before
you understand the previous unit. In case you need help contact the
course tutors.
You are expected to do all the self marked activities and one tutor
marked assignment. You are required to summit the assignment to
the nearest resource centre in your district. This module has five
units.
We strongly recommend that you read the overview carefully
before starting your study.
The module content
The module is broken down into units. Each unit comprises:
 An introduction to the unit content.
 Unit outcomes..
1
BIO 241: Plant and Animal Physiology
 Core content of the unit with a variety of learning activities.
 A unit summary.
 Assignments and/or assessments, as applicable.
Resources
For those interested in learning more on this subject, we provide
you with a list of additional resources at the end of this module;
these may be books, articles or web sites.
Your comments
After completing this module we would appreciate if you would
take a few moments to give us your feedback on any aspect of this
course. Your feedback might include comments on:
 Course content and structure.
 Course reading materials and resources.
 Course assignments.
 Course assessments.
 Course duration.
 Course support (assigned tutors, technical help, etc.)
Your constructive feedback will help us to improve and enhance
this course.
2
BIO 241: Plant and Animal Physiology
Module outcomes
Upon completion of this module you will be able to:
 Discuss water relations in plants.
 Describe photosynthesis in green plants.
Outcomes
 Explain mineral nutrition in plants.
 Discuss hormonal regulation of plant growth and development.
Timeframe
This module is expected to be covered within a period of 100
hours. The 100 hours will include studying the actual module
including all the activities.
How long?
Study skills
As an adult learner your approach to learning will be different to
that from your school days: you will choose what you want to
study, you will have professional and/or personal motivation for
doing so and you will most likely be fitting your study activities
around other professional or domestic responsibilities.
Essentially you will be taking control of your learning
3
BIO 241: Plant and Animal Physiology
environment. As a consequence, you will need to consider
performance issues related to time management, goal setting, stress
management, etc. Perhaps you will also need to reacquaint yourself
in areas such as essay planning, coping with exams and using the
web as a learning resource.
Your most significant considerations will be time and space i.e. the
time you dedicate to your learning and the environment in which
you engage in that learning.
We recommend that you take time now—before starting your selfstudy—to familiarize yourself with these issues.
Need help?
Should you require help in the course of your studies, do not
hesitate to contact the following course tutors
Help
Mr Muma E
Cell: 0977-185244 / 0969735302
e-mail address: [email protected]
Mrs Muzeya J.M. Cell: 0955553877/0977474449
4
BIO 241: Plant and Animal Physiology
Assignments
You will be expected to write at least two assignments in an
academic year. The first assignment is this module
The assignments should be handed in to course tutors during the
Assignments
residential sessions.
You will be required to submit the assignments in the order in
which they are given to you.
Assessments
You will be expected to write two tutor- marked test which will be
written during each residential session. You are also expected to
Assessments
answer the self- marked assessments in each unit of this module.
5
BIO 241: Plant and Animal Physiology
Getting around this Module
Margin icons
While working through this module you will notice the frequent
use of margin icons. These icons serve to “signpost” a particular
piece of text, a new task or change in activity; they have been
included to help you to find your way around the module.
A complete icon set is shown below. We suggest that you
familiarize yourself with the icons and their meaning before
starting your study.
Activity
Assessment
Assignment
Case study
Discussion
Group
activity
Help
Note it!
Outcomes
Reading
Reflection
Study skills
Summary
Terminology
Time
Tip
6
BIO 241: Plant and Animal Physiology
Unit 1
Movement of Materials into and out of the Cells in a Plant
1.0 Introduction
Your knowledge of movement of materials into and out of the cells
of plants is fundamental to your understanding of how biologically
significant molecules such as water reach the sites where they are
required for metabolic processes such as photosynthesis. In this
unit, you shall look first, at the processes that are responsible for
the movement of water into and out of a given cell under specified
concentration relative to that of its immediate environment. Then,
you shall discuss the concept of water potential in detail. This is
due to the fact that knowledge of water potential of a cell is also
important because it will enable you to deduce the direction of
water movement
During and upon completion of this unit you will be able to:
 Define deferential permeable membrane.
 Discuss diffusion and osmosis in plants.
Outcomes
 Differentiate between end-osmosis and exo-osmosis.
 Analyse the components of water potential with respect to water
relations in plants.
7
BIO 241: Plant and Animal Physiology
1.1 Principal Methods of Movement of Substances within Cells
The transport of water and other types of molecules across
membranes is key to many processes in living organisms. Many of
these transport processes proceed by diffusion through membranes
which are selectively permeable, allowing small molecules to pass
but blocking larger ones. These processes, including osmosis and
diffusion, are sometimes called passive transport since they do not
require any active role for the membrane. Other types of transport,
called active transport, involve properties of a cell membrane to
selectively "pump" certain types of molecules across the
membrane.
1.1.1 Diffusion
Let us now turn our attention to diffusion. What do you understand
by this term ‘diffusion’. Well, this process is defined as the net
movement of particles or molecules of a substance (gas or liquid)
from a region of higher concentration to a region of lower
concentration in order to reach an equillibrium. It is one of the
principle methods of movement of substances within cells, as well
as the method for essential small molecules to cross the cell
membrane. In this process, the diffusing particles have a certain
pressure called the diffusion pressure which is directly proportional
to the number of the diffusing particles. Therefore, diffusion takes
place from a region of higher diffusion pressure (area where there
are more particles) to a region of lower diffusion pressure (area
where there are fewer particles). That is, along diffusion pressure
gradient. The greater (steeper) the concentration gradient, the faster
the movement of the diffusing particles. If nothing intervenes, the
movement will continue until the concentration gradient is
eliminated. No energy in the form of ATP is used in transporting
the molecules; it is simply a natural random movement of the
8
BIO 241: Plant and Animal Physiology
particles involved. Diffusion can either be passive or facilitated
diffusion. Simple passive diffusion occurs when small molecules
pass through the lipid bilayer of a cell membrane. Facilitated
diffusion depends on carrier proteins imbedded in the membrane
to allow specific substances to pass through, that might not be able
to diffuse through the cell membrane.
(a) Distinguish between Simple passive diffusion and
Facilitated diffusion. You should spend not more than ten
minutes.
..........................................................................................................
.........................................................................................................
.............................................................................................................
..........................................................................................................
(b) Write short notes on the importance of the process of diffusion
to organisms.
................................................................................................
..................................................................................................
..................................................................................................
...................................................................................................
Well done, you can now move on to look at the factors that affect
diffusion especially across a plasma membrane
9
BIO 241: Plant and Animal Physiology
Factors affecting diffusion across a plasma membrane
The factors that you should consider regarding diffusion across a
lipid bilayer should include the following:
(a) The greater the solubility of the diffusing particles, the
more permeable the membrane will be
(b) All else being equal, smaller particles will diffuse more
rapidly than larger particles. Examples O2, H2O, CO2, rapidly
diffuse across lipid bilayer.
(c ) Larger hydrophilic unchanged molecules, such as sugars, do
not freely diffuse.
Rate of diffusion
The rate of diffusion is affected by properties of the cell, the
diffusing molecule, and the surrounding solution. We can use simple
equations and graphs to examine how particular molecules and their
concentration affect the rate of diffusion. We can also compare
simple and facilitated diffusion. The transport of gases across
membranes depends upon diffusion and the solubility of the gases
involved. In life science applications of such transport is
characterized by Graham's Law and Fick's Law. The rate of
diffusion is controlled by Fick's law. Fick's Law states that the rate
of diffusion across a membrane is directly proportional to the
concentration gradient of the substance on the two sides of the
membrane and inversely related to the thickness of the membrane
The relative diffusion rate for one molecular species is then given
by
10
BIO 241: Plant and Animal Physiology
The relative diffusion rate for two different molecular species is
then given by
It should be emphasized here that diffusion of one compound is
independent from diffusion of other compound.
Arising from the above discussion, you would expect an increase in
the rate of diffusion if,
(i) The diffusion pressure is increased
(ii) The temperature is increased
(iii) Density of the diffusion particles is lesser
(iv) The medium through which diffusion occurs is less
concentrated.
1.1.2. Osmosis
If you separate two solutions of different concentrations using a
semi-permeable membrane (a membrane which is permeable to the
smaller solvent molecules but not to the larger solute molecules),
then the solvent molecules will tend to move across the membrane
from the less concentrated to the more concentrated solution. This
process is called osmosis. Osmosis is the net movement of water
across a selectively permeable membrane driven by a difference in
solute concentrations on the two sides of the membrane. A
selectively permeable membrane is one that allows unrestricted
passage of water, but not solute molecules or ions. This therefore,
entails that Osmosis is a selective diffusion process driven by the
internal energy of the solvent molecules.
11
BIO 241: Plant and Animal Physiology
Demonstration of Osmosis
The phenomenon of osmosis can be demonstrated by the following
simple experiment.
Fig 1.2: Demonstration of Osmosis
If you tie the mouth of a thistle funnel with goat bladder to serve as
a semi-permeable membrane and then fill the thistle funnel with
concentrated sugar solution whose level is marked on its narrow
neck. Next, place the thistle funnel into a beaker of water. What
would happen to the level of the sugar solution in the thistle
funnel?
In osmosis water flows from the solution with the lower solute
concentration into the solution with higher solute concentration
through a semi-permeable membrane. This means that water flows
in response to differences in molarity across a membrane.
The size of the solute particles does not influence osmosis. If pure
water were on both sides of the membrane, the osmotic pressure
difference would be zero. That is, a state of equilibrium would be
said to have been reached once sufficient water has moved to
12
BIO 241: Plant and Animal Physiology
equalize the solute concentration on both sides of the membrane,
and at that point, net flow of water ceases.
In case, there are two solutions of different concentrations,
separated by the semi- permeable membrane, the diffusion of
solvent will take place from the less concentrated solution in to the
more concentrated. In this case osmosis can be said to be a
special kind of diffusion that pertains specifically to water: the
movement of water across a selectively permeable membrane that
permits the passage of water but inhibits the movement of the
solute. The water moves down a concentration gradient from the
region of its higher concentration of free water molecules (less
solutes) to the region of its lower concentration of free water
molecules (more solutes), or from high pressure to low pressure.
Solution types relative to cell
In comparing the relationship of the cell contents to those of the
surroundings, three terms are used:
(a) isotonic:
The two solutions have the same concentration of solutes,
(Solute concentration of solution equal to that of the cell).
hence the same amount of water moves into the cell as
moves out; No net water movement.
(b) hypotonic:
The water outside the cell has less solute (hypo = less),
tonic = dissolved particles
and therefore more free water with the result that water
moves into the cell at a greater rate than it moves out; Cell
expands (and may burst)
(c) hypertonic: The water outside the cell has more solute
(hyper = more), (tonic= dissolve) and therefore less free
water with the result that water moves out of the cell at a
13
BIO 241: Plant and Animal Physiology
greater rate than it moves in. A cell which experiences this
outward movement of water from its interior part shrinks,
and such a cell is said to be plasmolysed. You will learn
more on the concept of plasmolysis later in this unit.
Plasma membrane permeable to water but not to solute.
Water moves from solution with lower concentration. Of
dissolved particles to solution with higher concentration of
dissolved particles.
Water moves from dilute solute to concentrated solute.
Define the following terms:
a) isotonicity
…………………………………………………………………
…………………………………………………………………
b) hypotonicity
…………………………………………………………………
…………………………………………………………………
c) hypertonity
…………………………………………………………………
………………………………………………………………….
When a biological cell is in a hypotonic environment, the cell
interior accumulates water, water flows across the cell membrane
into the cell, causing it to expand. In plant cells, the cell wall
restricts the expansion, resulting in pressure on the cell wall from
within called turgor pressure.
14
BIO 241: Plant and Animal Physiology
Hypertonic solutions are those in which more solute (and hence
lower water potential) is present. Hypotonic solutions are those
with less solute (again read as higher water potential). Isotonic
solutions have equal (iso-) concentrations of substances. Water
potentials are thus equal, although there will still be equal amounts
of water movement in and out of the cell, the net flow is zero.
You should by now know that diffusion of water across a
membrane generates a pressure called osmotic pressure. If the
pressure in the compartment into which water is flowing is raised
to the equivalent of the osmotic pressure, movement of water will
stop. This pressure is often called hydrostatic ('water-stopping')
pressure.
In osmosis, water moves from a hypotonic solution to a hypertonic
through a selectively permeable membrane. Water will diffuse
across a selectively permeable membrane until the concentrations
are the same on both sides (i.e. isotonic). If pressure is applied to
the hypertonic side (the side into which the water is moving), it is
possible to stop the inward flow of water. The amount of pressure
needed to do so is called the osmotic pressure of the solution and
is determined by the concentration of total solutes in the solution.
Osmosis doesn't depend on the kinds of molecules or ions in
solution, only on the amount of solutes.
Significance of Osmosis in Plants
The following are some of the reasons as to why osmosis is
important.
(a) Large quantities of water are absorbed by roots from the soil
by Osmosis.
(b) Cell to cell movement of water and other substance
dissolved in it involves this process.
(c ) Opening and closing of stomata depend upon the turgor
15
BIO 241: Plant and Animal Physiology
pressure of the guard cells.
(d)The resistance of plants to drought and frost increases with
increase in Osmotic pressure of their cells.
(e)Turgidity of the cell of the young seedlings allows them to
come out of the soil.
1.2. Osmotic Pressure
As a result of the separation of solution from its solvent or the two
solutions by the semi-permeable membrane, a pressure is
developed in solution due to the presence of dissolved solutes in it.
This is called Osmotic pressure (O.P). Osmotic pressure is
measured in terms of atmosphere. Osmotic pressure is directly
proportional to the concentration of dissolved solutes in the
solution. A more concentrated solution has higher osmotic pressure
than one which is less concentrated.
Osmotic pressure does not increase by the addition of insoluble
solute to a solution Thus. Osmotic diffusion of solvent molecules
will not take place if the two solutions separated by the semipermeable membrane are of equal concentration having equal
Osmotic pressures.
Osmotic pressure therefore, the pressure which is needed to be
applied to a solution in order to prevent the inward flow of water
across a semi-permeable membrane.
Osmosis is a selective diffusion process driven by the internal
energy of the solvent molecules. A solution has a "high" osmotic
pressure while pure water has zero osmotic pressure. The energy
which drives the fluid transfer is the thermal energy of the water
molecules, and that amount of this energy is higher in the pure
solvent since there are more water molecules.
16
BIO 241: Plant and Animal Physiology
Fluid transport is from high "pressure" to low pressure just like a
normal fluid flows through pipes from high pressure to low
pressure.
The phenomenon of osmotic pressure arises from the tendency of a
pure solvent to move through a semi-permeable membrane and into
a solution containing a solute to which the membrane is
impermeable. This process is of vital importance in biology as the
cell's membrane is selective towards many of the solutes found in
living organisms.
In order to visualize this effect, imagine a U shaped clear tube with
equal amounts of water on each side, separated by a membrane at
its base that is impermeable to sugar molecules (made from dialysis
tubing). Sugar has been added to the water on one side. The height
of the water on each side will change proportional to the pressure
of the solutions.
Osmotic pressure causes the height of the water in the compartment
containing the sugar to rise, due to movement of the pure water
from its compartment into the compartment containing the sugar
water. This process will stop once the pressures of the water and
sugar water toward both sides of the membrane are equated.
One approach to the measurement of osmotic pressure is to
measure the amount of hydrostatic pressure necessary to prevent
fluid transfer by osmosis. In other words, you can measure osmotic
pressure by determining how much hydrostatic pressure on the
solution is required to prevent the transport of water from a pure
source across a semi-permeable membrane into the solution. A
positive pressure must be exerted on the solution to prevent
osmotic transport, again congruent with the concept that the
osmotic pressure of the pure solvent is relatively "high".
17
BIO 241: Plant and Animal Physiology
Importance of Osmotic pressure
Osmotic pressure is an important factor affecting cells.
Osmoregulation is the homeostasis mechanism of an organism to
reach balance in osmotic pressure. Osmotic pressure is necessary
for many plant functions. It is the resulting turgor pressure on the
cell wall that allows herbaceous plants to stand upright, and how
plants regulate the aperture of their stomata. In animal cells which
lack a cell wall however, excessive osmotic pressure can result in
cytolysis.
Osmotic pressure is the basis of filtering ("reverse osmosis"), a
process commonly used to purify water. The water to be purified is
placed in a chamber and put under an amount of pressure greater
than the osmotic pressure exerted by the water and the solutes
dissolved in it. Part of the chamber opens to a differentially
permeable membrane that lets water molecules through, but not the
solute particles. The osmotic pressure of ocean water is about 27
atm. Reverse osmosis desalinates fresh water from ocean salt
water.
1.3. Potential Osmotic Pressure
Potential osmotic pressure is the maximum osmotic pressure that
could develop in a solution if it were separated from distilled water
by a selectively permeable membrane. It is determined by the
number of particles in a unit volume of solution
Osmosis produces a physical force
Movement of water into a cell puts pressure on plasma membrane.
Animal cells will expand and may burst. Plant cells will not burst.
Why? An example of a case where osmosis may lead to the
generation of a physical force occurs in paramecium have
18
BIO 241: Plant and Animal Physiology
contractile vacuoles which serve as little pumps to expel excess
water out of the cell.
(a) Organisms with a cell wall, such as plants, do not burst.
(b) When a cell becomes turgid its cell membrane presses
against cell wall of the cell.
(c) The rigid cell wall resists due to its own structural
integrity.
(d) These opposing forces creates turgidity which keeps
young ones upright.
Let us now compare the behaviour of a plant cell and an animal cell
when each one of them is placed first, in concentrated salt solution
and then placed in distilled water.
Fig 1.2: Water relations and cell shape in blood cells.
19
BIO 241: Plant and Animal Physiology
Fig 1.3: Water relations in a plant cell.
Plants cells as Osmotic System
Living cells in plants form Osmotic systems due to the presence of
semi-permeable plasma membrane and the cell sap having a certain
Osmotic pressure. Plasma-membrane actually is not truly semipermeable as it allows certain solutes to pass through it and hence,
it is known as selectively permeable or differentially permeable
membrane. The cell wall is permeable.
The tonoplast or the vacuole membrane also possesses the same
nature.
The solvent in case of plants is always water. If a living plant cell
or tissue is placed in water or hypotonic solution (where O.P, is
lower than that of a cell sap) water enters into the cell sap by
Osmosis. This process is called end-Osmosis.
As a result of entry of the water into the cell sap, a pressure is
develop which prones the protoplasm against the cell wall and the
cell becomes turgid. This pressure is called turgar pressure.
At a given time, turgor pressure (T.P) equals the wall pressure
(W.P).
20
BIO 241: Plant and Animal Physiology
(a) If on the other hand, the plant cell or tissue is placed in
hypertonic solution (whose S.P is higher than that of cell
sap) the water comes out of the cell sap into the outer
solution and the cell becomes flaccid. This process is called
exosmosis.
(b) Cell or tissue will remain as such in isotonic solution.
You should note that water in the cell (mostly in the central
vacuole) exerts a turgor pressure against the cell wall, which, in
turn, exerts inwardly a mechanical wall pressure against the
protoplast. The two equal and opposing pressures give strength to
the cell and columns of water-filled cells keep the plant erect.
Before we move to another concept write any observations that you
would make if on a sunny day you forget to water chiness cabbage
plant.
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
……………………………………………………………………..
If you had made careful observations, you would probably see the
leaves of your plant drooping. One reason for that would probably
be that
21
BIO 241: Plant and Animal Physiology
the cells of the leaves lost water. As a result of this the turgor and
wall pressure of the cells of the leaves became less. Consequently,
the cells become limp (flaccid), and the whole plant if allowed to
remain in that condition would wither. With this background at
hand, let us now discuss plasmolysis.
1.3. Plasmolysis
In normal condition the protoplasm is tightly pressed against the
cell wall. If this plant cell or tissue is placed in a hypertonic
Solution, water comes out from the cell sap into the outer solution
due to ex-osmosis and the protoplasm begins to contract from the
cell wall. This is called incipient plasmolysis.
If the outer hypertonic solution is very much concentrated in
comparison to the cell sap, the process of ex-osmosis and
concentration or shrinkage of protoplasm continues and ultimately
the protoplasm separates from the cell wall and assumes a spherical
form.
The phenomenon is called plosmolysis and the cell of the tissue is
said to be plasmolysed.
Because of the permeable cell wall the space in between the cell
and plasma-membrane in plasmolysed cell is filled with outer
hypertonic solution.
As you have noted above, plasmolysis occurs due to the movement
of water from the cells. The movement of water from the cell
causes the cytoplasm of the cell to shrink away from the wall and
collapses into an interior clump.
22
BIO 241: Plant and Animal Physiology
When water moves into a plant cell by osmosis, the internal turgor
pressure developed pushes on the wall. What does this do to your
understanding of a neglected houseplant?
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………
Advantages of plasmolysis
The following are some of the advantages of plasmolysis:
1. This phenomenon is utilized in setting of meat and fishes
and addition of concentrated sugar solution to jams and
jellies to check the growth of fungi and bacteria which
become plasmolysed in concentration solution.
2. It indicates the semi- permeable nature of the plasma
membrane.
3. It is also used in determining the O.P of the sell sap.
23
BIO 241: Plant and Animal Physiology
1.4. Concept of Water Potential and Osmotic Relations of Plant
Cells
Water potential is the tendency of water to move from an area of
higher concentration to one of lower concentration Osmotic
movement of water involves certain work done and infact the main
driving force behind this movement is the difference between free
energies of water on the two sides of the semi- permeable
membrane. For non-electrolytes, free energy per mole, is known as
chemical potential. With reference to water this is called water
potential.
Energy exists in two forms: potential and kinetic. Water molecules
move according to differences in potential energy between where
they are and where they are going. Gravity and pressure are two
enabling forces for this movement. These forces also operate in the
hydrologic (water) cycle. Remember in the hydrologic cycle that
water runs downhill (likewise it falls from the sky, to get into the
sky it must be acted on by the sun and evaporated, thus needing
energy input to power the cycle).
Water potential is the potential energy of water per unit volume
relative to pure water in reference conditions. Water potential
quantifies the tendency of water to move from one area to another
due to osmosis, gravity, mechanical pressure, or matrix effects such
as surface tension. Water potential has proved especially useful in
understanding water movement within plants, animals, and soil.
Water potential is typically expressed in potential energy per unit
volume and very often is represented by the Greek letter Ψ.
24
BIO 241: Plant and Animal Physiology
Water potential integrates a variety of different potential drivers of
water movement, which may operate in the same or different
directions. Within complex biological systems, it is common for
many potential factors to be important. For example, the addition of
solutes to water lowers the water's potential (makes it more
negative), just as the increase in pressure increases its potential
(makes it more positive). If possible, water will move from an area
of higher water potential to an area that has a lower water potential.
One very common example is water that contains a dissolved salt,
like sea water or the solution within living cells. These solutions
typically have negative water potentials, relative to the pure water
reference. If there is no restriction on flow, water molecules will
proceed from the locus of pure water to the more negative water
potential of the solution.
Water potential is expressed in SI unit of pressure called pascals.
The water potential for pure water is arbitrarily fixed as Zero at one
atmosphere and a particular temperature.
Water potential is lowered by the addition of solutes and because
water potential volume is Zero for pure water, all other water
potential will be negative. In other words, the movement of water
will take place in Osmotic or other systems from a region of higher
water potential (Less negative) to a region of lower water potential
(more negative).
Osmotic potential is the opposite of water potential, which is the
degree to which a solvent tends to stay in a liquid.
25
BIO 241: Plant and Animal Physiology
1.4.1. Water Potential and Vascular Plants
When a water potential gradient is established between two areas,
water will spontaneously diffuse from the high end (soil) to the low
end (air). This gradient is necessary for plants to transport water.
Water potential may be established by: increasing the concentration
of solutes. Pure water has the highest potential while a saturated
solution of ions etc. would have the lowest potential. converting
water to a gas. Water potential is highest when water is a liquid and
lowest when water is a gas in air.
1.4.2. Components of water potential
Many different factors may affect the total water potential, and the
sum of these potentials determines the overall water potential and
the direction of water flow:
Ψ = Ψ0 + Ψπ + Ψp + Ψs + Ψv + Ψm
where:
Ψ0 is the reference condition,
Ψπ is the solute potential,
Ψp is the pressure component,
Ψs is the gravimetric component,
Ψv is the potential due to humidity, and
Ψm is the potential due to matrix effects (e.g., fluid cohesion and
surface tension).
All of these factors are quantified as potential energies per unit
volume, and different subsets of these terms may be used for
particular applications (e.g., plants or soils). Different conditions
are also defined as reference depending on the application: for
26
BIO 241: Plant and Animal Physiology
example, in soils, the reference condition is typically defined as
pure water at the soil surface.
1.4.3. Pressure Potential
Pressure potential is based on mechanical pressure, and is an
important component of the total water potential within plant cells.
Pressure potential increases as water enters a cell. As water passes
through the cell wall and cell membrane, it increases the total
amount of water present inside the cell, which exerts an outward
pressure that is retained by the structural rigidity of the cell wall.
By creating this pressure, the plant can maintain turgor, which
allows the plant to keep its rigidity. Without turgor, plants lose
structure and wilt.
The pressure potential in a living plant cell is usually positive. In
plasmolysed cells, pressure potential is almost zero. Negative
pressure potentials occur when water is pulled through an open
system such as a plant xylem vessel. Withstanding negative
pressure potentials (frequently called tension) is an important
adaptation of xylem vessels.
1.4.4. Solute potential
Pure water is usually defined as having a solute potential (Ψπ) of
zero, and in this case, solute potential can never be positive. The
relationship of solute concentration (in molarity) to solute potential
is given by the van 't Hoff equation:
Ψπ = − MiRT
where M is the concentration in molarity of the solute, i is the van 't
Hoff factor, the ratio of amount of particles in solution to amount
of formula units dissolved, R is the ideal gas constant, and T is the
absolute temperature.
27
BIO 241: Plant and Animal Physiology
For example, when a solute is dissolved in water, water molecules
are less likely to diffuse away via osmosis than when there is no
solute. A solution will have a lower and hence more negative water
potential than that of pure water. Furthermore, the more solute
molecules present, the more negative the solute potential is.
Solute potential has important implication for many living
organisms. If a living cell with a smaller solute concentration is
surrounded by a more concentrated solution, the cell will tend to
lose water to the more negative water potential (Ψw) of the
surrounding environment. This is often the case for marine
organisms living in sea water and halophytic plants growing in
saline environments. In the case of a plant cell, the flow of water
out of the cell may eventually cause the plasma membrane to pull
away from the cell wall, leading to plasmolysis. It can be measured
in plant cells using the Pressure bomb. Most plants, however, have
the ability to increase solute inside the cell to drive the flow of
water into the cell and maintain turgor.
This effect can be used to power an osmotic power plant .
1.4.5. Matrix potential (Matric potential)
When water is in contact with solid particles (e.g., clay or sand
particles within soil), adhesive intermolecular forces between the
water and the solid can be large and important. The forces between
the water molecules and the solid particles in combination with
attraction among water molecules promote surface tension and the
formation of menisci within the solid matrix. Force is then required
to break these meniscus. The magnitude of matrix potential
depends on the distances between solid particles—the width of the
menisci (see also capillary action)--and the chemical composition
28
BIO 241: Plant and Animal Physiology
of the solid matrix. In many cases, matrix potential can be quite
large and comparable to the other components of water potential
discussed above.
It is worth noting that matrix potentials are very important for plant
water relations. Strong (very negative) matrix potentials bind water
to soil particles within very dry soils. Plants then create even more
negative matrix potentials within tiny pores in the cell walls of their
leaves to extract water from the soil and allow physiological
activity to continue through dry periods. Germinating seeds have a
very negative matric potential. This causes water uptake in even
somewhat dry soils and hydrates the dry seed.
Osmotic pressure (O.P) in a solution results due to the presence of
solutes and the later lower the water potential. Therefore, O.P is a
quantitative index of the lowering of water potential in a solution
and pusing thermodynamic terminogy is called Osmotic potential
(4s). Osmostic pressure and Osmotic potential values are
numerically equal but while the former has positive sign, the later
carries a negative sign (if O.P =20 atm, the 4s = -20 atm ).
In an open osmotic system, the water potential and the osmotic
potential values are numerically similar and also have same sign
i.e., negative (similar will be the case in plasmolysed cells).
In plant cells a pressure is imposed on water which increases the
water potential. In plants this pressure is called turgor pressure (or
pressure potential).
This is the actual pressure with positive sign and ranges between
zero and numerical osmotic potential value.
The potential created by such pressures is called hydrostatic
pressure or pressure potential (4p).
In this case, water potential is equal to osmotic potential plus
pressure potential.
29
BIO 241: Plant and Animal Physiology
4w=4s+4p
Water potential values of plant cells under different Osmotic
conditions are as follows:
1) 4w=4s (as ψp = nil) ……………………… in plasmolysed
or flaccid cell (lowest)
2) 4w = 4s + 4p ………………………. In partially turgid cell
(higher)
3) 4w = zero (as 4p numerically ………………… in fully
turgid cell
EXPERIMENTAL WORK (LAB 1.1): TO DEMONSTRATE
OSMOSIS BY MEANS OF POTATO OSMOSCOPE
Skin of a large sized potato tuber is removed and one side of it is
cut to make a flat base. A cavity is made in the centre of the potato
tuber nearly up to the flat base and is filled with concentrated sugar
solution whose level is marked with a pin.
30
BIO 241: Plant and Animal Physiology
Thereafter place potato tuber in a beaker of water on its flat base.
Observe the experiment for at least two hours.
Questions
1. What would be the nature of results if you now fill the potato
cavity with distilled water and then place the potato tuber in a
beaker containing concentrated sugar solution. Give reason(s) for
answer.
2. Are the cell surface membrane of the potato tuber partially
permeable? Explain your answer.
3. Would you still get the same result(s) if the potato tuber was
replaced by semi-permeable membrane made up of copper
ferrocyanide. Justify your answer.
4.Explain why foods can be preserved by storing them in strong
solutions of salt or sugar.
EXPERIMENTAL WORK (LAB 1.2): EFFECT OF
TEMPERATURE AND ALCOHOL ON PERMEABILITY OF THE
PLASMA MEMBRANE
Small equal sized cylinders of beet root tissue are cut with the help
of a cork-borer and thoroughly washed with water. Each of these
cylinders is placed in a separate test tube containing water at
different temperature e.g. 00C , 100C, 200C, 300C, 400C, 600C,
700C and 800C. One of the cylinders is placed in the test tube
containing alcohol instead of water at ordinary temperature. Then,
observe the experiment for about thirty minutes.
31
BIO 241: Plant and Animal Physiology
Write down your observations and inferences of the experiment.
Use a table
(a)(i) Lower temperature than room temperature
(ii) Room temperature
(iii) Slightly higher temperature than room temperature
(iV) Higher temperatures (e.g. 600C, 700C and 800C).
(b) Explain result noted in (iv).
(c ) (i)write down the colour of the alcohol (cylinder placed in test
tube containing alcohol) after about thirty minutes.
(ii) why is observation noted in c(i) similar to that in a (iv).
1.3. Structure of a lab report in bio 241
The following are the salient components of a lab report. You
should arrange the components in the order they appear below
when you write all lab reports in this course.
Title
Is a statement of the type and extent of the investigation/a summary
of what the study is about.
Aim
Is a statement which specifically describes what you are going to
investigate in the lab or field. A specific area within a given
topic/title.
Introduction/Theory/Background

it gives relevant background to the topic and the rationale
for the investigation

a synopsis of the current state of knowledge about the topic.
32
BIO 241: Plant and Animal Physiology

It includes a clear aim.
Procedure/Method

It’s a step- by- step description of how the laboratory/field
activity was done.

It describes how materials (apparatus and chemicals) were
used (that is, as part of the experimental procedure(s)
involved); if appropriate it includes how variables were
manipulated, measured, monitored etc

It includes details of how data was collected.

Written in the past tense and in active voice and in
paragraph form.
Results

Gives a full description of the results. This section should
not discuss the results.

Includes the raw data ( e.g in a table)

Where necessary, the raw data have been averaged or
transformed

Includes graphs (where appropriate).

Each figure (table, graph, drawing, photo) has a title and is
numbered in a way that makes it possible to refer to it in
the text (Fig. 1 etc)

Written in the past tense and, where appropriate, in the
active voice.
33
BIO 241: Plant and Animal Physiology
Discussion

An interpretation (analysis) of the results written in
paragraph form

Includes a discussion of the findings in light of the concepts
involved.

It includes a description of, where appropriate, trends and
patterns in the data collected.

It also includes comments on sources of error, limitations of
data, assumptions made by the investigator about the
system and where possible, ideas for further investigations.

Where appropriate indicate precision of your results using
standard deviation.
Answers to questions

Write your answers to specific questions raised in the lab
schedules here.

Provide a title ‘Answers to questions’ before answering the
question(s)

Number each question.
Conclusion

Is a statement that describes whether or not the results of
the investigation support the aim.

Provides the reader with the investigator’s analysis of the
results.

It answers the aim of the lab activity.
34
BIO 241: Plant and Animal Physiology
Bibliography or Reference

A list of all sources of information and assistance.

This includes citations of written material ( e.g. texts,
journals), web pages, and practical and advisory help.
35
BIO 241: Plant and Animal Physiology
Unit summary
In this unit you have learnt about two principal methods that are
responsible for the movement of substances into and out of cell,
Summary
namely, diffusion and osmosis. We defined diffusion as the net
movement of particles or molecules of a substance (gas or liquid)
from a region of higher concentration to a region of lower
concentration in order to reach equilibrium. Then, osmosis was
said to be the net movement of water across a selectively permeable
membrane driven by a difference in solute concentrations on the
two sides of the membrane. We noted that osmosis was a special
case of diffusion in which water moves down its concentration
gradient. In addition to this, we studied the behavior of both plant
and animal cells when placed, first in water and then subsequently
placed in concentrated sugar solution and came to differentiate
end-osmosis and exo-osmosis.
Finally, we looked at water potential and its role in determining the
direction of water movement when a cell is placed in isotonic
solution; when a cell is placed in hypertonic solution; when a cell is
placed in hypotonic solution.
36
BIO 241: Plant and Animal Physiology
Assessment
Answer the following questions in the spaces provided
Assessment
1. (a) Explain in your own words what is meant by the term ‘ water
potential.’
(b) How does water potential govern the movement of molecules
by osmosis?
(c ) Name two factors which affect water potential and write an
equation to show their relationship.
(d) Which is the greater water potential, Ψ = 0 kPa or Ψ = -1kPa?
2. Given that the three cells X,Y and Z are adjacent as indicated in
the diagram below.
Fig. 1.5: Adjacent cells X,Y and Z
(a) In which direction will osmosis occur between these cells?
(b) Between which pair of cells will the net rate of water
Movement be greatest ? Justify your answer.
37
BIO 241: Plant and Animal Physiology
Unit 2
Transport in plants
2.0 Introduction
Welcome to unit 2 of this module. In this unit you will learn about
transport of materials in plants. In unit 4 of this module you will
learn how organic compounds are made during photosynthesis.
These organic compounds, made mainly in the leaves must be
moved from leaves to other parts of the plant. Photosynthetic cells
need water and inorganic ions, both of which are usually available
only in the soil. Hence movement of water and inorganic ions from
the roots to sites of photosynthesis is essential.
Diffusion and osmosis would not be sufficient to supply materials
to cells located far away from the source. The named processes can
only carry materials to very near places- a few centimeters from the
soil: water and mineral ions are transported from the roots. Oxygen
and carbon dioxide can move by diffusion and not transported large
distances within the plant. It is essential for to have the basic
understanding right from the start that plants do not transport
materials in a single transport system; water and dissolved mineral
ions are transported upwards in a plant mainly in the xylem. As for
water, inorganic and organic solutes both move are upwards and
downwards in the phloem tissues.
Having outlined the need for transport system in plants, we shall
look at the various levels of transport in plants. Then, we shall
consider the movement of water through cells. You will finally
discuss in some detail the mechanism that is responsible to pull
xylem sap up the plant.
38
BIO 241: Plant and Animal Physiology
During and upon completion of this unit you will be able to:
 Give an outline of absorption and subsequent movement of
water in the roots of a plant.
Outcomes
 Discuss water movement in xylem through TACT
mechanisms
 Discuss the merits and de-merits of Cohension-adhension theory
concening long-distance transport of water
 Explain the phloem transport of sucrose from source to sink
.
2.1. Water Potential re-visited
Before you learn about the various aspects of transport in plants,
you should review unit 1 of this module. This is necessary because
you will need the knowledge of unit 1 for you to understand this
unit.
As a reminder, survival of the plant depends on balancing water
uptake and water loss. As you are already aware, in an animal cell,
water flows from hypotonic to hypertonic solutions, but in a plant
cell, there is the added presence of the pressure created by the cell
wall. The combination of solute concentration differences and
physical pressure are incorporated into water potential,
abbreviated with the Greek letter psi ( ).
You also noticed in unit 1 of this module that water will flow
through a membrane from a solution of high water potential to a
solution of low water potential. We further discussed that pure
water has a water potential of 0 MPa ( = 0 MPa). In this regard
39
BIO 241: Plant and Animal Physiology
the addition of solutes lowers water potential ( = -ve MPa for
instance) and so, an increase in pressure (by lowering a piston for
example) will raise water potential.
You further observed that these two forces named above (water
potential due to pressure, and water potential due solute
concentration) to combine form the following equation:
= p+ s
Where = total water potential, p = water potential due to
pressure (May be positive or negative), and s = water potential
due solute concentration (also known as Osmotic Potential) Always
negative or zero
Having gone through the major concepts of water potential, you
should now be ready to consider how water is absorbed in higher
plants. This will be followed by transport of water and other
substances.
Let us start by considering absorption of water in higher plants.
2.2. Absorption of Water by Roots
In higher plants water is absorbed through root hairs which are in
contact with soil water and form a root hair zone a little behind the
root tips. Root hairs are thin-walled extensions of the epidermal
cells in roots . When epidermis bears root hairs it is known as
piliferous layer of the roots. The walls of the root hairs are
permeable to water and consist of pectic substances and cellulose
which are strongly hydrophilic (water loving) in nature. Root hairs
contain vacuoles filled with cell sap.
40
BIO 241: Plant and Animal Physiology
Mechanism of water absorption
The mechanism of water absorption is of two types, namely,
i.
Active absorption of water
ii.
Passive absorption of water
Active absorption of water: In this process the root cells play
active role the absorption of water and metabolic energy released
through respiration is consumed. Active transport establishes a
lower water potential and helps the root hairs take in the
necessary minerals dissolved in soil water. A lower water
potential allows water to be drawn into the root cells by osmosis.
Active absorption may be of two types:
a. Osmotic absorption, that is, when water is
absorption from the soil into the xylem of the roots
according to the osmotic gradient.
b. Non-osmotic absorption, that is, when water is
absorbed against the osmotic gradient.
Passive Absorption of Water: It is mainly due to transpiration, the
root cells do not play active role and remain passive.
The flow of water and minerals from the soil to the cells of the root
is accomplished by transpirational pull, active transport and a
special layer of cells called the casparian strip. Transpiration will
be addressed later in the module.
41
BIO 241: Plant and Animal Physiology
Minerals enter the root by active transport into the symplast of
epidermal cells and move toward and into the stele through the
plasmodesmata connecting the cells.
They enter the water in the xylem from the cells of the pericycle (as
well as of parenchyma cells surrounding the xylem) through
specialized transmembrane channels.
2.3. Movement of Water through Cells: Two routes, the Symplast
and the Apoplast
Soil water enters the root through its epidermis. It appears that
water then travels in both
a. the cytoplasm of root cells — called the symplast
(extracellular route) that is, it crosses the plasma
membrane and then passes from cell to cell through
plasmodesmata.
b. in the nonliving parts of the root — called the apoplast
(intracellular route) — that is, in the spaces between the
cells and in the cells walls themselves. This water has
not crossed a plasma membrane.
However, the inner boundary of the cortex, the endodermis, is
impervious to water because of a band of suberized matrix called
the casparian strip. Therefore, to enter the stele, apoplastic water
must enter the symplasm of the endodermal cells. From here it can
pass by plasmodesmata into the cells of the stele.
42
BIO 241: Plant and Animal Physiology
The Endodermis - The Root's Border Guard
Water flowing through the apoplast contains many minerals that
the plant needs - it may also contains toxins and substances that the
plant may not want. However, since the water is flowing through
the apoplast, there is no way to prevent the passive transport of
these toxins, until the water hits the endodermis.
Endodermis
Cells of the endodermis possess cell walls that are ringed by the
Casparian Strip, a waxy layer (composed of suberin). The root
endoderm because of a waxy layer of subrin has the ability to
actively transport ions in one direction only.

The Casparian Strip is a wax and therefore prevents the
apoplastic flow of water

Water must pass through the plasma membrane and enter
the symplast

The plasma membrane of the endodermal cells contain
many transport proteins to actively transport some
molecules in and others to pump other molecules out

Once water passes under the Casparian Strip in the
endodermal cells, it is free to enter the apoplast again on its
way to the xylem.
43
BIO 241: Plant and Animal Physiology
Fig 2.1 Symplastic and apoplastic routes
Alternatively, you can represent the two routes involved in the
movement of water in the roots as shown below.
Fig 2.2: Symplastic and apoplastic routesup up to xylem
44
BIO 241: Plant and Animal Physiology
2.4. Water Movement in Xylem through TACT Mechanism
Before you start this section, attempt to answer the following
question.
Using your knowledge from high school biology, what do you
think are the forces that make water move through the Xylem?
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………...............................................................................................
Good try. Do not worry if you were not able to answer very well.
In order for you to answer this question, there is need for you to
first look at the mechanism involved. As you study the text on
water movement, compare your answer to the question with the
major points of the discussion about this mechanism. The
mechanism is based on purely physical forces because the xylem
vessels and tracheids are lifeless.
For water to move though the xylem vessels, roots are not needed.
This was demonstrated over a century ago by a German botanist
45
BIO 241: Plant and Animal Physiology
who sawed down a 21 meters oak tree and placed the base of the
trunk in a barrel of picric acid solution. The solution was drawn up
the trunk, killing nearby tissues as it went.
However, when the acid reached the leaves and killed them, the
upward movement of water ceased showing that leaves are needed
in the upward transport of water.
Removing a band of bark from around the trunk — a process called
girdling — did not stop the upward flow of water. Girdling
removes only the phloem, not the xylem. The leaves do not wilt
because the xylem which transports the water is still present.
*Xylem is the water transporting tissue in plants that is dead when
it reaches functional maturity*. Tracheids are long, tapered cells of
xylem that have end plates on the cells that contain a great many
crossbars. Tracheid walls are (festooned) with pits. Vessels, an
improved form of tracheid, have no (or very few) obstructions
(crossbars) on the top or bottom of the cell. The functional diameter
of vessels is greater than that of tracheids.
2.4.1. Forces that aid water transport in xylem
Four important forces that combine to transport water solutions
from the roots, through the xylem elements, and into the leaves are:
transpiration, adhesion, cohesion and tension. These forces are
simply referred to as TACT forces. These letters refer to the
following:

Transpiration

Adhesion

Cohesion

Tension
46
BIO 241: Plant and Animal Physiology
A combination of adhesion, cohesion, and surface tension (see
below) allow water to climb the walls of small diameter tubes like
xylem. This is called capillary action.
Transpiration involves the pulling of water up through the xylem
of a plant utilizing the energy of evaporation and the tensile
strength of water. You will learn more about transpiration in unit 3.
Adhesion is the attractive force between water molecules and other
substances. Because both water and cellulose are polar molecules
there is a strong attraction for water within the hollow capillaries of
the xylem.
Cohesion is the attractive force between molecules of the same
substance. (The property of water molecules to cling to each other
through the hydrogen bonds they form). Water has an unusually
high cohesive force again due to the 4 hydrogen bonds each water
molecule potentially has with any other water molecule. It is
estimated that water's cohesive force within xylem give it a tensile
strength equivalent to that of a steel wire of similar diameter.
Tension can be thought of as a stress placed on an object by a
pulling force. This pulling force is created by the surface tension
which develops in the leaf's air spaces. A meniscus has a tension
that is inversely proportional to the radius of the curved water
surface.
In other words, as the water surface becomes more curved tension
increases. Tension is a negative pressure - a force that pulls water
from locations where the water potential is greater.
The bulk flow of water to the top of a plant is driven by solar
energy since evaporation from leaves is responsible for
transpiration pull. Root pressure can only provide a modest push in
47
BIO 241: Plant and Animal Physiology
the overall process of water transport. Its greatest contribution may
be to re-establish the continuous chains of water molecules in the
xylem which often break under the enormous tensions created by
transpiration.
Let us now look at a theory in which the above named forces
interact together to pull the water up the stem of a plant from its
roots.
2.4.2. Cohesion-Adhesion Theory
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly
proposed that water is pulled up the plant by tension (negative
pressure) from above.
Transpiration exerts a pull on the water column within the xylem.
The lost water molecules are replaced by water from the xylem of
the leaf veins, causing a tug on water in the xylem. Adhesion of
water to the cell walls of the xylem facilitates movement of water
upward within the xylem. This combination of cohesive and
adhesive forces is referred to as the Cohesion-Adhesion Theory.
As you learnt in your first year biology, stomata usually open up
during the day to let carbon dioxide in and inadvertently let water
escape. So, during transpiration, water vapor leaves the air spaces
of the plant via the stomata. This is due to the point that there is a
gradient in water potential, high water potential in the soil and very
low water potential in the air. This water lost through transpiration
is replaced by evaporation of the thin layer of water that clings to
the mesophyll cells. That is, as the water leaves, it is replaced by
water clinging to the inside of the cell walls. This creates a tension
(pulling) on the water in the xylem and gently pulls the water
toward the direction of water loss. The cohesion of water is strong
enough to transmit this pulling force all the way down to the roots.
48
BIO 241: Plant and Animal Physiology
Adhesion of water to the cell wall also aids in resisting gravity. As
we said before, the water column in the tallest trees can be 100m the tension created by evaporation of water coupled with the
cohesive and adhesive forces is enough to support this column
against the forces of gravity
Root Pressure: A Mechanism to "Push" Xylem Sap Up the
Plant
We have just learnt that stomata open during the day. As a result,
water is lost through them through transpiration. A sensible
question you might ask yourself is that: what happens then at night
when the stomata in the leaves are generally closed? In order to
answer this question well, let us start by considering the following:
at night, transpiration is almost nil. However, the root cells
continue to actively transport minerals into the stele (the root stele
is basically everything surrounded by the endodermis - primarily
the xylem and the phloem). This active transport lowers the water
potential within the stele. Water passively flows into the roots,
pushing the water up against gravity. Water that reaches the leaves
is often forced out, causing a beading of water upon the leaf tips
known as guttation. In most plants, however, root pressure is not
the primary mechanism for transporting the xylem. Tall trees
generate almost no root pressure (the weight of the water pushing
down on the xylem more than counteracts any generated root
pressure). So, what is your final answer?
2.4.3. Problems with the theory
When water is placed under a high vacuum, any dissolved gases
come out of solution as bubbles (as we saw above with the rattan
vine. This is called cavitation. Any impurities in the water enhance
the process. So measurements showing the high tensile strength of
49
BIO 241: Plant and Animal Physiology
water in capillaries require water of high purity — and sap in the
xylem is not pure.
So might cavitation break the column of water in the xylem and
thereby interrupt its flow? Probably not, so long as the tension does
not greatly exceed ~1.9 x 103 kPa.
By spinning branches in a centrifuge, it has been shown that water
in the xylem avoids cavitation at negative pressures exceeding ~1.6
x 103 kPa. And the fact that sequoias can successfully lift water 109
m — which would require a tension of ~1.9 x 103 kPa — indicates
that cavitation is avoided even at that value.
However, such heights may be approaching the limit for xylem
transport. Measurements close to the top of the tallest living
sequoia 113 m high show that the high tensions needed to get water
up there have resulted in:

smaller stomatal openings, causing

lower concentrations of CO2 in the needles, causing

reduced photosynthesis, causing

reduced growth (smaller cells and much smaller needles).
So, the limits to which water can be transported subsequently limits
the ultimate height which trees can reach. The tallest tree ever
measured, a Douglas fir, was 125.9 meters high.
2.5. Phloem Transport: Flow from Source to Sink in Angiosperms
Food, primarily sucrose is transported by the vascular tissue called
phloem from a source to a sink. A source being the part of a plant
where the food is made, and a sink being the part of a plant where
the food is needed or utilised.
50
BIO 241: Plant and Animal Physiology
Phloem consists of several types of cells: sieve tube cells (aka sieve
elements), companion cells, and the vascular parenchyma. Sieve
cells are tubular cells with end walls known as sieve plates. Most
lose their nuclei but remain alive, leaving an empty cell with a
functioning plasma membrane.
Companion cells load sugar into the sieve element (sieve elements
are connected into sieve tubes). Fluids can move up or down within
the phloem, and are translocated from one place to another. Sources
are places where sugars are being produced. Sinks are places where
sugar is being consumed or stored. Translocation - the process of
moving photosynthetic product through the phloem
Food moves through the phloem by a Pressure-Flow Mechanism.
Sugar moves (by an energy-requiring step) from a source (usually
leaves) to a sink (usually roots) by osmotic pressure. Translocation
of sugar into a sieve element causes water to enter that cell,
increasing the pressure of the sugar/water mix (phloem sap). The
pressure causes the sap to flow toward an area of lower pressure,
the sink. In the sink, the sugar is removed from the phloem by
another energy-requiring step and usually converted into starch or
metabolized.
Unlike transpiration's one-way flow of water sap, food in phloem
sap can be transported in any direction needed so long as there is a
source of sugar and a sink able to use, store or remove the sugar.
The source and sink may be reversed depending on the season, or
the plant's needs. Sugar stored in roots may be mobilized to
become a source of food in the early spring when the buds of trees,
the sink, need energy for growth and development of the
photosynthetic apparatus.
51
BIO 241: Plant and Animal Physiology
Phloem sap is mainly water and sucrose, but other sugars,
hormones and amino acids are also transported. The movement of
such substances in the plant is called translocation.
The Pressure Flow or Mass Flow Hypothesis
The accepted mechanism needed for the translocation of sugars
from source to sink is called the pressure flow hypothesis. (see
diagram below)
As glucose is made at the source (by photosynthesis for example) it
is converted to sucrose (a dissacharide). The sugar is then moved
into companion cells and into the living phloem sieve tubes by
active transport. This process of loading at the source produces a
hypertonic condition in the phloem.
Water in the adjacent xylem moves into the phloem by osmosis. As
osmotic pressure builds the phloem sap will move to areas of lower
pressure.
At the sink osmotic pressure must be reduced. Again active
transport is necessary to move the sucrose out of the pholem sap
and into the cells which will use the sugar -- converting it into
energy, starch, or cellulose. As sugars are removed osmotic
pressure decreases and water moves out of the phloem.
52
BIO 241: Plant and Animal Physiology
Fig 2.3: The movement of sugars in the phloem
The movement of sugars in the phloem begins at the source, where
(a) sugars are loaded (actively transported) into a sieve tube.
Loading of the phloem sets up a water potential gradient that
facilitates the movement of water into the dense phloem sap from
the neighboring xylem (b). As hydrostatic pressure in the phloem
sieve tube increases, pressure flow begins (c), and the sap moves
through the phloem. Meanwhile, at the sink (d), incoming sugars
are actively transported out of the phloem and removed as complex
carbohydrates. The loss of solute produces a high water potential in
the phloem, and water passes out (e), returning eventually to the
xylem.
53
BIO 241: Plant and Animal Physiology
In this unit you looked at the absorption of water and
mineral salts from the soil by the root hair cells. Thereafter,
Summary
you learnt about short-distance transport of substances from
cell to cell. Movement of water through root cells takes
place through two routes. These being: the symplast and the
apoplast. Symplastic movement is one which involves the
movement of water and solutes through the continuous
connection of cytoplasm (though plasmodesmata).
Apoplastic movement is the movement of water and solutes
through the cell walls and the intercellular spaces. You also
looked at long-distance transport of water and minerals salts
photosynthate in the xylem and phloem respectively. We
observed that various forces are responsible for pulling the
water in the xylem. However, the most important factor in
lifting the water up the plant through the xylem was
attributed to the cohension-adhension theory. As for
photosynthate, we noted that mass flow hypothesis had been
suggested for its movement from the leaves (source) to
other parts of the plant (sink).
54
BIO 241: Plant and Animal Physiology
Answer the following questions in the spaces provided
Assessment
1.
With the help of a well labelled diagram, explain how water
movement through the symplast and apoplast occurs.
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
2. Analyse the forces that contribute to the lifting of water through
the xylem vessels
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
55
BIO 241: Plant and Animal Physiology
Unit 3
Transpiration
3.0 Introduction
Welcome to this unit. In this unit you will look at how plants lose
water to their immediate environment. We shall then consider the
physiological importance of the process of transpiration to plants.
This unit will also address the de-merits of the said process. You
will then discuss the role anti-transpirants in reducing the amount
of transpiration. In this unit you will be exposed to a number of
practical work – this will definitely you in filling the gaps between
theory and practical work.
Before you start exploring the subject of transpiration, let us first
acquaint yourself with the meaning of the term ‘transpiration’.
56
BIO 241: Plant and Animal Physiology
During and upon completion of this unit you will be able to
57
BIO 241: Plant and Animal Physiology
 State the differences between transpiration and evaporation in
plant leaves
Outcomes
 Discuss the role of transpiration in the pulling of water from the
roots up the plants.
 Discuss factors responsible for triggering the change in the
shape of guard cells.
 Suggest strategies developed by some plants to reduce the rate
of transpiration.
58
BIO 241: Plant and Animal Physiology
3.1 What is Transpiration
What do you understand by the term ‘transpiration’?
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………
I am sure your response looks very similar to this ‘Transpiration is
the process where water contained in liquid form in plants is
converted to vapour and released to the atmosphere.’ This
definition, however, appears to equate transpiration to evaporation.
It is difficult to separate the processes of evaporation and
transpiration, so this transfer of water is sometimes simply called
evapotranspiration.’ The major difference between the two
processes is that transpiration is physiological while evaporation is
physical. Against this background, a better definition of
transpiration and which we shall adopt in this unit is that
transpiration is a vital physiological process in plants in which
water is lost from their aerial parts in form of water vapour and for
which living tissues are. The said loss of water occurs chiefly at the
leaves while their stomata are open for the passage of CO2 and O2
during photosynthesis or transpiration. It can be demonstrated by
the following experiment:
59
BIO 241: Plant and Animal Physiology
Fig. 3.1: Demonstration of transpiration
3.2. Kinds of Transpiration
At this level of your studies, it is important for you to be familiar
with the nature of transpiration that is occurring in plants. If you
own a garden this will be of great use. Generally, there are three
types of transpiration, namely, stomatal transpiration, cuticular
transpiration and lenticular transpiration. Let us now examine each
one of them:
a) Stomatal Transpiration. About ninety five percent of
transpiration takes place through stomata. Stomata are
usually distributed in more numbers on the lower surface
of the leaves. In monocotyledous e.g. grasses they are
equally distributed on all sides. While in aquatic plants
with floating leaves they are present on the upper surface.
60
BIO 241: Plant and Animal Physiology
b) Cuticular Transpiration. (Peristomatal Transpiration).
Although cuticle is impervious to water. Still some water
may be lost through it. It may contribute a maximum of
about 10%. of the total transpiration.
c) Lenticular Transpiration. Some water may be lost by
woody stems through lenticels which is called Lenticular
Transpiration.
As you have already seen, the stomata are the vehicle for
transpiration. Transpiration through stomata on the leaves is called
foliar transpiration.
3.3 Transpiration as a necessary evil
The volume of water lost in transpiration can be very high. It has
been estimated that over the growing season, one acre of corn
(maize) plants may transpire 400,000 1.5 million liters of water. As
liquid water, this would cover the field with a lake 38 cm deep. An
acre of forest probably does even better.
But air that is not fully saturated with water vapor (100% relative
humidity) will dry the surfaces of cells with which it comes in
contact. So the photosynthesizing leaf loses substantial amount of
water by evaporation. This transpired water must be replaced by
the transport of more water from the soil to the leaves through the
xylem of the roots and stem.
61
BIO 241: Plant and Animal Physiology
Besides unnecessary wastage of energy in water absorption due to
transpiration, it may be sometimes harmful to them in other
respects also. For example:

Very often, when the rate of transpiration is high and soil
deficient in water an internal water deficit is created in the
plants which may affect metabolic processes.

Many xerophytes have to develop structural modifications
and adaptations to check transpiration.

Deciduous trees have to shed their leaves during autumn to
check loss of water.
But, inspite of the various disadvantages the plants cannot avoid
transpiration due to their peculiar internal structure particularly
those of leaves. Their internal structure although basically meant
for gaseous exchange for respiration, photosynthesis etc is such that
it cannot check the evaporation of water. Therefore, many workers
like Curtis (1926) have called transpiration as necessary evil.
3.4 Supposed advantages of Transpiration
Considering what you learnt in the previous unit, you might be
‘tempted’ to think of transpiration as simply a hazard to plant life.
Contrary to this view, transpiration is usually thought to be the
"engine" that pulls water up from the roots to the leaves. Plants also
seem to benefit from the process of transpiration in several other
ways. The supposed advantages of transpiration include the
following:
a) supply photosynthesis (1%-2% of the total); (It is the
‘engine" that pulls water up from the roots up the plant).
b) bring minerals from the roots for biosynthesis within the
leaf;
62
BIO 241: Plant and Animal Physiology
c) transpiration tends to cool the leaf.
d) maintains the plant's shape and structure by keeping cells
turgid.
3.5 Mechanism of Stomatal Transpiration
In section 3.2 of this module we came to know that there are three
types of transpiration. Having done that, we should now be happy
to move on to the mechanism of transpiration. In this section, we shall
only consider the mechanism of stomatal transpiration. In order for
us to appreciate how stomatal transpiration occurs, it is important
for us to first study the structure of a stoma and thereafter, keep
referring to it as we look at key events leading to its opening and
closure.
Fig 3.2: Structure of guard cells
According to the diagram above, you should be able to notice that a
stoma has:
a) two guard cells that are fused at their ends.
b) the inner cell walls of these guard cells which form the stoma
are thicker than the outer walls.
63
BIO 241: Plant and Animal Physiology
c ) The guard cells has cellulose microfibrils that are oriented
radially rather than longitudinally.
Upon examining the above fig, it should now be easy for you to
notice that a stoma is a physical gap between two special epidermal
cells called guard cells. When the pair of guard cells are turgid.
That is, full of water, they bow in such a way as to increase the gap
(stoma) between them.
Let us now apply our knowledge on the structure of a stoma to
answer the following activity below.
Explain why guard cells curve when they are turgid. (You should
not spend more than 10 minutes on this task).
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
..........................................................................................................
64
BIO 241: Plant and Animal Physiology
3.5.1 Theories on opening and closing of the stomata
Let us now turn our attention to how you could explain the opening
and closing of the stomata. The following theories will be used in
our description of the opening and closing of the stomata:
a) Photosynthetic theory
b) Starch-sugar inter-conversion theory:
Photosynthetic theory
This theory assumes that, for the stoma to open, photosynthesis
should be taking place in the guard cells. As you previously learnt
in you foundation biology that guard cells have chloroplasts, so
these cells carry out photosynthesis in the presence of sunlight.
Photosynthesis manufactures ‘glucose’ which increases the osmotic
pressure in the guard cells as compared to the epidermal cells that
surround them. As a result, water moves into the guard cells by
osmosis hence increasing its turgidity. The inner walls of the guard
cells are thicker than the hence the outer walls stretch more than the
inner wall causing the inner wall to bulge outwards. In this process,
the stomata opens.
This theory further assumes that In absence of light (at night), no
photosynthesis takes place in the guard cells of the leafs.
The glucose in the cells that was manufactured in the day is
converted into starch; which lowers the osmotic pressure of the
guard cells than that of the epidermal cells surrounding them. Due
to this difference in the osmotic potential between the guard cell’s
interior, water moves from the guard cells into the surrounding
fluid outside it. As a result of this, the epidermal cells withdraws
water from the guard cells through osmosis, making the guard cell
flaccid. The thinner outer wall shrinks and the curvature of the
inner wall reduces; then the stomata closes.
65
BIO 241: Plant and Animal Physiology
Starch-sugar inter-conversion theory
Photosynthesis occurs during the day due to the presence of light.
This lowers the concentration of Carbon dioxide which is a raw
material for the above process. This reduces the acidity of the
guard. This condition favours conversion of starch to glucose
(sugar); which then increases the guard cells’ osmotic pressure;
water from the nearby epidermal cells will move by osmosis to the
guard cell making it more turgid.The thinner outer walls stretches
more causing the guard cells to bulge out hence opening the
stomata.
At night when there is no light, no photosynthesis takes place that
means the level of carbon dioxide in the guard cells increases
increasing acidity. Acidic condition promotes conversion of
glucose to starch; and the osmotic pressure of the guard cells
reduces than that of the neighbouring cells hence loses water
through osmosis. The cell thus become flaccid and the stomata
then closes.
If the plant loses water, the guard cells become flaccid and the gap
closes
3.5.2 The mechanism for opening and closing of a
stoma
Let us now attempt to explain how a stoma opens and closes. To
start with, when a pair of guard cells is fully turgid, they (guard
cells) bow in such a way as to increase the gap themselves. The
entry of water into these same guard cells causes an increase in
turgor pressure. Turgor pressure increases because of a negative
66
BIO 241: Plant and Animal Physiology
water potential due to an influx of potassium ions (K+) into the
guard cells from its neighbouring epidermal cells. As a result of
this accumulation of K+ from the neighboring epidermal cells,
these guard cells become hypertonic to its environment.
The reversible uptake of K+ ions takes place because of the
membrane potential created when H+ are actively pumped out of
the cell. This reversible movement of K+ ions, from cell's interior to
the outside, makes the cell's interior become negative compared to
its surroundings.
Fig 3.1: Diagramatic presentation of the opening and closing of a
stoma
The stoma is closed at night
when the large central
vacuole is isotonic, even
hypotonic to surrounding
fluids. K+ ions are outside
of the cell, and H+ ions by
and large remain attached to
the weak organic acids
within the cell.
Blue light is absorbed
by a membrane
protein which
somehow causes an
increase in the
activity of proton
pumps which use
ATP to transport H+
out of the cell.
With H+ on the outside K+
readily diffuse into the cell
to compensate for the
negative electrical
potential. The hypertonic
conditions within the cell
attract water molecules and
the stoma opens as turgor
pressure increases.
The changes in turgor pressure result primarily through the
reversible uptake of K+ ions. As a result of this, stomata open when
guard cells accumulate K+ from neighboring epidermal cells. How
67
BIO 241: Plant and Animal Physiology
does this change the water potential ( ) in the guard cells? Well,
the, stomata close when K+ leaves the guard cells into the
neighboring epidermal cells. The transport of K+ is probably
coupled to the transport of H+ in an antiport system.
3.5.3 What factors trigger the change of shape in
guard cells
The factors that trigger the change of shape in guard cells include
the following:
a) increase in blue light at dawn - a blue light sensitive
receptor activates proton pumps and turgor pressure
increases. Light also stimulates the photosynthetic
production of ATP
b) Absence of CO2
c) Circadian rhythms - All eukaryotic cells have chemical
based metabolic clocks entrained to the day-night cycle. A
common 24 hour biological clock is responsible for
circadian rhythms (circa, about - dies, day)
3.5.3 Strategies for maximizing the availability of
CO2 while minimizing water loss
Strategies for maximizing the availability of CO2 have evolved in
land plants. These are as follows:
In C4 Photosynthesis: C4 plants are twice as efficient as C3
varieties in terms of fixing carbon (making sugar). A C4 plant will
lose "only" 300 grams of water by evaporation for every gram of
CO2 fixed by photosynthesis whereas C3 plants lose 600 grams of
water for the same grams of CO2 fixed.
68
BIO 241: Plant and Animal Physiology
CAM Photosynthesis: A CAM plant is adapted to the hot dry
conditions prevalent in desert climes. These plants have a unique
strategy in which their stomata remain closed most of the day when
water loss is highest, but can maintain a healthy rate of
photosynthesis even though CO2 is not supplied by gas exchange
through these pores. How can this be?
These plants absorb and store CO2 at night when stomata are open.
Water loss is reduced and the acids which are used to sequester the
CO2 readily release it during the day as needed.
3.5.4 Factors that affect the rate of stomatal
movements
Let us now move on to consider factors that affect the rate of
stomatal movements. In this module, you shall use stomatal
movements interchangeably with opening and closing of stomata.
The factors that greatly influence and control stomatal movements,
include the following: Light, Temperature, Carbon dioxide
concentration, and Water deficits and abscisic acid (ABA)
Light
Plants transpire more rapidly in the light than in the dark. This is
largely because light stimulates the opening of the stomata. Light
also speeds up transpiration by warming the leaf. The amount of
light required to achieve maximal stomatal openings varies with
species. Light may have controlling influence on stomatal opening
in the following ways among several others:
(i) Photosynthesis reduces the CO2 conc. In guard cells which
has powerful stimulus for opening of stomata.
(ii) Osmotically active substances such as soluble sugars sre
synthesized during photosynthesis which may contribute in
decreasing the water potential of guard cells.
69
BIO 241: Plant and Animal Physiology
Temperature
Usually an increase in temperature results in increased stomatal
opening provided water does not become a limiting factor.
Stomata of some plants e.g. camellia do not open at very low
temperatures (below 0 0C) even in strong light. At 30°C, a leaf may
transpire three times as fast as it does at 20°C. On the other hand in
some plant species the stomata tend to close even at high temp.
(more than 30°C). This may be due to increased CO2 conc. Inside
the leaves caused by increased respiration rate at high temp. and
heat-impaired photosynthesis in the latter category of plants.
Carbon dioxide concentration
Conc. of CO2 has pronuounced effect on stomatal movement.
Reduced CO2 conc. favours opening of stomata while an increase
in CO2 conc. promotes stomata closing. Why ?
Water deficits and abscisic acid (ABA)
When rate of transpiration exceeds the rate of absorption of water,
a water deficit is created in plants. Such plants begin to show signs
of wilting and are known as water stressed plants. Most of the
mesophytes under such conditions close their stomata quite tightly
and completely in order to protect them from the damage which
may result due to extreme water shortage. The stomata reopen only
when water potential of these plants is stored. This type of control
of stomatal movement by water is called hydropassive control.
Accumulation of absciscic acid (ABA) in the guard cells of many
different water-stressed plants is now established. The ABA causes
stomata of such plants to close. When water potential of the waterstressed plant is restored, the stomata reopen and ABA gradually
disappears from guard cells. this type of control of stomata by
70
BIO 241: Plant and Animal Physiology
water (mediated through ABA) has been called hydroactive
control. Externally applied ABA to leaves of normal plants is also
known to induce closure of stomata and the idea is growing that
ABA is a primary regulator of stomatal action in water-stressed
plants.
3.6 Measurement of Transpiration
The following methods are usually employed for the measurement
of transpiration in plants.
(i) Weighing methods
(ii) Cobalt chloride method
(iii) Collecting and weighing transpired water
(iv) Ganong’s potometer
In our discussion, we shall employ the Ganong’s photometer. The
movement of the air bubble in the capillary tube is noted on the
scale which measures the rate of transpiration.
Fig 3.3 Ganong’s photometer
71
BIO 241: Plant and Animal Physiology
3.7 Leaves: Transpiration and Pulling of Water
As you already know from your high school biology,
photosynthesis requires water. The system of xylem vessels from
root to leaf vein can supply the needed water. What force does a
plant use to move water molecules into the leaf parenchyma cells
where they are needed? Let us consider the nature of forces that
contribute to the actual movement of water up a tree. Well, several
forces combine to overcome the pull of gravity. The pulling forces
and energy needed involves:
a) free energy of the water potential gradient
b) free energy of evaporation
c) force of surface tension
d) force of hydrogen bonding between water molecules .
These combined forces culminate in the process called
transpiration. Ultimately water is pulled, molecule by molecule
into the leaf. Each force can be communicated to the next because
water forms a strong continuous chain from root to leaf.
a) Water moves in the direction it does (root to leaf) because
of the water potential gradient. The gradient is highest in
the water surrounding the roots and lowest in the air space
within the spongy parenchyma of the leaf. (liquids have
higher potential than gases and the purer the liquid the
higher its potential)
b)
The energy of evaporation is needed to to pull molecules
away from the film of water coating air spaces within the
spongy parenchyma. (see diagram below)
c)
As more molecules evaporate from the film coating the air
spaces the curvature of the meniscus increases which
increases the surface tension. Water from surrounding cells
72
BIO 241: Plant and Animal Physiology
and air spaces will then be pulled towards this area to reduce
the tension.
d)
Finally these forces are communicated to water molecules
within the xylem because each water molecule is bound to
the next by hydrogen bonds.
Water Potential and the leaf
Evaporation from the leaf sets up a water potential gradient
between the outside air and the leaf's air spaces. The gradient is
transmitted into the photosynthetic cells and on to the water-filled
xylem in the leaf vein.
Fig 3.4: Water Potential and Evaporation of water from the leaf
3.8. Factors affecting the rate of Transpiration
Using a potometer you can study the effect of various
environmental factors on the rate of transpiration. As you
previously observed, when water is transpired or otherwise used by
the plant it has to be replaced from the soil. In this section, you
shall discuss factors that tend to affect the rate of transpiration in a
plant can be divided into the following components, namely,
a) External factors
b) internal factors
73
BIO 241: Plant and Animal Physiology
3.8.1 External Factors
(i) Atmospheric humidity
The rate of diffusion of any substance increases as the
difference in concentration of the substances in the two
regions increases. When the surrounding air is dry,
diffusion of water out of the leaf goes on more rapidly.
(ii) Temperature
An increase in temperature brings about an increase in the
rate of transpiration by lowering the relative humidity, and
opening the stomata widely.
(iii)Wind
.When the wind is stagnant (i.e. not blowing) the rate of
transpiration remains normal.
When the wind is blowing gently the rate of transpiration
increase because it removes moisture from the vicinity of
the transpiring parts of the plant., thus facilitating the
diffusion of water vapour from the intercellular spaces of
the leaves to the outer atmosphere through stomata.
When the wind is blowing violently the rate of transpiration
is decreased because it creates hindrance in the outward
diffusion of water vapour from the transpiring parts and it
may also close the stomata.
(iv) Light increases the rate of transpiration because: (a) in light
stomata open, and
(b) it increases the temperature.
74
BIO 241: Plant and Animal Physiology
(v) Ultimate effect of atmospheric pressure on the rate
of transpiration is nil. The positive effect of low
atmosphere pressure is neutralized by the low temperature
with it. Similarly, the negative effect of high atmosphere in
plants on the rate of transpiration is neutralized by
comparatively higher temperature of the plants.
(vi) Availability of soil water
Rate of transpiration will decrease if there is not enough
water in the soil from which it can easily be absorbed by
the root. That is, a plant cannot continue to transpire
rapidly if its water loss is not made up by replacement from
the soil. When absorption of water by the roots fails to keep
up with the rate of transpiration, loss of turgor occurs, and
the stomata close. This immediately reduces the rate of
transpiration (as well as of photosynthesis). If the loss of
turgor extends to the rest of the leaf and stem, the plant
wilts.
(vii) Carbon dioxide
An increase in CO2 conc. in the atmosphere (over the
usual conc.) more so inside the
leaf, leads towards stomatal closure and hence, it retards
transpiration.
3.8.2 Internal Factors
(i) Internal water condition
Deficiency of water in the plants will result in decrease of
75
BIO 241: Plant and Animal Physiology
transpiration rate. Increased rates of transpiration
continuing for longer periods often creates internal water
deficit in plants because absorption of water does not keep
pace with it.
(ii) Structural features
The number, size, position and the movement of stomata
affect rate of transpiration. In dark stomata are closed and
stomata transpiration is checked. Sunken stomata help in
reducing the rate of stomata transpiration. When they are
situated in grooved and sometimes protected by hairs, the
rate of transpiration is further decreased. In xerophytes the
leaves are reduced in size or may even fall to check foliar
transpiration. Thick cuticle or presence of wax coating on
exposed parts reduces cuticular transpiration.
3.8.3 Diurnal fluctuations in the rate of
stomatal transpiration .
You are already aware that the rate of transpiration is not
the same through out the day. With respect to stomatal
transpiration, its rate varies throughout the daily period of
24 hours. Generally, The fluctuations (changes) in the rate
during the day are as follows:
(i)
In the morning, when light falls on plants the
stomata begin to open and transpiration starts at a
certain rate.
(ii)
Stomata gradually open widely increasing the rate of
transpiration till it reaches its maximum a little
before noon.
76
BIO 241: Plant and Animal Physiology
(iii)
At about noon, internal water deficit is created
because absorption of water fails to keep pace with
the rate of transpiration. This lowers the turgor
pressure of the guard cells which now become
flaccid to close the stomata. Both these factors
result in sharp decline in the rate of transpiration,
(leaves at this stage may even fade out.
(iv)
Internal water deficit in plants is made good in the
afternoon gradually due to the absorption of more
water by the roots. Stomata again open and the rate
of transpiration increases (but is not maximum).
(v)
In the evening the stomata begin to close due to
diffused light and the rate of transpiration falls.
(vi)
At night the stomata are closed and the stomatal
transpiration is almost completely stopped.
In some plants such as tomato, strawberry, watery drops ooze out
from the uninjured margins of leaves where a main vein ends. This
is called gutation. The phenomenon is associated with the presence
of special types of stomata at the margins of the leaves which are
called water stomata or hydathodes. Each hydathode consists of a
water pore which remains permanently open. Usually gutation
takes place early in the morning when the rate of absorption and the
root pressure are higher while the transpiration is very low.
3.9. The Control of Transpiration
As long as plants can pull water from the soil as fast as it leaves
from the leaves, there is no problem. It is important to notice that
when water loss exceeds water uptake, the plants will wilt as the
77
BIO 241: Plant and Animal Physiology
leaves lose turgor pressure. The conditions that favor wilting are
hot, sunny, and windy days. Adaptations to reduce transpiration loss
in plants growing in dry conditions (xerophytes) are as indicated
below.
a)
Succulent (thick) leaves - store water
b)
Loss of leaves/reduction of leaves to form spines - light is
not limiting, so photosynthesis can be carried out by the
shoot
c)
White leaves/spines - light colors reflect light and heat,
thereby cooling the plant
d)
Trichomes (hairs) - create a more humid microenvironment
to reduce evaporative water loss
e)
Sunken stomates - like trichomes, a more humid
microenvironment is created
f)
CAM photosynthesis - stomates open during the night
(when it is cooler) and fix CO2 into four-carbon acids
g)
Thick cuticles - prevent water loss from epidermal cells
A number of substances are known which when applied to plants
retard their transpiration. Such substances are called
antitranspirants. Some antitranspirants include the following:
colourless plastics, silicon oils and low viscosity waxes when
sprayed on leaves, form a thin film which is impermeable to water
but not to CO2 or O2. CO2.
An increase in CO2 conc. in atmosphere from the usual 0.03% to
about 0.05% causes partial closure of stomata. In very high conc.
however, it may cause complete stomatal closure and thus retard
photosynthesis too. Use of CO2 as antitranspirant is economical and
practically feasible only in glasshouses.
78
BIO 241: Plant and Animal Physiology
EXPERIMENTL WORK (LAB 3.1): To compare the rate of
absorption of water with the rate of transpiration
Take a large glass bottle connected with a graduated side tube. Fill
the apparatus with water and fix a small rooted plant in the bottle
through a hole in the cork. Make the cork airtight and put a few
drops of oil on the surface of the water in the side tube.
Set the apparatus as arranged below.
Fig. 3.5: Apparatus to compare the amount of water transpired
with that absorbed by the roots
Weigh whole of the apparatus and note the level of water in the
graduated tube. Observe the experiment for a few hours. Then,
note the difference between the initial and final water levels; the
difference between initial and final water level will be equal to the
amount of water absorbed by the plant through the roots.
79
BIO 241: Plant and Animal Physiology
Now reweigh whole of the apparatus. The difference between the
initial and final weight will be equal to the amount of water
transpired by the plant.
Questions
1. Give a reason for putting a few oil drops on the surface of the
water in the side tube.
2. What is the amount of water absorbed by the plant through its
root?
3. What is the amount of water transpired by the plant?
4. Compare the rate of absorption of water with the rate of
transpiration.
EXPERIMENTL WORK (LAB 3.2): Stomatal transpiration
and cuticular transpiration in the leaves
Take four fresh leaves from a dicot tree. Tie their petioles with a
thread whose both ends in turn are tied with two strands so that the
leaves hang freely.
The four leaves were arranged as shown below.
Fig. 3. Four leaves experiment
80
BIO 241: Plant and Animal Physiology
Now cover both the surfaces of the first leaf by Vaseline. Apply
Vaseline only on the lower side of the second leaf and on the upper
side of the third leaf. Do not apply Vaseline on the fourth leaf.
Keep them in sunlight. Observe the experiment for a few hours.
Questions
1. What are your observations on each of the leaves? Give
reason(s) for your answers.
2. Compare the amount of water (in relative terms) lost in stomatal
transpiration and cuticular
transpiration as noted in the four leaves in this experiment.
3. Explain how humidity, temperature and light affect transpiration.
4. Explain stomatal regulation of transipiration.
81
BIO 241: Plant and Animal Physiology
Unit summary
In this unit you looked at how plants lose water to their immediate
environment. We discussed that plants lose water to the
Summary
atmosphere mainly through the process of transpiration. Three
kinds of transpiration were discussed. These being, stomatal
transpiration, cuticular transpiration and lenticular transpiration.
We defined transpiration as being a vital physiological process in
plants in which water is lost from their aerial parts in form of
water vapour and for which living tissues are. The said loss of
water occurs chiefly at the leaves while their stomata are open for
the passage of CO2 and O2 during photosynthesis. The water is lost
from the plant through stomata. We came to learn that Stomata
open when guard cells accumulate K+ from neighboring epidermal
cells. Then, stomata close when K+ leaves the guard cells into the
neighboring epidermal cells. The transport of K+ is probably
coupled to the transport of H+ in an antiport system.
Despite transpiration being harmful to the plant, the process helps
in pulling the water up the plant in the xylem, thus, supplying
water and mineral ions for biosynthesis. The process also helps in
cooling the plant.
Various factors tend to affect the rate at which transpiration occurs
and these include the following temperature, wind, atmospheric
humidity, leaf structure, and carbon dioxide. We also noted that
some plants have developed adaptations to reduce transpiration.
Finally, you learnt that antitranspirant were substances that tend
to reduce or retard transpiration.
82
BIO 241: Plant and Animal Physiology
Answer the following question in the spaces provided
Assessment
Design an experiment that would help you to determine the
rate of transpiration.
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
...........................................................................................................
83
BIO 241: Plant and Animal Physiology
Assessment
Answer the following questions in the spaces provided
Assessment
84
BIO 241: Plant and Animal Physiology
Unit 4
Photosynthesis
4.0 Introduction
You already know that photosynthesis uses the energy from
sunlight to produce sugar. Photosynthesis is also sometimes called
carbon assimilation. The conversion of unusable sunlight energy
into usable chemical energy is associated with the actions of the
green pigment chlorophyll. Most of the time, the photosynthetic
process uses water and release the oxygen that we absolutely must
have to stay alive. Oh yes, we humans need the food as well!
There is still much about the process that is not known, but in the
last decades enormous strides have been made in analyzing the
chemical pathways involved. It is not our purpose here to discuss in
detail all the chemistry of photosynthesis or to mention all the
reactions and compounds now thought to be involved in it; but
photosynthesis is so fundamental to life that you ought to
understand at least the broad outlines of the process.
During and upon completion of this unit you will be able to:
85
BIO 241: Plant and Animal Physiology
 Discuss the role of photosynthetic pigments in higher plants.
 Describe the structure of chloroplast
Outcomes
 Explain action and absorption spectra of light of various
photosynthetic pigment
 Discuss the role of the ‘light-harvesting system’ in green plants.
 Discuss the mechanism of photosynthesis in green plants.
 State the salient features of C3 carbon reduction cycle.
 Interpret transport of photoassimilates; source –sink relations.
4.0 Introduction
4.1 Organisms that are able to photosynthesize
Photosynthesis occurs in plants, algae and many species of
bacteria, but not in archaea. Photosynthetic organisms are called
photoautotroph. Although photosynthesis can happen in different
ways in different species, some features are always the same, e.g.
the process always begins when energy from light is absorbed by
proteins called photosynthetic pigments.
Leaves of higher green plants are the principal organs of
photosynthesis. Photosynthesis takes place within the chloroplasts
of the mesophyll tissue.
Photosynthesis is represented by the following traditional equation:
86
BIO 241: Plant and Animal Physiology
4.2. Significance of Photosynthesis
a) It maintains the normal level of oxygen in the atmosphere
b) Nearly all life either depends on it directly or indirectly as
the ultimate source of energy in their food
c) It provides vast reserves of energy to man as fuel such as
coal, oil, peat, wood and dung. (Photosynthesis is a source
of the carbon in all the organic compounds within
organisms’ bodies.
The reactions of photosynthesis can be divided into two
distinct stages, namely, light-dependent and lightindependent reactions. In order for you to understand the
series of reactions of these two distinct stages of
photosynthesis, there is need for us, to first look at how
light energy is trapped, and then discuss how this trapped
light energy is converted into chemical energy in the form
of ATP and NADPH.
4.3. Photosynthetic Pigments
A pigment is any substance that absorbs light. The color of the
pigment comes from the wavelengths of light reflected (in other
words, those not absorbed). Chlorophyll, the green pigment
common to all photosynthetic cells, absorbs all wavelengths of
visible light except green, which it reflects to be detected by our
eyes. Black pigments absorb all of the wavelengths that strike
them. White pigments/lighter colors reflect all or almost all of the
energy striking them. Pigments have their own characteristic
absorption spectra, the absorption pattern of a given pigment.
Chlorophyll is a complex molecule. Several modifications of
chlorophyll occur among plants and other photosynthetic
87
BIO 241: Plant and Animal Physiology
organisms. All photosynthetic organisms (plants, certain protistans,
prochlorobacteria, and cyanobacteria) have chlorophyll a.
Accessory pigments absorb energy that chlorophyll a does not
absorb. Accessory pigments include chlorophyll b (also c, d, and e
in algae and protistans), xanthophylls, and carotenoids (such as
beta-carotene). Chlorophyll a absorbs its energy from the VioletBlue and Reddish orange-Red wavelengths, and little from the
intermediate (Green-Yellow-Orange) wavelengths.
4.3.1 Types of Photosynthetic pigments
There are three types of photosynthetic pigments in higher plants.
(i) Chlorophylls
(ii) Carotenoids and
(iii) Phycobillins
Chlorophyll and Carotenoids are insoluble in water and can be
extracted only with organic solvents. Phycobillins are soluble in
water. Carotenoids include carotenes and xanthophylls. The latter
are also called carotenols. Different pigments absorb light of
different wavelengths and show characteristic absorption peaks in
vivo and in vitro.
88
BIO 241: Plant and Animal Physiology
Table 4.1: Distribution of Photosynthetic Pigments in Plant
Kingdom
Pigment
Distribution in Plant
Kingdom
Chlorophylls
chlorophyll a
all photosynthesizing plants
chlorophyll b
higher plants and algae
chlorophyll c
diatoms and brown algae
chlorophyll d
in some red algae
chlorophyll e
in tribonema and zoospores of
vancheria
bacterio chlorophyll a
purple and green bacteria
bacterio chlorophyll b
purple bacterium
Carotenoids
Carotenes
Mostly in algae and higher
plants
Xanthophylls (carotenols)
Mostly in algae and higher
plants
Phycobillins
Phycoerythrine
In blue-green and red algae
Phycocyanine
Allophycocyania
89
BIO 241: Plant and Animal Physiology
4.3.2. The structure of the chloroplast and
photosynthetic membranes
The thylakoid is the structural unit of photosynthesis. Both
photosynthetic prokaryotes and eukaryotes have thylakoids. They
are flattened sacs/vesicles containing photosynthetic chemicals.
Only eukaryotes have chloroplasts with a surrounding membrane.
Thylakoids are stacked like pancakes in stacks known collectively
as grana. The areas between grana are referred to as stroma. While
the mitochondrion has two membrane systems, the chloroplast has
three, forming three compartments. These three compartments
being

intermembrane- between the two membranes of the
chloroplast envelope

stroma

thylakoid lumen
Fig 4.1: structure of Chlorophyll
90
BIO 241: Plant and Animal Physiology
4.3.3. How the structure of chloroplast is adapted to
carry out photosynthesis
The chloroplasts of higher plants are discoid or ellipsoidal in shape;
the chloroplast is bounded by two membranes; chloroplasts have a
partial genetic autonomy. Internally the chloroplast is filled with
stroma in which is embedded granum.
Thylakoid membranes of grana are the primary photochemical
reaction centres; Light-dependent reaction occurs on these
thylakoid membranes; Dark reactions of photosynthesis occur in
the stroma. The stroma region of chloroplasts contains all the
enzymes, sugars and organic acids which are needed for the lightindependent reactions or dark reactions as they are popularly
referred to.
Thylakoids within chloroplasts have a relatively large
surface area which allows as much light energy as possible to be
absorbed. The chloroplast has an envelope (inner and outer
membrane) which contains the reactants for photosynthesis and
keeps them close to the reaction sites. The thylakoid membranes
contain many ATP synthase molecules for the production of ATP
in the light-dependent reaction.
4.4. Action and Absorption Spectra
4.4.1. Action Spectrum
An action spectrum is the rate of a physiological activity plotted
against wavelength of light.
In 1881, the German plant physiologist T. W. Engelmann placed a
filamentous green alga under the microscope and illuminated it
with a tiny spectrum of visible light.
In the medium surrounding the strands were motile, aerobic
bacteria.
91
BIO 241: Plant and Animal Physiology
After a few minutes, the bacteria had congregated around the
portions of the filament illuminated by red and blue light.
Assuming that the bacteria were congregating in regions where
oxygen was being evolved in photosynthesis, Engelmann
concluded that red
Fig 4.2: The action spectra for photosynthesis
and blue light are the most effective colors for photosynthesis.
With modern instruments, a plot of the rate of photosynthesis as a
function of wavelength of light produces a graph like this. More
precise than Engelmann's but telling the same story.
The action spectrum of photosynthesis is the relative effectiveness
of different wavelengths of light at generating electrons. If a
pigment absorbs light energy, one of three things will occur.

Energy is dissipated as heat.

The energy may be emitted immediately as a longer
wavelength, a phenomenon known as fluorescence.
The action spectrum of photosynthesis is the relative effectiveness
of different wavelengths of light at generating electrons. If a
pigment absorbs light energy, one of three things will occur.

Energy is dissipated as heat.
92
BIO 241: Plant and Animal Physiology

The energy may be emitted immediately as a longer
wavelength, a phenomenon known as fluorescence.

Energy may trigger a chemical reaction, as in
photosynthesis. Chlorophyll only triggers a chemical
reaction when it is associated with proteins embedded in a
membrane (as in a chloroplast) or the membrane infoldings
found in photosynthetic prokaryotes such as cyanobacteria
and prochlorobacteria
4.4.2 Absorption Spectrum
An absorption spectrum is a spectrum of radiant energy whose
intensity at each wavelength is a measure of the amount of energy
at that wavelength that has passed through a selectively absorbing
substance.
The absorption of radiation by a substance can be quantified with
an instrument called a spectrophotometer. This is a device that

produces a beam of monochromatic ("single-color")
radiation that can be shifted progressively across the
spectrum;

passes the beam through a solution of the substance, and

measures the radiation that gets through.
The graph shows the absorption spectrum of a mixture of
chlorophyll a and chlorophyll b in the range of visible light.
Carotenoids and chlorophyll b absorb some of the energy in the
green wavelength. Why not so much in the orange and yellow
wavelengths? Both chlorophylls also absorb in the orange-red end
of the spectrum (with longer wavelengths and lower energy). The
origins of photosynthetic organisms in the sea may account for this.
Shorter wavelengths (with more energy) do not penetrate much
below 5 meters deep in sea water. The ability to absorb some
energy from the longer (hence more penetrating) wavelengths
93
BIO 241: Plant and Animal Physiology
might have been an advantage to early photosynthetic algae that
were not able to be in the upper (photic) zone of the sea all the
time.
Note that both chlorophylls absorb light most strongly in the red
and violet portions of the spectrum. Green light is poorly absorbed
so when white light (which contains the entire visible spectrum)
shines on leaves, green rays are transmitted and reflected giving
leaves their green color.
Fig4.3: The absorption spectra for chlorophyll a
and chlorophyll b
The similarity of the action spectrum of photosynthesis and the
absorption spectrum of chlorophyll tells us that chlorophylls are the
most important pigments in the process. The spectra are not
identical, though, because carotenoids, which absorb strongly in the
blue, play a role as well.
The carotenoids help fill in the absorption gaps of chlorophyll so
that a larger part of the sun's spectrum can be used. The energy
absorbed by these "antenna pigments" is passed to chlorophyll a
where it drives the light reactions of photosynthesis.
94
BIO 241: Plant and Animal Physiology
4.5. Light-Harvesting Complexes (LHC) in Green Plants
Light harvesting complex is a set of photosynthetic pigment
molecules that absorb light and channel the energy to the
photosynthetic reaction centre, where the light reactions of
photosynthesis occur. It is used by plants and photosynthetic
bacteria to collect more of the incoming light than would be
captured by the photosynthetic reaction center alone. Lightharvesting complexes are found in a wide variety among the
different photosynthetic species. The complexes consist of proteins
and photosynthetic pigments and surround a photosynthetic
reaction center to focus energy, attained from photons absorbed by
the pigment, toward the reaction center using resonance energy
transfer.
Light-harvesting complexes are located around the reaction center
and add additional pigments, carotenoids, and chlorophylls to
funnel absorbed energy to the special pair via resonance energy
transfer. The main function of the light-harvesting complexes is to
gather light energy and to transfer this energy to the reaction
centers for the photo-induced redox processes. LH complex funnels
the excitation energy into its reaction centers.
4.5.1. The light-harvesting system, LH
A common misconception is that photosynthesis relies only on
chlorophyll pigments. The truth is that photosynthesis would be
rather inefficient using only chlorophyll molecules. Chlorophyll
molecules absorb light only at specific wavelengths (see image). A
large gap is present in the middle of the visible regions between
approximately 450 and 650 nm. This gap corresponds to the peak
of the solar spectrum, so failure to collect this light would
constitute a considerable lost opportunity. That's why
95
BIO 241: Plant and Animal Physiology
photosynthesis organisms have developed a light-harvesting
system, which bundles different pigments to create a much wider
absorption spectrum.
The light harvesting-system is composed of numerous lightharvesting complexes that completely surround the reaction center
where the photoinduced charge separation takes place. Chlorophyll,
carotenes, and xanthophylls are arranged in such light-harvesting
complexes or LHC-proteins. These pigments are referred to as
accessory pigments and funnel the energy to a special pigment in
the reaction center of PSI or PSII.
If a pigment molecule absorbs a photon, an electron in the molecule
becomes excited. For most compounds that absorb light, the
electron simply returns to the ground state and the absorbed energy
is converted into heat and/or fluorescence. But, in a LHC-protein,
the pigments are so arranged that the excitation energy can be
transferred from one molecule to a nearby molecule. The rate of
this process, called resonance energy transfer, depends strongly on
the distance between the energy donor and energy acceptor
molecules. For reasons of conservation of energy, energy transfer
must be from a donor in the excited state to an acceptor of equal or
lower energy. If the energy of a photon becomes lower, the
wavelength also becomes longer. When chlorophyll is isolated
from the enzymes it is associated with, the second scenario can be
seen to happen. The pigments in an LHC-protein are so arranged
that pigments are very close to each other and a pigment is near
another pigment that absorbs photons with a longer wavelength. As
a consequence, the pigment in the reaction center has to absorb
photons with the longest wavelength and cannot transfer this
energy to another pigment. The function of LHC-proteins is to
create a constant supply of excitation-energy to the reaction center
pigment. Every reaction center has a couple of LHC-proteins.
96
BIO 241: Plant and Animal Physiology
When the excited molecule has a nearby neighbour molecule, the
excitation energy may also be transferred, through electromagnetic
interactions, from one molecule to another. This process is also a
resonance energy transfer, and the rate depends strongly on the
distance between the energy donor and energy acceptor molecules.
Light-harvesting complexes have their pigments specifically
positioned to optimize these rates.
Chlorophylls and carotenoids are important in light-harvesting
complexes present in plants. Chlorophyll b is almost identical to
chlorophyll a except it has a formyl group in place of a methyl
group. This small difference makes chlorophyll b absorb light with
wavelengths between 400 and 500 nm more efficiently.
Carotenoids are long linear organic molecules that have alternating
single and double bonds along their length. Such molecules are
called polyenes. Two examples of carotenoids are lycopene and βcarotene. These molecules also absorb light most efficiently in the
400 – 500 nm range. Due to their absorption region, carotenoids
appear red and yellow and provide most of the red and yellow
colours present in fruits and flowers
In most purple bacteria, the photosynthetic membranes contain two
types of light-harvesting complexes: light harvesting complex I
(LH-I) and light harvesting complex II (LH-II) [1]. While LH-I is
tightly bound to the photosynthetic reaction centres, LH-II is not
directly associated with the reaction centres, but transfers energy to
the reaction centres via LH-I [1].
.
97
BIO 241: Plant and Animal Physiology
The carotenoids help fill in the absorption gaps of chlorophyll so
that a larger part of the sun's spectrum can be used. The energy
absorbed by these "antenna pigments" is passed to chlorophyll a
where it drives the light reactions of photosynthesis.(See fig…
below)
Carotenoids also serve a secondary function, that of suppressing
damaging photochemical reactions, in particular those including
oxygen, which can be induced by bright sunlight. Laboratory tests
have shown that plants lacking carotenoids quickly die on exposure
to light and oxygen. So, the carotenoid molecules also serve a
safeguarding function
4.5.2. Photosynthetic reaction centre
The excitation P680 → P680*of the reaction center pigment P680
occurs here. These special chlorophyll molecules embedded in PS
II absorb the energy of photons, with maximal absorption at
680 nm. A reaction center comprises several protein subunits (>10
or >11), providing a scaffold for a series of cofactors. The latter can
be pigments (like chlorophyll, pheophytin, carotenoids), quinones,
or iron-sulfur clusters.
98
BIO 241: Plant and Animal Physiology
Fig 4.4: Photosynthetic reaction centre
Electrons within these molecules are promoted to a higher-energy
state. This is one of two core processes in photosynthesis, and it
occurs with astonishing efficiency (greater than 90%) because, in
addition to direct excitation by light at 680 nm, the energy of light
first harvested by antenna proteins at other wavelengths in the
light-harvesting system is also transferred to these special
chlorophyll molecules.
This is followed by the step P680*→ pheophytin, and then on to
plastoquinone, which occurs within the reaction center of PS II.
High-energy electrons are transferred to plastoquinone.
Plastoquinone is then released into the membrane as a mobile
electron carrier.
This is the second core process in photosynthesis. The initial stages
occur within picoseconds, with an efficiency of 100%. The
seemingly impossible efficiency is due to the precise positioning of
molecules within the reaction center. This is a solid-state process,
not a chemical reaction. It occurs within an essentially crystalline
environment created by the macromolecular structure of PS II. The
usual rules of chemistry (which involve random collisions and
random energy distributions) do not apply in solid-state
environments.
4.6. Photosystems
Photosystems (ancient Greek: phos = light and systema =
assembly) are functional and structural units of protein complexes
involved in photosynthesis that together carry out the primary
photochemistry of photosynthesis: the absorption of light and the
transfer of energy and electrons. In other words, photosystems are
arrangements of chlorophyll and other pigments packed into
99
BIO 241: Plant and Animal Physiology
thylakoids that together carry out the primary photochemistry of
photosynthesis. They are found in the thylakoid membranes of
plants, algae and cyanobacteria (in plants and algae these are
located in the chloroplasts), or in the cytoplasmic membrane of
photosynthetic bacteria.
At the heart of a photosystem lies the Reaction Center, which is an
enzyme that uses light to reduce molecules. In a photosystem, this
Reaction Center is surrounded by light-harvesting complexes that
enhance the absorption of light and transfers the energy to the
Reaction Centers. Light-Harvesting and Reaction Center
complexes are membrane protein complexes that are made of
several protein-subunits and contain numerous cofactors. Light
harvesting is a set of photosynthetic pigment molecules that absorb
light and channel the energy to the photosynthetic reaction centre,
where the light reactions of photosynthesis occur. Then a lightharvesting complex is a complex of subunit proteins that may be
part of a larger supercomplex of a photosystem, the functional unit
in photosynthesis. It is used by plants and photosynthetic bacteria
to collect more of the incoming light than would be captured by the
photosynthetic reaction center alone. Light-harvesting complexes
are found in a wide variety among the different photosynthetic
species. The complexes consist of proteins and photosynthetic
pigments and surround a photosynthetic reaction center to focus
energy, attained from photons absorbed by the pigment, toward the
reaction center using resonance energy transfer.
In the photosynthetic membranes, reaction centers provide the
driving force for the bioenergetic electron and proton transfer
chain. When light is absorbed by a reaction center (either directly
or passed by neighbouring pigment-antennae), a series of oxidoreduction reactions is initiated, leading to the reduction of a
terminal acceptor. Two families of reaction centers in photosystems
exist: type I reaction centers (like photosystem I (P700) in
10
0
BIO 241: Plant and Animal Physiology
chloroplasts and in green-sulphur bacteria) and type II reaction
centers (like photosystem II (P680) in chloroplasts and in nonsulphur purple bacteria).
Eukaryotes have Photosystem II plus Photosystem I. Photosystem I
uses chlorophyll a, in the form referred to as P700. Photosystem II
uses a form of chlorophyll a known as P680. Both "active" forms of
chlorophyll a function in photosynthesis due to their association
with proteins in the thylakoid membrane. Photosystem I and II are
very similar in structure and function. They use special proteins,
called light-harvesting complexes, to absorb the photons with very
high effectiveness. If a special pigment molecule in a
photosynthetic reaction center absorbs a photon, an electron in this
pigment attains the excited state and then is transferred to another
molecule in the reaction center. This reaction, called photoinduced
charge separation, is the start of the electron flow and is unique
because it transforms light energy into chemical forms.
Each photosystem can be identified by the wavelength of light to
which it is most reactive (700 and 680 nanometers, respectively for
PSI and PSII in chloroplasts), the amount and type of lightharvesting complexes present and the type of terminal electron
acceptor used. Type I photosystems use ferredoxin-like iron-sulfur
cluster proteins as terminal electron acceptors, while type II
photosystems ultimately shuttle electrons to a quinone terminal
electron acceptor. One has to note that both reaction center types
are present in chloroplasts and cyanobacteria, working together to
form a unique photosynthetic chain able to extract electrons from
water, creating oxygen as a byproduct.
Photosystem II
PS II is an extremely complex, highly organized transmembrane
structure that contains a water-splitting complex, chlorophylls and
10
1
BIO 241: Plant and Animal Physiology
carotenoid pigments, a reaction center (P680), pheophytin (a
pigment similar to chlorophyll), and two quinones. It uses the
energy of sunlight to transfer electrons from water to a mobile
electron carrier in the membrane called plastoquinone:
Fig 4.6: Photosystem 2
H2O → P680 → P680* → plastoquinone
Plastoquinone, in turn, transfers electrons to b6f, which feeds them
into PS I.
The water-splitting complex
The step H2O → P680 is performed by a poorly-understood
structure embedded within PS II called the water-splitting complex
or the oxygen-evolving complex. It catalyzes a reaction that splits
water into electrons, protons and oxygen:
2H2O → 4H+ + 4e- + O2
The electrons are transferred to special chlorophyll molecules
(embedded in PS II) that are promoted to a higher-energy state by
the energy of photons.
10
2
BIO 241: Plant and Animal Physiology
Photosystem I
This photosystem produces only ATP. It takes place in the
thylakoid and makes 6ATP's to fulfill the uneven ratio in demand
by the dark reactions. PS I accepts electrons from plastocyanin and
transfers them either to NADPH (noncyclic electron transport) or
back to cytochrome b6f (cyclic electron transport):
Fig 4.5: Photosystem I
PS I, like PS II, is a complex, highly organized transmembrane
structure that contains antenna chlorophylls, a reaction center
(P700), phylloquinine, and a number of iron-sulfur proteins that
serve as intermediate redox carriers.
The light-harvesting system of PS I uses multiple copies of the
same transmembrane proteins used by PS II. The energy of
absorbed light (in the form of delocalized, high-energy electrons) is
funneled into the reaction center, where it excites special
chlorophyll molecules (P700, maximum light absorption at
700 nm) to a higher energy level. The process occurs with
astonishingly high efficiency.
10
3
BIO 241: Plant and Animal Physiology
Electrons are removed from excited chlorophyll molecules and
transferred through a series of intermediate carriers to ferredoxin, a
water-soluble electron carrier. As in PS II, this is a solid-state
process that operates with 100% efficiency.
There are two different pathways of electron transport in PS I. In
noncyclic electron transport, ferredoxin carries the electron to the
enzyme ferredoxin NADP+ oxidoreductase that reduces NADP+ to
NADPH. In cyclic electron transport, electrons from ferredoxin are
transferred (via plastoquinone) to a proton pump, cytochrome b6f.
They are then returned (via plastocyanin) to P700.
NADPH and ATP are used to synthesize organic molecules from
CO2. The ratio of NADPH to ATP production can be adjusted by
adjusting the balance between cyclic and noncyclic electron
transport.
It is noteworthy that PS I closely resembles photosynthetic
structures found in green sulfur bacteria, just as PS II resembles
structures found in purple bacteria.
10
4
BIO 241: Plant and Animal Physiology
Fig 4.5: Relationship between Photosystems I and II
Comparisons between Photosystems I and II
For oxygenic photosynthesis, both photosystems I and II are
required. Oxygenic photosynthesis can be performed by plants and
cyanobacteria, which are believed to be the progenitors of the
photosystem-containing chloroplasts of eukaryotes. Photosynthetic
bacteria that cannot produce oxygen have a single photosystem
called BRC, bacterial reaction center.
The photosystem I was named "I" since it was discovered before
photosystem II, but this does not represent the order of the electron
flow.
10
5
BIO 241: Plant and Animal Physiology
When photosystem II absorbs light, electrons in the reaction-center
chlorophyll are excited to a higher energy level and are trapped by
the primary electron acceptors. To replenish the deficit of electrons,
electrons are extracted from water by a cluster of four Manganese
ions in photosystem II and supplied to the chlorophyll via a redoxactive tyrosine.
Photoexcited electrons travel through the cytochrome b6f complex
to photosystem I via an electron transport chain set in the thylakoid
membrane. This energy fall is harnessed, (the whole process
termed chemiosmosis), to transport hydrogen (H+) through the
membrane, to the lumen, to provide a proton-motive force to
generate ATP. The protons are transported by the plastoquinone. If
electrons only pass through once, the process is termed noncyclic
photophosphorylation.
When the electron reaches photosystem I, it fills the electron deficit
of the reaction-center chlorophyll of photosystem I. The deficit is
due to photo-excitation of electrons that are again trapped in an
electron acceptor molecule, this time that of photosystem I.
ATP is generated when the ATP synthetase transports the protons
present in the lumen to the stroma, through the membrane. The
electrons may either continue to go through cyclic electron
transport around PS I or pass, via ferredoxin, to the enzyme
NADP+ reductase. Electrons and hydrogen ions are added to
NADP+ to form NADPH. This reducing agent is transported to the
Calvin cycle to react with glycerate 3-phosphate, along with ATP
to form glyceraldehyde 3-phosphate, the basic building-block from
which plants can make a variety of substances.
10
6
BIO 241: Plant and Animal Physiology
4.7. Photosynthetic Electron Transport Chains in Chloroplasts
Fig 4.6: Photosynthetic Electron Transport Chains in Chloroplasts
The photosynthesis process in chloroplasts begins when an electron
of P680 of PSII attains an higher-energy level. This energy is used
to reduce a chain of electron acceptors that have subsequently
lowered redox-potentials. This chain of electron acceptors is known
as an electron transport chain. When this chain reaches PS I, an
electron is again excited, creating a high redox-potential. The
electron transport chain of photosynthesis is often put in a diagram
called the z-scheme, because the redox diagram from P680 to P700
resembles the letter z.[1]
The final product of PSII is plastoquinol, a mobile electron carrier
in the membrane. Plastoquinol transfers the electron from PSII to
the proton pump, cytochrome b6f. The ultimate electron donor of
PSII is water. Cytochrome b6f proceeds the electron chain to PSI
through plastocyanin molecules. PSI is able to continue the electron
transfer in two different ways. It can transfer the electrons either to
plastoquinol again, creating a cyclic electron flow, or to an enzyme
called FNR, creating a non-cyclic electron flow. PSI releases FNR
into the stroma, where it reduces NADP+ to NADPH.
Activities of the electron transport chain, especially from
cytochrome b6f, lead to pumping of protons from the stroma to the
lumen. The resulting transmembrane proton gradient is used to
make ATP via ATP synthase.
10
7
BIO 241: Plant and Animal Physiology
The overall process of the photosynthetic electron transport chain
in chloroplasts is:
H2O → PS II → plastoquinone → cytb6f → plastocyanin → PS I
→ NADPH
PS II is a transmembrane structure found in all chloroplasts. It
splits water into electrons, protons and molecular oxygen. The
electrons are transferred to plastoquinone, which carries them to a
proton pump. Molecular oxygen is released into the atmosphere.
The emergence of such an incredibly complex structure, a
macromolecule that converts the energy of sunlight into potentially
useful work with efficiencies that are impossible in ordinary
experience, seems almost magical at first glance. Thus, it is of
considerable interest that, in essence, the same structure is found in
purple bacteria.
Cytochrome b6f
PS II and PS I are connected by a transmembrane proton pump,
cytochrome b6f complex (plastoquinol—plastocyanin reductase;
EC 1.10.99.1. Electrons from PS II are carried by plastoquinone to
b6f, where they are removed in a stepwise fashion and transferred to
a water-soluble electron carrier called plastocyanin. This redox
process is coupled to the pumping of four protons across the
membrane. The resulting proton gradient (together with the proton
gradient produced by the water-splitting complex in PS II) is used
to make ATP via ATP synthase.
The similarity in structure and function between cytochrome b6f (in
chloroplasts) and cytochrome bc1 (Complex III in mitochondria) is
striking. Both are transmembrane structures that remove electrons
from a mobile, lipid-soluble electron carrier (plastoquinone in
10
8
BIO 241: Plant and Animal Physiology
chloroplasts; ubiquinone in mitochondria) and transfer them to a
mobile, water-soluble electron carrier (plastocyanin in chloroplasts;
cytochrome c in mitochondria). Both are proton pumps that
produce a transmembrane proton gradient.
4.8. Stages of Photosynthesis
Photosynthesis is a two stage process. The first
process is the Light Dependent Process (Light
Reactions), requires the direct energy of light to make energy
carrier molecules that are used in the second process. The Light
Independent Process (or Dark Reactions) occurs when the products
of the Light Reaction are used to form C-C covalent bonds of
carbohydrates. The Dark Reactions can usually occur in the dark, if
the energy carriers from the light process are present. Recent
evidence suggests that a major enzyme of the Dark Reaction is
indirectly stimulated by light, thus the term Dark Reaction is
somewhat of a misnomer.
The Light Reactions occur in the grana of the chloroplasts while
the Dark Reactions take place in the stroma of the chloroplasts.
In order for you to understand the series of reactions of these two
distinct stages of photosynthesis, there is need for you, to first look
at how light energy is trapped, and then, how this trapped light
energy is converted into chemical energy in the form of ATP and
NADPH. You shall learn Light Dependent and the Light
Independent reactions later in the unit.
4.8.1. Light Reactions
The light-dependent reactions, or light reactions, are the first stage
of photosynthesis, the process by which plants capture and store
energy from sunlight. In this process, light energy is converted into
chemical energy, in the form of the energy-carrying molecules ATP
10
9
BIO 241: Plant and Animal Physiology
and NADPH. In the light-independent reactions, the formed
NADPH and ATP drive the reduction of CO2 to more useful
organic compounds, such as glucose. However, although lightindependent reactions are, by convention, also called dark
reactions, they are not independent of the need of light, for they are
driven by ATP and NADPH, products of light.
The light-dependent reactions of photosynthesis, take place on the
thylakoid membrane inside a chloroplast. The inside of the
thylakoid membrane is called the lumen, and outside the thylakoid
membrane is the stroma, where the light-independent reactions take
place. The thylakoid membrane contains some integral membrane
protein complexes that catalyze the light reactions. There are four
major protein complexes in the thylakoid membrane: Photosystem I
(PSI), Photosystem II (PSII), Cytochrome c6f complex, and ATP
synthase. These four complexes work together to ultimately create
the products ATP and NADPH.
The two photosystems absorb light energy through proteins
containing pigments, such as chlorophyll. The light-dependent
reactions begin in photosystem II. When a chlorophyll a molecule
within the reaction center of PSII absorbs a photon, an electron in
this molecule attains a higher energy level. Because this state of an
electron is very unstable, the electron is transferred from one to
another molecule creating a chain of redox reactions, called an
electron transport chain (ETC). The electron flow goes from PSII
to cytochrome b6f to PSI. In PSI, the electron gets the energy from
another photon. The final electron acceptor is NADP. In oxygenic
photosynthesis, the first electron donor is water, creating oxygen as
a waste product. In anoxygenic photosynthesis various electron
donors are used.
11
0
BIO 241: Plant and Animal Physiology
Fig 4.7: Photosystem I and II, and Redox chain
Cytochrome b6f and ATP synthase work together to create ATP.
This process is called photophosphorylation, which occurs in two
different ways. In non-cyclic photophosphorylation, cytochrome
b6f uses the energy of electrons from PSII to pump protons from
the stroma to the lumen. The proton gradient across the thylakoid
membrane creates a proton-motive force, used by ATP synthase to
form ATP. In cyclic photophosphorylation, cytochrome b6f uses
the energy of electrons from not only PSII but also PSI to create
more ATP and to stop the production of NADPH. Cyclic
phosphorylation is important to create ATP and maintain NADPH
in the right proportion for the light-independent reactions.
The net-reaction of all light-dependent reactions in oxygenic
photosynthesis is:
2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP
11
1
BIO 241: Plant and Animal Physiology
Fig 4.8: Net reaction of light-dependent reaction in oxygenic
photosynthesis.
Photophosphorylation
Photophosphorylation is the process of creating ATP using a
Proton gradient created by the Energy gathered from sunlight. The
process of creating the Proton gradient resembles that of the
electron transport chain of Respiration. But since formation of this
proton gradient is light-dependent, the process is called
Photophosphorylation.
Chemiosmosis - Chemiosmosis is the process of using Proton
movement to join ADP and Pi. This is accomplished by enzymes
called ATP synthases or ATPases. The CF1-ATPase of the
Thylakoid membrane is shown on the left. As protons pass through
this enzyme ADP and Pi are joined to make ATP. The movement
of the Protons through this enzyme provides the Energy needed to
make ATP.
Non cyclic Photophosphorylation really refers to the ATP
generated by protons moved across the thylakoid membranes
11
2
BIO 241: Plant and Animal Physiology
during the Z-scheme. The Cytb6-f complex acts as an electron
transport chain. As the electrons lose Energy (during a series of
re/dox reactions) Protons are moved into the Thylakoid space. This
Proton gradient can be used to generate ATP chemiosmotically.
Fig 4.9: Noncyclic photophosphorylation
Fig 4.10: cyclic photophosphorylation
11
3
BIO 241: Plant and Animal Physiology
4.8.2. Non-cyclic photophosphorylation compared to cyclic
photophosphorylation.
As you have observed, PSI and PSII work as a unit. They do have
some similarities and differences in their structure and the way they
function which are worth mentioning.
Similarities
They both consist of a complex of molecules embedded in
thylakoid membranes of the chloroplast. They both contain
chlorophyll molecules, which can convert light energy into
chemical energy.
In both photosystems, a photon causes an electron to reach a high
energy level and the energized electron is passed to a chlorophyll
molecule in the reaction center before it can leave the photosystem.
Both contain carotenoid molecules.
Differences
The chlorophyll molecules in the reaction center of photosystem II
are P680 (sensitive to wavelengths up to about 680 nm), whereas
those in photosystem I are P700, which respond to slightly longer
wavelengths.
Photosystem II, unlike photosystem I, contains plastoquinone,
which passes the energetic electron to cytochromes b6 and f, but
photosystem I passes the electron to ferredoxin.
In non-cyclic photophosphorylation, photosystem II is associated
with the photolysis of water and subsequent synthesis of ATP;
11
4
BIO 241: Plant and Animal Physiology
photosystem I is associated with the conversion of NADP+ to
NADPH.
In cyclic photophosphorylation, only photosystem I produces an
energized electron on receipt of a photon. Instead of producing
NADPH, this electron travels to plastoquinone, and then to
cytochromes b6 and f, as in the non-cyclic process.
4.8.2 Dark reactions
The C3 carbon reduction cycle is the primary pathway of carbon
fixation in all photosynthetic organisms, reducing CO2 from the
atmosphere to form carbohydrates; in higher plants it takes place in
the chloroplast stroma.
Role of Ribulose-1,5-bisphosphate carboxylase oxygenase
Ribulose-1,5-bisphosphate carboxylase oxygenase, most
commonly known by the shorter name RuBisCO, is an enzyme
involved in the Calvin cycle that catalyzes the first major step of
carbon fixation, a process by which the atoms of atmospheric
carbon dioxide are made available to organisms in the form of
energy-rich molecules such as glucose. RuBisCO catalyzes either
the carboxylation or the oxygenation of ribulose-1,5-bisphosphate
(also known as RuBP) with carbon dioxide or oxygen.
RuBisCO is usually active only during the day because ribulose
1,5-bisphosphate is not being produced in the dark, due to the
regulation of several other enzymes in the Calvin cycle. In addition,
the activity of RuBisCO is coordinated with that of the other
enzymes of the Calvin cycle in several ways.
RuBisCO is very important in terms of biological impact because it
catalyzes the primary chemical reaction by which inorganic carbon
permanently enters the biosphere. Many autotrophic bacteria and
11
5
BIO 241: Plant and Animal Physiology
archaea fix carbon via the reductive acetyl CoA pathway, the 3hydroxypropionate cycle or the reverse Krebs cycle, but they make
up a relatively minor portion of global net primary production.
Phosphoenolpyruvate carboxylase PEPC only temporarily fixes
carbon. RuBisCO is also the most abundant protein in leaves, and
is considered to be the most abundant protein on Earth.[2][3] It
accounts for 50% of soluble leaf protein in C3 plants (20-30% of
total leaf nitrogen) and 30% of soluble leaf protein in C4 plants (59% of total leaf nitrogen).[3] Given its important role in the
biosphere, there are currently efforts to genetically engineer crop
plants so as to contain more efficient RuBisCO (see below).
The Calvin Cycle
The Calvin Cycle involves the fixation and reduction of Carbon.
It’s as easy as 1, 2, 3 below.
1. Carbon is Fixed CO2 + RuBP -> 2 PGA molecules (1 + 5 = 2 X 3)
2. The phosphoglyceric acid is reduced (using NADPH) to
phosphoglyceraldehyde.
3. The fixed, reduced Carbon is rearranged to make Glucose and
regenerate RuBP.
How it happens :

CO2 combines with the phosphorylated 5-carbon sugar
ribulose bisphosphate

This reaction is catalyzed by the enzyme ribulose
bisphosphate carboxylase oxygenase (RUBISCO an
enzyme which can fairly claim to be the most abundant
protein on earth)

The resulting 6-carbon compound breaks down (real fast)
into two molecules of
3-phosphoglyceric acid (PGA)
11
6
BIO 241: Plant and Animal Physiology

The PGA molecules are further phosphorylated (by ATP)
and are reduced (by NADPH) to form
phosphoglyceraldehyde (G3P, also called PGAL)

Phosphoglyceraldehyde serves as the starting material for
the synthesis of glucose and fructose, or it can be used to
make more RuBP.

Glucose and fructose make the disaccharide sucrose, which
travels in solution to other parts of the plant (e.g., fruit,
roots) or is used in the synthesis of the polysaccharides
starch and cellulose
Fig. 4.11:Simplied Calvin cycle
These Energy rich molecules were formed during the Light
reactions of Photosynthesis: This is the 5-Carbon molecule to
which Carbon (CO2) is fixed. This molecule can be used to form
Glucose or to regenerate RuBP. This reaction is called "Carbon
Fixation". This reaction requires Energy in the form of ATP.
These 2 molecules are formed from the breakdown of the product
11
7
BIO 241: Plant and Animal Physiology
of Carbon Fixation.
During this reaction Carbon is reduced.
The sum of the Calvin cycle:
1. Carboxylation - CO2 is covalently linked to a carbon skeleton (RuBP)
2. Reduction - carbohydrate is formed at the expense of ATP and
NADPH
3. Regeneration - the CO2 acceptor RuBP reforms at the expense of ATP
The immediate products of one turn of the Calvin cycle are 2
glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2
NADP+ (ADP and NADP+ are regenerated in the Light-dependent
reactions). Each G3P molecule is composed of 3 carbons. In order
for the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate)
must be regenerated. So, 5/6 carbon from the 2 G3P molecules are
used for this purpose. Therefore, there is only 1 net carbon
produced to play with for each turn. To create 1 surplus, G3P
requires 3 carbons, and therefore 3 turns of the Calvin cycle. To
make one glucose molecule (which can be created from 2 G3P
molecules) would require 6 turns of the Calvin cycle. Surplus G3P
can also be used to form other carbohydrates such as starch,
sucrose, and cellulose, depending on what the plant needs.
Hexose (six-carbon) sugars are not a product of the Calvin cycle.
Although many texts list a product of photosynthesis as C6H12O6,
this is mainly a convenience to counter the equation of respiration,
where six-carbon sugars are oxidized in mitochondria. The
carbohydrate products of the Calvin cycle are three-carbon sugar
phosphate molecules, or "triose phosphates," namely,
glyceraldehyde-3-phosphate (G3P).
11
8
BIO 241: Plant and Animal Physiology
4.9. Photoassimilate
In botany, a photoassimilate is one of a number of biological
compounds formed by assimilation using light-dependent reactions.
This term is most commonly used to refer to the energy-storing
monosaccharides produced by photosynthesis in the leaves of
plants.
Only NADPH, ATP and water are made in the "light" reactions.
Monosaccharides, though generally more complex sugars, are
made in the "dark" reactions.The term "light" reaction can be
confusing as some "dark" reactions require light to be active.
Photoassimilate movement through plants from "source to sink"
using xylem and phloem is of biological significance. This
movement is mimicked by many infectious particles - namely
viroids - to accomplish long ranged movement and consequently
infection of an entire plant.
11
9
BIO 241: Plant and Animal Physiology
Unit summary
In this unit we discussed the process of photosynthesis in
green plants. You have seen why photosynthesis is defined
Summary
as a process by which plants, some bacteria, and some
protistans use the energy from sunlight to produce sugar. It
is this sugar which is converted into ATP, the "fuel" used by
all living things, in cellular respiration. Photosynthesis is
sometimes called carbon assimilation. During
photosynthesis, light energy is absorbed by photosynthetic
pigments, a mixture of chlorophylls and accessory pigments
such as xanthophylls. All the energy absorbed by other
pigments are funnelled to the reaction centre chlorophyll,
a.
You learnt that this process involves two phases, namely,
light-dependent reactions and light-independent reactions.
The products of light-dependent reaction were noted as
ATP, NADPH and oxygen. ATP provides the energy to
drive the light-dependent reaction and NADPH provides the
‘reducing power’ needed to covert carbon dioxide into
carbohydrate. The carbohydrate produced by these reactions
are exported to the cell cytosol, as a three-carbon sugar
phosphate which forms the starting point for many of the
plant cell’s synthetic pathways.
12
0
BIO 241: Plant and Animal Physiology
12
1
BIO 241: Plant and Animal Physiology
Answer the following questions in the spaces provided
Assessment
1.
What is the likely evolutionary advantage of two
photosystems over one photosystem in photosynthesis?
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
............................................................................................................
2. Which of the following are products of the light-independent
stage of photosynthesis?
(a) NADH
(b) O2
(c ) NADPH
(d) ATP
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
………………………………………………………………………
3. Compare and contrast cyclic and non-cyclic
Photophosphorylation.
Unit 5
Mineral Nutrition in Plants
5.0 Introduction
12
2
BIO 241: Plant and Animal Physiology
Unlike animals (which obtain their food from what they eat) plants
obtain their nutrition from the soil and atmosphere. Using sunlight
as an energy source, plants are capable of making all the organic
macromolecules they need by modifications of the sugars they
form by photosynthesis. However, plants must take up various
minerals through their root systems for use.
In your o-level biology you learnt that living organisms need
materials for the synthesis of new protoplasm for growth and
repair, and materials that will serve them as sources of energy.
They obtain these substances from the environment which is both
water and air. Plants absorb most of their nutrients from the soil
water. In this unit, you will study the nutrient requirement for green
plants
Upon completion of this unit you will be able to:
12
3
BIO 241: Plant and Animal Physiology
 Define the terms ‘essential and non-essential elements.
 Discuss the general functions of mineral elements and
Outcomes
effects on their deficiencies on plant.
 Determine the essentiality of mineral elements* scientific
notation t*
 Describe the pathway for converting atmospheric nitrogen in
legumes
5.1 Essential and non-essential Elements in plant Nutrition
Chemical analysis of the plant ash ( the residue left after the dry
matter of the plant has been burnt), has shown that plants contain
about 40 different elements. Some of them are indispensible or
necessary for the normal growth and development of the plant and
are called essential elements. The rest of the elements are called
non-essential elements.
According to Epstein (1972), there are two main criteria to judge
essentiality of an element for plant growth and development:
i.
An element is essential if the plant cannot complete its life
cycle ( that is, form viable seeds) in the absence of that
element and
ii.
An element is essential if it forms part of the molecule or
constituent of the plant that is itself essential for the plant
(for instance Nitrogen in proteins and Mg in chlorophylls).
It is now known that the following 17 elements are essential for
majority of the plants;
12
4
BIO 241: Plant and Animal Physiology
C, H, O, N, P, K, Ca, S, Mg, Zn, B, Cu, Ni, Cl, and Mo. Besides
these, Al, Si, Se, Na, Co, V and Ga may be essential for some
plants.
Essential elements may be classified into two groups:
i.
Major elements (or macronutrients)
ii.
Minor elements ( or micronutrients or trace elements)
Carbon, Hydrogen, and Oxygen are considered the essential
elements. Nitrogen, Potassium, and Phosphorous are obtained from
the soil and are the macronutrients. Calcium, Magnesium, and
Sulphur are the secondary macronutrients needed in lesser quantity.
The micronutrients, needed in very small quantities and toxic in
large quantities, include Iron, Manganese, Copper, Zinc, Boron,
and Chlorine. A complete fertilizer provides all three primary
macronutrients and some of the secondary and micronutrients. The
label of the fertilizer will list numbers, for example 5-10-5, which
refer to the percent by weight of the primary macronutrients.
5.1.1 Major elements ( macronutrients)
The essential elements which are required by the plant in
comparatively large amounts (1000 mg or more/kg of dry matter)
are called major elements or macronutrients. They are C, H, O, N,
P, K, Ca, S, and Mg.
5.1.2 Minor elements (or micronutrients or trace
elements)
12
5
BIO 241: Plant and Animal Physiology
The essential elements which are required in very small amounts
(less than 100mg/kg of dry matter) or traces, by the plants are
called minor elements ( micronutrients or trace elements). They
include the following: Fe, Mn, Zn, B, Cu, Ni, Cl and Mo.
Trace elements should not be confused with the tracer elements.
Tracer elements are usually radioactive or heavy isotopes which are
used to trace out some metabolic pathway. Some examples of
commonly used tracer elements in plant physiology are Carbon-14,
Nitrogen-15, Oxygen-18, Pottasium-42, Calcium-45, Phosphorus32 and Sulphur-35.
5.2 General Functions of Essential Elements in Plants
The general functions of the essential elements are as follows:
Constituents of protoplasm and cell walls
C, H, O, N and P, are very important and permanent constituents
of the protoplasm and the cell wall. C, H and O form most of the
part of plant body. N is important constituent element of proteins
and nucleic acids, S of proteins, and P of nucleic acids. Besides
these, Mg is an important constituent of chlorophylls while Ca is
present in middle lamella in the form of calcium pectate.
Influence on the osmotic Pressure of plant cells
12
6
BIO 241: Plant and Animal Physiology
Osmotic pressure and other osmotic relations of the plant cells are
maintained due to the presence of organic compounds and mineral
salts dissolved in the cell sap.
Catalytic function
Many elements like Fe, Cu, Zn, Mo, Mg, Mn, Cl etc., are required
in catalytic amounts to carry on various enzymatic reactions in the
cells. These elements may be part of prosthetic group of the
enzymes or co-enzymes, or may act as activators.
Antagonistic or balancing function
Some elements like Ca, Mg, K etc., counteract the toxic effects of
other mineral elements by maintaining ionic balance.
Table 5.1. Essential plant nutrients: their relative amounts in
plants, functions and classification
Name
Chemical Relative %
symbol
in plant*
Nitrogen
N
100
Phosphorus
P
6
Potassium
K
25
Calcium
Ca
12.5
Magnesium
Mg
8
Function in
Nutrient
plant
category
Proteins, amino
acids
Nucleic acids,
Primary
ATP
macronutrients
Catalyst, ion
transport
Cell wall
component
Part of
Secondary
macronutrients
chlorophyll
12
7
BIO 241: Plant and Animal Physiology
Sulfur
S
3
Iron
Fe
0.2
Copper
Cu
0.01
Manganese
Mn
0.1
Amino acids
Chlorophyll
synthesis
Component of
enzymes
Activates
enzymes
Micronutrients
Zinc
Zn
0.03
Boron
B
0.2
Molybdenum
Mo
0.0001
Chlorine
Cl
0.3
Activates
enzymes
Cell wall
component
Involved in N
fixation
Photosynthesis
reactions
5.3 Determination of Essentiality of Mineral Elements
The essentiality of a particular mineral element for normal growth
and development of the plant can be determined by solution
culture method. The roots of the plant which is to be studied are
immersed in a container. The stem of the plant is kept projected
through a hole in the cover of the container. The stem is protected
from the sharp edge of the hole with a pad of non-absorbant cotton.
12
8
BIO 241: Plant and Animal Physiology
The nutrient solution is aerated for optimum growth of the roots
and mineral absorption. The plant below serves as control.
Fig.5.1: Solution culture
A second similar plant is now fixed in another container in which
the nutrient solution lacks a particular mineral element whose
essentiality is to be determined. The effect of the deficiency of this
mineral element is then observed on the growth of the plant. If the
plant deviates from its normal growth, that is, it shows the
deficiency symptoms, the mineral element missing from the nutrient
solution will be an essential element for the growth and
development of that plant.
In this experiment great care should be taken to avoid
contamination of the nutrient solution, as much as possible,
particularly with trace elements. These elements are often present
as impurities in
a) Container’s wall
b) Water used in preparing the nutrient solution
12
9
BIO 241: Plant and Animal Physiology
c) Reagents used
d) Rooting medium
e) The dust from the surrounding atmosphere.
Table 5.2. Generalized Symptoms of Plant Nutrient Deficiency or
Excess.
Plant
Nutrient
Type
Visual symptoms
Light green to yellow appearance of leaves,
Deficiency especially older leaves; stunted growth; poor fruit
development.
Nitrogen
Dark green foliage which may be susceptible to
Excess
lodging, drought, disease and insect invasion. Fruit
and seed crops may fail to yield.
Deficiency
Leaves may develop purple coloration; stunted
plant growth and delay in plant development.
Phosphorus
Excess
Deficiency
Excess phosphorus may cause micronutrient
deficiencies, especially iron or zinc.
Older leaves turn yellow initially around margins
and die; irregular fruit development.
Potassium
Excess
Excess potassium may cause deficiencies in
magnesium and possibly calcium.
Reduced growth or death of growing tips;
Calcium
Deficiency blossom-end rot of tomato; poor fruit development
and appearance.
13
0
BIO 241: Plant and Animal Physiology
Excess
Excess calcium may cause deficiency in either
magnesium or potassium
Initial yellowing of older leaves between leaf
Deficiency veins spreading to younger leaves; poor fruit
development and production.
Magnesium
High concentration tolerated in plant; however,
Excess
imbalance with calcium and potassium may
reduce growth.
Initial yellowing of young leaves spreading to
Deficiency whole plant; similar symptoms to nitrogen
deficiency but occurs on new growth.
Sulfur
Excess
Excess of sulfur may cause premature dropping of
leaves.
Initial distinct yellow or white areas between veins
Deficiency of young leaves leading to spots of dead leaf
Iron
tissue.
Excess
Possible bronzing of leaves with tiny brown spots.
Deficiency Interveinal yellowing or mottling of young leaves.
Manganese
Excess
Deficiency
Zinc
Excess
Older leaves have brown spots surrounded by a
chlorotic circle or zone.
Interveinal yellowing on young leaves; reduced
leaf size.
Excess zinc may cause iron deficiency in some
13
1
BIO 241: Plant and Animal Physiology
plants.
Deficiency
Boron
Death of growing points and deformation of leaves
with areas of discoloration.
Leaf tips become yellow followed by necrosis.
Excess
Leaves get a scorched appearance and later fall
off.
Adapted from: W.F. Bennett (editor), 1993. Nutrient Deficiencies &
Toxicities in Crop Plants, APS Press, St. Paul, Minnesota.
5.4 Role of soil in Plant Nutrition
Fertile soil contains the nutrients in a readily available form that
plants require for growth. The roots of the plant act as miners
moving through the soil and bringing needed minerals into the
plant roots.
Plants use these minerals in:
1. Structural components in carbohydrates and proteins
2. Organic molecules used in metabolism, such as the
Magnesium in chlorophyll and the Phosphorous found in
ATP
3. Enzyme activators like potassium, which activates possibly
fifty enzymes
4. Maintaining osmotic balance.*
5.4 Pathway for Converting Atmospheric Nitrogen in Legumes
Plants need nitrogen for many important biological molecules
including nucleotides and proteins. However, the nitrogen in the
atmosphere is not in a form that plants can utilize. Many plants
13
2
BIO 241: Plant and Animal Physiology
have a symbiotic relationship with bacteria growing in their roots:
organic nitrogen as rent for space to live. These plants tend to have
root nodules in which the nitrogen-fixing bacteria live.
The bacteria live symbiotically with the plant. All the nitrogen in
living systems was at one time processed by these bacteria, who
took atmospheric nitrogen (N2) and modified it to a form that living
things could utilize (such as NO3 or NO4; or even as ammonia, NH3
Not all bacteria utilize the above route of nitrogen fixation. Many
that live free in the soil, utilize other chemical pathways.
While root hairs greatly enhance the surface area (hence absorbtion
surface), the addition of symbiotic mycorrhizae fungi vastly
increases the area of the root for absorbing water and minerals from
the soil.
5.5 Soilless Growth or Hydroponics
The practice of growing plants in nutrient-enriched water without
soil is caled soilless growth or hydroponics. However, the term
hydroponics is now applied to plants rooted in sand, gravel or
other similar matter such as verimiculite or expanded clay (that is,
Kity liter) which is soaked with a recycling flow of nutrientenriched water.
The plants are grown in large tanks containing nutrient solution and
are supported by wire netting. The tanks are provided with the
solution-regulatng and pumping system and are placed in green
houses under controlled environment. After about a month in the
greenhouse when the plants flower, they are further supported by
strings attached to the greenhouse roof. Because the plants are
grown in large tanks, this process of soilless cultivation is also
known as tank farming.
13
3
BIO 241: Plant and Animal Physiology
Fig.5.2: Systems for growing plants in nutrient solutions (A)
Hydroponic system
13
4
BIO 241: Plant and Animal Physiology
Nutrient film growth
Plant roots lie on the surface of a slanting trough and the nutrient
solution is pumped as a thin film over the roots. This system
ensures adequate supply of oxygen to roots and the composition
and Ph of the nutrient solution can automatically controlled. This
technique is often used in commercial production.
Aeroponic growth system
Plants roots are suspended over the nutrient solution in a nutrient
mist chamber. With the help of a motor driven rotor, the nutrient
solution is whipper
13
5
BIO 241: Plant and Animal Physiology
Unit summary
In this unit you looked at mineral nutrients in plants. You
have learnt that some mineral nutrients are essential, while
Summary
others are non-essential. An element is essential if the plant
cannot complete its life cycle (that is, form viable seeds) in
the absence of that element. Essential elements may be
classified as: major elements ( macronutrients) and minor
elements (micronutrients or trace elements).You are also
aware of the functions of the essential elements which
included : Catalytic function, Influence on the osmotic
Pressure of plant cells, Antagonistic or balancing function
and forming Constituents of protoplasm and cell walls.
13
6
BIO 241: Plant and Animal Physiology
Assessment
Answer the following questions in the spaces provided
Assessment
1. ‘All elements that are present in a plant need not be
essential to its survival’. Comment.
2. Why is purification of water and nutrient salts so important
in studies involving mineral nutrition using hydroponics.
3. Explain with examples: macronutrients, micronutrients,
beneficial nutrients, toxic elements and essential elements.
4. Name at least five different deficiency symptoms in plants.
Describe them and correlate them with the concerned
mineral deficiency.
5. How are the minerals absorbed by the plants?
6. What are the conditions necessary for fixation of
atmospheric nitrogen by Rhizobium. What is their role in
N2 -fixation?
13
7
BIO 241: Plant and Animal Physiology
Answers to Assessment
UNIT 6
Answer to Q1. The criteria for essentiality of an element are given
below:
(a) The element must be absolutely necessary for supporting
normal growth and reproduction. In the absence of the element the
plants do not complete their life cycle or set the seeds.
(b) The requirement of the element must be specific and not
replaceable by another element. In other words, deficiency of any
one element cannot be met by supplying some other element.
(c) The element must be directly involved in the metabolism of the
plant.
All elements that are present in a plant do not fulfill these criteria
hence cannot be essential for plant survival.
Answer to Q2. In 1860, Julius von Sachs, a prominent German
botanist, demonstrated, for the first time, that plants could be
grown to maturity in a defined nutrient solution in complete
absence of soil. The essence of all these methods involves the
culture of plants in a soil-free, defined mineral solution. These
methods require purified water and mineral nutrient salts.
Purification of water and nutrient salt is important to rule out other
influencing factors. The presence of pure nutrients will give the
clear cut scientific results. This will help in making a sound basis
for the right prediction.
Answer to Q3.
Macronutrients: Macronutrients are generally present in plant
tissues in large amounts (in excess of 10 mmole Kg–1 of dry
matter). The macronutrients include carbon, hydrogen, oxygen,
nitrogen, phosphorous, sulphur, potassium, calcium and
magnesium. Of these, carbon, hydrogen and oxygen are mainly
obtained from CO2 and H2O, while the others are absorbed from
the soil as mineral nutrition.
Micronutrients: Micronutrients or trace elements, are needed in
very small amounts (less than 10 mmole Kg–1 of dry matter). These
13
8
BIO 241: Plant and Animal Physiology
include iron, manganese, copper, molybdenum, zinc, boron,
chlorine and nickel.
Beneficial Elements: In addition to the 17 essential elements
named above, there are some beneficial elements such as sodium,
silicon, cobalt and selenium. They are required by higher plants.
Toxic Elements: The requirement of micronutrients is always in
low amounts while their moderate decrease causes the deficiency
symptoms and a moderate increase causes toxicity. In other words,
there is a narrow range of concentration at which the elements are
optimum. Any mineral ion concentration in tissues that reduces the
dry weight of tissues by about 10 per cent is considered toxic. Such
critical concentrations vary widely among different micronutrients.
The toxicity symptoms are difficult to identify. Toxicity levels for
any element also vary for different plants.
Answer to Q4.
Iron Deficiency:
Iron (Fe) deficiency is a plant disorder also known as "limeinduced chlorosis". It can be confused with manganese deficiency.
A deficiency in the soil is rare but iron can be unavailable for
absorption if soil pH is not between about 5 and 6.5. A common
problem is when the soil is too alkaline (the pH is above 6.5). Also,
iron deficiency can develop if the soil is too waterlogged or has
been overfertilised. Elements like calcium, zinc, manganese,
phosphorus, or copper can tie up iron if they are present in high
amounts.
Iron is needed to produce chlorophyll, hence its deficiency causes
chlorosis. For example, iron is used in the active site of glutamyltRNA reductase, an enzyme needed for the formation of 5Aminolevulinic acid which is a precursor of heme and chlorophyll.
Symptoms: Symptoms include leaves turning yellow or brown in
the margins between the veins which may remain green, while
young leaves may appear to be bleached. Fruit would be of poor
quality and quantity. Any plant may be affected, but raspberries
and pears are particularly susceptible, as well as most acid-loving
plants such as azaleas and camellias.
Treatment: Iron deficiency can be avoided by choosing
appropriate soil for the growing conditions (e.g., avoid growing
acid loving plants on lime soils), or by adding well-rotted manure
or compost.
13
9
BIO 241: Plant and Animal Physiology
Potassium Deficiency:
Plants require potassium ions (K+) for protein synthesis and for the
opening and closing of stomata, which is regulated by proton
pumps to make surrounding guard cells either turgid or flaccid. A
deficiency of potassium ions can impair a plant's ability to maintain
these processes.
Symptoms: The deficiency most commonly affects fruits
vegetables, notably potatoes, tomatoes, apples, currants,
gooseberries, and typical symptoms are brown scorching
curling of leaf tips, and yellowing of leaf veins. Purple spots
also appear on the leaf undersides.
and
and
and
may
Deficient plants may be more prone to frost damage and disease,
and their symptoms can often be confused with wind scorch or
drought.
Prevention and Cure: Prevention and cure can be achieved in the
shorter term by feeding with home-made comfrey liquid, adding
seaweed meal, composted bracken or other organic potassium-rich
fertilisers. In the longer term the soil structure should be improved
by adding plenty of well rotted compost or manure. Wood ash has
high potassium content, but should be composted first as it is in a
highly soluble form.
Calcium Deficiency:
Calcium (Ca) deficiency is a plant disorder that can be caused by
insufficient calcium in the growing medium, but is more frequently
a product of a compromised nutrient mobility system in the plant.
This may be due to water shortages, which slow the transportation
of calcium to the plant, or can be caused by excessive usage of
potassium or nitrogen fertilizers.
Symptoms: Calcium deficiency symptoms appear initially as
generally stunted plant growth, necrotic leaf margins on young
leaves or curling of the leaves, and eventual death of terminal buds
and root tips. Generally the new growth of the plant is affected
first. The mature leaves may be affected if the problem persists.
Treatment: Calcium deficiency can be rectified by adding
Agricultural lime to acid soils, aiming at a pH of 6.5, unless the
plant in question specifically prefers acidic soil. Organic matter
should be added to the soil in order to improve its moistureretaining capacity.
14
0
BIO 241: Plant and Animal Physiology
Plant damage is difficult to reverse, so take corrective action
immediately. Make supplemental applications of calcium nitrate at
200 ppm nitrogen. Test and correct the pH if needed because
calcium deficiency is often associated with low pH.
Nitrogen Deficiency:
Nitrogen (N) deficiency in plants can occur when woody material
such as sawdust is added to the soil. Soil organisms will utilise any
nitrogen in order to break this down, thus making it temporarily
unavailable to growing plants. 'Nitrogen robbery' is more likely on
light soils and those low in organic matter content, although all
soils are susceptible. Cold weather, especially early in the season,
can also cause a temporary shortage.
Symptoms: All vegetables apart from nitrogen fixing legumes are
prone to this disorder. Symptoms include poor plant growth, leaves
are pale green or yellow in the case of brassicas. Lower leaves
show symptoms first. Leaves in this state are said to be etiolated
with reduced chlorophyll. Flowering and fruiting may be delayed.
Prevention and Control: Prevention and control of nitrogen
deficiency can be achieved in the short term by using grass
mowings as a mulch, or foliar feeding with manure, and in the
longer term by building up levels of organic matter in the soil.
Sowing green manure crops such as grazing rye to cover soil over
the winter will help to prevent nitrogen leaching, while leguminous
green manures such as winter tares will fix additional nitrogen from
the atmosphere
Manganese Deficiency:
Manganese (Mn) deficiency is a plant disorder that is often
confused with, and occurs with, iron deficiency. Most common in
poorly drained soils, also where organic matter levels are high.
Manganese may be unavailable to plants where pH is high.
Symptoms: Affected plants include onion, apple, peas, French
beans, cherry and raspberry, and symptoms include yellowing of
leaves with smallest leaf veins remaining green to produce a
‘chequered’ effect. The plant may seem to grow away from the
problem so that younger leaves may appear to be unaffected.
Brown spots may appear on leaf surfaces, and severely affected
leaves turn brown and wither.
Prevention: Prevention can be achieved by improving soil
structure. Do not over-lime.
14
1
BIO 241: Plant and Animal Physiology
Answer to Q5. Plants uptake essential elements from the soil
through their roots and from the air through their leaves. Nutrient
uptake in the soil is achieved by cation exchange, wherein root
hairs pump hydrogen ions (H+) into the soil through proton pumps.
These hydrogen ions displace cations attached to negatively
charged soil particles so that the cations are available for uptake by
the root. In the leaves, stomata open to take in carbon dioxide and
expel oxygen. The carbon dioxide molecules are used as the carbon
source in photosynthesis.
Though nitrogen is plentiful in the earth's atmosphere, relatively
few plants engage in nitrogen fixation (conversion of atmospheric
nitrogen to a biologically useful form). Most plants therefore
require nitrogen compounds to be present in the soil in which they
grow.
Answer to Q6. Rhizobia are unique because they live in a
symbiotic relationship with legumes. Common crop and forage
legumes are peas, beans, clover, and soy.
Infection and signal exchange
The symbiotic relationship implies a signal change between both
partners that leads to mutual recognition and development of
symbiotic structures. Rhizobia live in the soil where they are able
to sense flavonoids secreted by the root of their host legume plant.
Flavonoids trigger the secretion of Nod factors, which in turn are
recognized by the host plant and can lead to root hair deformation
and several cellular responses such as ion fluxes. The best known
infection mechanism is called intracellular infection, in this case
the rhizobia enter through a deformed root hair in a similar way to
endocytosis, forming an intracellular tube called the infection
thread. A second mechanism is called "crack entry", in this case no
root hair deformation is observed and the bacteria penetrate
between cells, though cracks produced by lateral root emergence.
Later on bacteria become intracellular and an infection thread is
formed like in intracellular infections. The infection triggers cell
division in the cortex of the root where a new organ, the nodule
appears.
Nodule formation and functioning
Infection threads grow to the nodule, infect its central tissue and
release the rhizobia in these cells where they differentiate
morphologically into bacteroids and fix nitrogen from the
atmosphere into a plant usable form, ammonium (NH4+), utilizing
the enzyme nitrogenase. In return the plant supplies the bacteria
with carbohydrates, proteins, and sufficient enough oxygen so as
14
2
BIO 241: Plant and Animal Physiology
not to interfere with the fixation process. Leghaemoglobins, plant
proteins similar to human hemoglobins help to provide oxygen for
respiration while keeping the free oxygen concentration low
enough not to inhibit nitrogenase activity.
The legume – Rhizobia symbiosis is a classic example of
mutualism — Rhizobia supply ammonia or amino acids to the plant
and in return receive organic acids (principally as the dicarboxylic
acids malate and succinate) as a carbon and energy source — but
its evolutionary persistence is actually somewhat surprising.
Because several unrelated strains infect each individual plant, any
one strain could redirect resources from nitrogen fixation to its own
reproduction without killing the host plant upon which they all
depend. But this form of cheating should be equally tempting for
all strains, a classic tragedy of the commons. It turns out that
legume plants guide the evolution of Rhizobia towards greater
mutualism by reducing the oxygen supply to nodules that fix less
nitrogen, thereby reducing the frequency of cheaters in the next
generation
Readings
There are a number of excellent resources on the web. A few suggested links
are:
 http://www.how-to-study.com/
The “How to study” web site is dedicated to study skills resources. You
will find links to study preparation (a list of nine essentials for a good
study place), taking notes, strategies for reading text books, using
reference sources, test anxiety.
 http://www.ucc.vt.edu/stdysk/stdyhlp.html
This is the web site of the Virginia Tech, Division of Student Affairs.
You will find links to time scheduling (including a “where does time go?”
link), a study skill checklist, basic concentration techniques, control of the
study environment, note taking, how to read essays for analysis, memory
skills (“remembering”).
14
3
BIO 241: Plant and Animal Physiology
 http://www.howtostudy.org/resources.php
Another “How to study” web site with useful links to time management,
efficient reading, questioning/listening/observing skills, getting the most
out of doing (“hands-on” learning), memory building, tips for staying
motivated, developing a learning plan.
The above links are our suggestions to start you on your way. At the time of
writing these web links were active. If you want to look for more go to
www.google.com and type “self-study basics”, “self-study tips”, “self-study
skills” or similar.
14
4