Download Transport in multicellular plants

What substances need transporting
in plants?
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Oxygen?
Carbon dioxide?
Water?
Minerals?
Nutrients? Sugars,amino acids.
Hormones?
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Transport in multicellular
plants
Carbon dioxide
Needed for photosynthesis in all
green parts of the plant, mainly
leaves.
It diffuses from the air into leaves
down the concentration
gradient(high conc in air low conc
in leaves)
The large surface area/volume
ratio of leaves helps this
It is NOT moved by the plant
transport system
CO2
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Oxygen
All plant cells need it for respiration
Photosynthesising cells produce their
own supplies
For other cells such as roots it must
diffuse in from air in the soil
Excess oxygen from leaves diffuse
out
Plants have much lower energy
needs than animals ,so need oxygen
much less rapidly
Oxygen is NOT transported around
the plant.
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Organic nutrients
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Photosynthetic cells make their own nutrients
These include glucose & amino acids
These need to be transported to other parts
of the plant
This involves the phloem
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Inorganic ions and water
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Ions such as magnesium,potassium,nitrates
etc are needed by cells
These including water are taken up by the
roots and transported in the xylem
The plant transport system
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Only WATER, MINERAL IONS and
ORGANIC NUTRIENTS need
transporting
Because of the LOW ACTIVITY of
plants these are not needed rapidly
The plant transport system is much
simpler than animals
Fluids move much more slowly
There is no pump to move the fluids
The cells transporting substances are
known as vascular tissue
There are two tissues consisting of
systems of tubes called xylem &
phloem
WATER
TRANSPORT
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In a plant water moves
down a water potential
gradient
From HIGH water
potential (ψ) in the soil
to
LOW water potential
(ψ) in the air around the
leaves
Because of this there is a
constant flow of water
through the plant
Soil to Root Hair
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Water is absorbed by the roots
Most of the root is impermeable
to water
Only the root hair cells have thin
permeable cell walls
These are found in a region
behind the root tip
They grow out between the soil
particles and provide a large
surface area.
They are very delicate and easily
damaged,often lasting only a few
days before being replaced
Mycorhizas
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Some trees have fungi
growing in their roots
These associations are called
mycorhizas
The fungi form a mass of
fine threads which help
absorb nutrients especially
phosphates
Some trees growing on poor
soils cannot survive without
the fungi
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Water enters the root hair
cell by osmosis
The root hair cell contains
dissolved nutrients and
minerals, this gives it a low
water potential
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These minerals have been
pumped into the cells by
active transport
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Soil water has higher water
potential
Water enters the cell by
osmosis from a high WP to
a low WP
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Water Transport : From root hair to
xylem
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Water enters the root hair
cells
It then passes through the
cortex and into the stele
This consists of the
endodermis,pericycle and
into the xylem
Water potential in the xylem
is lower than in the root hair
cells
Water moves down the
water potential gradient
Root Section
Ranunculus: t.s young root
Tissue plan
Apoplast & Symplast
There are three routes for the water
through the cortex cells
1. Apoplast pathway
 The water moves through the cell walls
 These are made of cellulose fibres and can
soak up water
 It moves between cells via the intercellular
spaces or via touching cell walls.
 It does not pass through any membranes,
meaning mineral ions can be transported
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2. Symplast pathway
 Here the water enters the cells
 It passes through the cytoplasm
 It passes from cell to cell through the
cytoplasm in plasmodesmata
 It is thought that this pathway is most
often used
3. Vacuolar pathway
 This is similar to the symplast but also
includes the vacuoles
 Apoplast may be used when a lot of
water is being lost from the plant and
flow through the root needs to be rapid
The endodermis
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When the water
reaches these cells it
must follow the
symplast pathway
This is because the cell
walls contain a thick
waterproof band called
a casparian strip.
This is made of a waxy
substance called
suberin
This blocks the flow of
water by the apoplast
route
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It is thought that this may allow control of
the minerals entering the xylem as they
must pass through cell membranes
Endodermis cells pump minerals into the
xylem by active transport
This lowers the ψ and helps move water in to
the xylem by osmosis
It may also help generate root pressure
Casparian strip also blocks water return out
of the xylem
Apoplast & Symplast
How does water move up the stem?
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Root pressure
Transpiration stream
Capillary action
Water transport: From root to
leaf
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Water passes
from the roots
into the stem
and then into
the leaves
It does this
through the
XYLEM TISSUE
Xylem Tissue
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Xylem tissue is a group
of cells that work
together to transport
water & minerals and
support the plant
It contains several
different types of cells
Xylem vessel
elements
Tracheids
(Sclerenchyma) fibres
Parenchyma cells
Xylem vessel
elements
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Xylem vessels transport
water
They are made up of many
elongated vessel elements.
These form a hollow pipe
These began life as a
normal plant cell
But a substance called
lignin has been laid down
in the walls
Lignification
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Lignin is a complex
carbohydrate
It is hard,strong and
impermeable to water
It can be stained red in
microscope slides
This thickening of the cell
wall with lignin kills the
cell
This leaves a hollow space,
a lumen, through which
water can pass
Types of lignification
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Thickening
starts off as
circular or
spiral,
then
reticulate
and finally
pitted
Types of
thickening
Pits
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Pits are not open
They have the original
cell wall in them
This is fully permeable to
water
Pits allow water to enter
and leave the xylem
They can also be used to
bypass blockages
Tracheid
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Also dead , hollow cells
with lignified walls
Their ends are not
completely open and
taper
Water passes between
cells via pits
They are the main
water conducting tissue
in more primitive plants
More modern
plants(angiosperms)
make more use of
xylem vessels
Fibres
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Small dead
elongated cells
Lignified walls
Small lumen
Main role is
supporting the
plant
Stem Vascular Structure
Vascular Bundle
Leaf Section(dicot TS)
Leaf section
Transpiration Stream
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To understand this we first need
to consider:
Movement of water: Leaf to air
The mesophyll cells of the leaf
have wet cell walls
Water evaporates into the
air spaces saturating them
Water will move by osmosis
from the xylem vessels (via
pits) across the cells of the
leaf to replace water lost
If the air outside the leaf has
a lower water potential than
inside, water vapour will
diffuse out of the leaf through
the stomata
This loss of water by
evaporation from the aerial
parts of plants is called
TRANSPIRATION
Transpiration: the price plants pay
for photosynthesis
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Carbon dioxide is needed for
photosynthesis
Stomata must be open so this can diffuse
into the leaf
This means water is lost through the open
stomata
If water is being lost too quickly the
stomata may be closed
Leaves also wilt giving less surface area
to lose water
Transpiration can also be important in
cooling plant leaves as evaporation use
heat
Transpiration Stream : root xylem to leaf xylem
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As water evaporates from the leaf cells water replaces it
from the leaf xylem vessels by osmosis
Removing water from the xylem lowers the
hydrostatic pressure
This is now lower than the pressure in the roots
causing water to move up the xylem (similar to being
sucked up a straw)
If you suck too hard on a straw the pressure causes the
walls to collapse
To prevent this xylem vessels walls are strong,
lignified.
The transpiration stream is a passive process, relying
on evaporation of water from the leaves.
Mass Flow
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This movement of water is called mass
flow
The water molecules move all together
as a mass
It can do this because the water
molecules are held together by H
bonds
This is called COHESION
They are also attracted to the lignin in
the walls of the xylem
This is ADHESION
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If the column of water broke an air bubble
would form and this would stop the mass flow
The small diameter of the xylem vessels makes
this less likely to happen
Capillary action
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If thin tubes are placed
in water the water will
move some way up the
tube
This is capillary action
due to adhesion &
cohesion
Root Pressure
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Hydrostatic pressure at the root end
of the xylem can be increased
Cells surrounding the xylem in the
roots pump mineral ions into the
xylem by active transport
This lowers the water potential and
more water enters the xylem from the
cells by osmosis
Hydrostatic pressure increases
This is not essential though and only
helps
Measuring the rate of transpiration
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The rate of transpiration varies
This will depend on:
Temperature:
The higher the temperature the faster the rate
of evaporation inside the leaves and the faster
the rate of diffusion out of the stomata
Water potential gradient between leaf and
air
The steeper the gradient the faster the water
vapour will diffuse out of the leaf
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Moving air will remove water vapour from around the leaf.
Keeping the gradient steep and speeding up diffusion
Dry air will mean a steeper gradient
Number of stomata
The more stomata the quicker water vapour will be lost
Position of stomata
Stomata on lower surface are not exposed to the heating effect
of the sun.So less water lost
Light
Stomata open in the light.
Soil water availability
Low levels of soil water may mean wilting,plant stress and
stomatal closure
Waxy cuticle
Its presence reduces water evaporation from leaf surface
Factors affecting the rate of transpiration
Factor affecting
transpiration rate
How it affects water loss
Using the potometer
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8.
Label the diagram fully
The potometer does not measure the
accurate uptake of water,it can only
compare water uptake.Why?
Why should the leafy shot stem be cut at
an angle under water and placed in the
potometer?
What must the potometer be to ensure it
works correctly?
Explain how you can estimate water
uptake using the potometer
Explain how you can change the
following conditions when using the
potometer: humidity, light intensity, air
currents, temperature.
Explain the role of the reservoir.
Plot a graph of your results and explain
them.
condition
Movement of meniscus each
minute(mm)
1
2
3
4
5
Using the potometer
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Fill the apparatus and syringe totally with water by placing it in a bowl – air
bubbles will block the water flow
Cut a shoot under water – this ensures the xylem does not get blocked with
air bubbles
Place the shoot in the apparatus under water
Ensure it is a tight fit and seal with vaseline – any leaks will let air into the
apparatus.
Set up on a stand
As water evaporates from the leaves water will be taken up from the
apparatus to replace it.
This will not give real total of water uptake as a small amount of the water
taken up will be used for photosynthesis
Measure the movement of the water level in a set time.
The water level can be reset by adding water from the syringe.
Try moving air, lower temperature, higher humidity to see their effect on
transpiration rate.
Xerophytes
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Plants adapted to living in very dry areas.
Explain using examples how xerophytic plants reduce
water loss by transpiration
Include:
Leaves reduced to spines
Waxy cuticle
Position of stomata
Sunken stomata
Hairy leaves
Rolled leaves
Phloem Tissue
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This tissue transports
ASSIMILATES
These are substances the plants
make themselves and include
sugars and amino acids
Phloem tissue consists of 2
main types of cells
Sieve elements
Companion cells
There are also parenchyma
cells and fibres
Sieve Elements
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Sieve elements are the single
cells which join up to form
sieve tubes
These are living cells
They have a
Cellulose cell wall
Cell membrane
Only a thin lining of
cytoplasm containing
Endoplasmic reticulum &
mitochondria
No nucleus,chloroplasts or
ribosomes
Sieve plates
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The ends of the cells are
not completely open but
have a sieve plate
containing open sieve
pores so materials are
free to move through
Companion cells
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These are next to the sieve elements
They have the “normal plant cell” structure
However they have more mitochondria and
ribosomes than normal
Smaller vacuole
There are many plasomdesmata between the
cells so materials can easily pass between the
cells
Microscopy
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Both light and electron
microscopy show
protein fibres in the
phloem
It was thought these
had a role to play
It has now been shown
they are only a
response to damage
by the cells when the
sections are taken
Translocation
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The transport of soluble organic
molecules within the plant
(assimilates)
Takes place through the sieve
tubes
It takes place by mass flow
Due to hydrostatic pressure
differences at either end of the
phloem
But this is an active process , not
passive as in transpiration
Assimilates are actively loaded
into the phloem and unloaded out
of it
Sources and Sinks
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The area where assimilates
are being made is called
the SOURCE
It is most often the leaf
where many organic
molecules are made during
photosynthesis
However it could be a
storage organ such as a
potato tuber where sugars
are being loaded into the
phloem
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Assimilates are transported from the
source to the SINK
This is where the assimilates are
unloaded and are being used by the
plant
They might include:
A growing point of the root or stem
A storage organ in the root, building up
starch
A developing fruit or flower
A nectary in a flower
As sinks could be above or below the
sources materials can move in both
directions in phloem
How translocation occurs
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Loading of assimilates
The most widespread
assimilate which is loaded
into the phloem is sucrose
This is formed from triose
sugars(C3) made in the
mesophyll leaf cells in
photosynthesis
This is a disaccharide that is
the transport
carbohydrate for plants
It moves across the leaves to
the phloem via apoplast or
symplast
This varies between species
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When it reaches the companion
cells it is moved into
them(loaded) by active
transport
A H+ protein pump moves
hydrogen ions out of the
companion cell by active
transport
This is against their
concentration gradient and
requires energy from ATP
This is provided by the many
mitochondria in the companion
cells
This means a high concentration
of H+ outside the cell
Another protein in the
membrane allows the H+ ions
back into the cell down their
concentration gradient
This is linked to bringing
sucrose into the cell
This protein is a co-transporter
protein
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Mass Flow
Sucrose can then diffuse into
the sieve tube via the
plasmodesmata
This lowers the water
potential of the sieve tube
Water enters them by
osmosis from the xylem
raising the hydrostatic
pressure
The sucrose solution is pushed
down the sieve tubes
At the sink sucrose diffuses
out of the phloem and water
follows
This lowers the hydrostatic
pressure
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Unloading at the sink
The mechanism of this is unclear
sucrose may just diffuse out into the
cells that need it
The sucrose may be converted to
glucose & fructose by the enzyme
invertase, and be used for
respiration
or changed to starch and stored
In both cases its concentration will
decrease maintaining a concentration
gradient
In some cases it is possible that sucrose
is unloaded by active transport
Evidence for the mechanism of
translocation
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Sources of evidence come from:
Light and electron microscopy
Composition of phloem sap
Speed of flow of sap and hydrostatic
pressure
Potential difference readings from
inside and outside cells
Phloem sap
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This is not easy to collect.
As soon as you cur phloem
tissue it rapidly blocks the
sieve pores with the protein
plug
Within hours this is replaced
by callose a polysaccharide
similar to cellulose
This “clotting” mechanism
stops loss of sap and entry
of microbes.
Castor oil sap can be
collected
Solute
Concentration
(m/l)
Sucrose
250
K+
80
Amino acids
40
Cl-
15
phosphate
10
Mg2+
5
Na+
2
ATP
0.5
nitrate
0
auxins
traces
Using Aphids
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Aphids can place their
proboscis into the phloem
sieve tube
The proboscis is so fine that it
does not activate the plants
protective mechanism
They can then be anaesthetised
and heads removed
Samples of sap will then exude
from the cut end of the stylet
Evidence for Translocation theory
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Phloem sap has a high pH about 8.0 , suggesting a low
concentration of H+ ions
This would be the case if they are being removed by
active transport.
There is a -150mv potential across the membrane, again
supporting the removal of H+ ions, making the inside
–ve.
ATP is present in the sap which would support active
transport
Flow rate of sap is 10,000x faster than it would be for
diffusion and match readings of hydrostatic pressure
differences at source & sink
Phloem sap is under pressure and will leak out if cut
Evidence against
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Why do the sieve pores still exist? They obstruct
mass flow and you would think evolution would
have removed them!
Phloem transports to several different sinks at
one time, not to the one with the lowest
hydrostatic pressure!
The differences between sieve
elements and xylem vessels
Do exercise SAQ 10.9 p147