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Biology
Preliminary Course
Stage 6
Patterns in nature
Part 5: Obtaining and transporting materials in plants
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Contents
Introduction ............................................................................... 2
Autotrophic and heterotrophic cells ........................................... 4
Obtaining nutrients in plants...................................................... 5
Photosynthesis .....................................................................................5
Function of leaves ..............................................................................11
The stem.............................................................................................13
Roots...................................................................................................16
Transport systems in plants .................................................... 18
Xylem ..................................................................................................19
Phloem................................................................................................28
Gas exchange in plants........................................................... 30
Suggested answers................................................................. 37
Exercises–Part 5 ..................................................................... 39
Part 5: Obtaining and transporting materials in plants
1
Introduction
Plants have specialised structures to obtain nutrients from their
environment. You may recall that plants and animals obtain nutrients
differently. Plants rely on the Sun to manufacture food by a process
called photosynthesis. Plants are autotrophic organisms. Animals cannot
manufacture their own food; they consume or eat other organisms in
order to gain the nutrients they require for life processes. Animals are
heterotrophic organisms.
Plants and animals have specialised cells, tissues and organs to obtain the
nutrition they require and carry out their body processes. Some of these
will be investigated in this part.
In this part you will be given opportunities to learn to:
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•
distinguish between autotrophs and heterotrophs in terms of nutrient
requirements
•
identify the materials required for photosynthesis and its role in
ecosystems
•
identify the general word equation for photosynthesis and outline
this as a summary of a chain of biochemical reactions
•
explain the relationship between the organisation of the structures
used to obtain water and minerals in a range of plants and the need to
increase the surface area available for absorption
•
explain the relationship between the shape of leaves, the distribution
of tissues in them and their role
•
outline the transport system in plants including:
–
root hair cells
–
xylem
–
phloem
–
stomates and lenticels
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In this part you will be given opportunities to:
•
plan, choose equipment or resources and perform first–hand
investigations to gather information and use available evidence to
demonstrate the need for chlorophyll and light in photosynthesis
•
perform a first–hand investigation and gather first–hand data to
identify and describe factors that affect the rate of transpiration
•
perform a first–hand investigation of the movement of materials in
xylem or phloem.
Extracts from Biology Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board’s website
at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/index.html.
This version November 2002.
To complete the practical activities in this part you will require the
following equipment. Alternative exercises have been included.
•
•
•
•
•
•
•
•
•
•
1 large beaker or saucepan
1 small beaker or glass jar
Bunsen burner or hot plate
tripod and gauze (if using
Bunsen)
250 mL water
50 mL methylated spirit
a few soft fleshy leaves such as a
geranium
aluminium foil
a variegated leaf plant
iodine solution
Part 5: Obtaining and transporting materials in plants
•
stick of celery
•
glass of water with food colouring
(red/blue works best)
•
knife, small kitchen type
•
hand lens or microscope with lamp
•
glass slides and cover slips if using
microscope
•
thin glass tubing or clear plastic
tubing
•
Vaseline® or petroleum jelly
•
soft, fleshy plant stem eg.
Impatiens
•
marker pen or sheet of graph paper
•
scissors
•
retort and clamp or similar.
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Autotrophic and heterotrophic cells
Cells can be classified as either autotrophic or heterotrophic depending
on how nutrition is obtained.
Autotrophic cells are those which can make their own food
(auto = self; trophic = feeding). Plant cells with chloroplasts are
autotrophic. The Sun’s energy is used to combine simple substances like
carbon dioxide and water. These two raw materials are used to make
glucose. Glucose can be changed into starch and other more complex
substances like cellulose.
Heterotrophic cells are those which cannot make their own food
(hetero = other). Heterotrophs depend on food made by others.
Heterotrophic cells include animal cells, fungal cells and some
bacterial cells.
Examples of autotrophic cells are:
_________________________________________________________
Examples of heterotrophic cells are:
_________________________________________________________
Check your answers.
Complete Exercise 5.1.
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Obtaining nutrients in plants
Photosynthesis
Plants carry out the food–making process called photosynthesis. In this
process plants convert the Sun’s energy into chemical energy stored in
sugars such as glucose. To do this, plants must have access to materials
such as carbon dioxide and water. Oxygen and water are also produced
in the process.
Inputs
carbon dioxide
water
light
Outputs
oxygen
water
sugars
What do you remember about photosynthesis?
1
Where does photosynthesis take place in a plant?
_____________________________________________________
2
Name the cell organelle where photosynthesis takes place.
_____________________________________________________
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3
Describe the conditions necessary for photosynthesis to take place.
______________________________________________________
______________________________________________________
Check your answers.
Plants are producers as they make their own food. Plants are the first
step in a food chain. They use light energy to produce carbohydrates like
glucose. These carbohydrates are eaten by animals (herbivores and
omnivores). In turn, herbivores are eaten by carnivores and so on
through the food chain. So, in an ecosystem photosynthesis is an
important step in the flow of energy.
The phases of photosynthesis
There are two main stages or phases in the photosynthetic process.
These are called the light and dark phases.
The light phase
During this phase, light is absorbed by chlorophyll and the splitting of
water molecules occurs. Water molecules are split to form oxygen and
hydrogen ions. Light acts on the chlorophyll. The energy is converted
from light to chemical energy. These reactions take place in the grana of
the chloroplast. Many enzymes are used to carry out the process.
light
chlorophyll a
ENERGY
oxygen
released
splits
H2O
H+
to the second phase
Simplified light phase of photosynthesis.
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The dark phase
The dark phase is often called the fixation of carbon phase.
Carbon dioxide is fixed into glucose molecules using hydrogen ions and
energy obtained during the light phase.
energy
from light
phase
hydrogen
from light
phase
H+
+
5C sugar
CO2
many steps
Simplified dark phase of photosynthesis.
This phase also involves a number of steps in the reaction where specific
enzymes are required. This series of reactions occur in the colourless
fluid of the chloroplast called the stroma that surround the grana.
Light is not required for these reactions. Glucose is synthesised during
the dark phase.
1
Outline what happens in the light phase of photosynthesis.
_____________________________________________________
_____________________________________________________
2
Outline what happens in the dark phase of photosynthesis.
_____________________________________________________
_____________________________________________________
Check your answers.
Word equations are used to describe reactions. They can also be used as
a summary for complex pathways. The general word equation for
photosynthesis is shown below.
light energy
carbon dioxide + water
sugar + oxygen
chlorophyll
You can see from the previous diagrams that photosynthesis is a complex
process. So, this equation is a summary of the biochemical reactions that
make up the process of photosynthesis. You do not need to learn these
biochemical reactions.
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A general equation for photosynthesis is:
6CO2 +6 H2O
C6H12O6 + 6O2
Radioactive tracing has shown that a more correct equation for
photosynthesis is:
6CO2 +12 H2O
C6H12O6 +6O2 + 6H2O
Complete Exercise 5.2.
Why do most plants appear green?
Photosynthetic pigment is a mixture of a number of different pigments,
including chlorophyll. Chlorophyll absorbs mostly blue–violet and red
light and reflects green. This characteristic of reflecting green light is
why most leaves appear green in colour.
Engelmann’s experiment
Thomas Englemann was a German biologist who investigated the
wavelength of light used by plants during photosynthesis. He used
bacteria to detect the presence of oxygen by observing the change in
bacteria numbers in different light environments. The environments
were created by projecting the whole spectrum onto a filament of algae.
The results indicated that red and violet wavelengths are absorbed by
chlorophyll pigments, resulting in the increased production of oxygen
and increased numbers of bacteria in the corresponding regions.
The need for chlorophyll and light in photosynthesis
In this activity you will be investigating the need for chlorophyll and light in
photosynthesis.
The first experiment examines the production of starch as an indicator
that photosynthesis has occurred in parts of a leaf.
The second experiment focuses on the production of starch in areas of
leaves that do not contain chlorophyll.
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Variegated leaves have areas that do not contain chlorophyll (Photo J West).
Aim
The aim of this experiment is to demonstrate the need for chlorophyll and
light in photosynthesis.
Materials required:
•
1 large beaker or saucepan
•
1 small beaker or glass jar
•
Bunsen burner or hot plate
•
tripod and gauze (if using Bunsen)
•
250 mL water
•
50 mL methylated spirit
•
a few soft fleshy leaves such as a geranium
•
aluminium foil
•
a variegated leaf plant (leaves with a mixture of colours – choose
one that is a mixture of yellow and green)
•
iodine solution.
Method:
1
Place aluminium foil over one half of your leaf on the plant, secure
with paperclips and leave overnight.
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2
Place in direct sunlight for several hours.
3
Place your variegated plant in sunlight for several hours.
4
Pick the leaves from both plants and place the leaves in a beaker
with the water. Remove the foil from the soft leaf. Heat the water
and leaves gently, until they go very limp.
5
Turn off the heat source.
6
Remove the leaves from the water and place into the small beaker or
jar with the methylated spirit.
Care should be taken using methylated spirit, as it is a flammable
substance. Avoid contact with a naked flame. Even methylated spirits
vapours are flammable.
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7
Place the small beaker or jar into the hot water and allow to stand for
approximately five minutes. The green pigment should be extracted
from the leaves after this time. If the methylated spirit has not become
very dark green, stir the leaves and leave for a few extra minutes.
8
After there has been sufficient chlorophyll extracted, remove the
leaves and wash them in water.
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Now place your two leaves onto a white surface and flood them with
iodine. Remember that iodine turns blue–black in the presence of
starch and starch is produced in areas that are actively
photosynthesising.
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Results
In the areas that produce starch and are carrying out photosynthesis the
iodine will turn blue–black or purple. In the yellow areas of variegated
plants there is no chlorophyll so these areas should not be coloured.
In the leaf that was covered with aluminium foil the light would not have
got under the foil and photosynthesis would not have occurred.
Conclusion
The experiment demonstrated that photosynthesis does not occur unless
there is both light and chlorophyll present.
Do Exercise 5.3 now.
Function of leaves
Go outside into the garden or take a walk to a park. Look at the leaves on
the plants and sketch three different ones on your own paper.
Note, on your drawings, three similarities and three differences in the
leaves you selected.
You will have noticed that most leaves are thin and flat. There are a
large range of leaf shapes. Being thin and flat means that leaves have a
large surface area to volume ratio which is important for the absorption
of light, oxygen and carbon dioxide.
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epidermis
cuticle
cells containing chloroplasts
palisade
mesophyll
layer
spongy
mesophyll
stomate
air space
xylem
phloem
cell wall
vascular bundle
Cross–section of a leaf. Source: Messel, H (chair). (1963.) Science for high
school students. The Foundation for Nuclear Energy. University of Sydney.
The chloroplasts are located in the mesophyll (middle leaf) region of the
leaf. Gases enter and leave the leaf through the stomates. Therefore, the
structure of the leaf ensures that the photosynthetic cells that contain
chlorophyll are close enough to the top of the leaf to receive light and
close enough to the stomates to gain the gases they require.
1
Outline the features of leaves.
_____________________________________________________
_____________________________________________________
2
Identify the major role of leaves.
______________________________________________________
______________________________________________________
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3
a) Identify the tissue where photosynthesis occurs in leaves. How
does the location of this tissue assist in photosynthesis?
_________________________________________________
_________________________________________________
b) Xylem carries water up through the plant. How does the
location of this tissue help in photosynthesis?
_________________________________________________
_________________________________________________
c) Xylem is associated with phloem in plants. Predict a role
related to photosynthesis for phloem in plants.?
_________________________________________________
_________________________________________________
4
Can you suggest a reason why leaves are thin and flat? (Hint: Think
about the effects of the SA:V.)
_____________________________________________________
_____________________________________________________
Check your answers.
The stem
You have looked at the structure of leaves. This is where photosynthesis
occurs. But how does the products of photosynthesis get to the other
parts of the plants. To answer that question you need to look at the
structure of the stem.
Sketch a plant from your garden and label the stem, leaves, flowers and
buds. Use your own paper.
Stems can be recognised because they have leaves and buds. Most stems are
above ground, forming part of the shoot system of plants. Some plants have
underground stems and their leaves may be reduced to scales.
Although the arrangement varies with different types of plants, stems
usually form a complex branching pattern. The leaves are spaced along
them to gain maximum exposure to sunlight.
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Tissues in stems
Stems are usually green when young and can carry out photosynthesis.
They may become woody when older. At the tip of each stem is a
terminal bud, a growing point for the plant. Stems also support the
flowers and fruit of the plant.
One of the main functions of stems is transport. Internally, stems contain
tubes of conducting tissue, the xylem and phloem. This vascular tissue
carries materials between the shoot and root systems. The conducting
tissue is arranged in a ring or scattered throughout the stem tissue.
The outer covering of stems, the epidermis, forms an impermeable layer
protecting the inner cells and preventing water loss. There are stomates
for the exchange of gases on young green stems and lenticels which
serve the same purpose on woody stems.
Cells in the cortex and pith usually store food but may also contain
chloroplasts and photosynthesise. There are air spaces between cells for
the circulation of gases. Some stems are hollow with little or no pith.
Cross–section of a stem showing vascular bundles. (Photo Jane West)
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Close–up of a vascular bundle showing xylem, cambium and phloem
(Photo Jane West)
Vascular bundles
Vascular bundles are groups of conducting tissue in a stem. Each bundle
contains three types of tissue: xylem, phloem and cambium.
Xylem forms long tubes up to 1 m in length. They are dead cells
(they have no nucleus). These long tubes are known as xylem vessels.
Xylem vessels are thickened with woody material, with cross walls that
have broken down. Xylem gives support, strength and rigidity to the
stem, and transports water and mineral ions upwards from the roots to the
leaves. Note; Water and mineral ions travel only in one direction in the
xylem–upwards.
Phloem consists of living sieve–tube cells forming long columns.
There are perforations in the cell walls so that the cytoplasm of the cells
connects along the tubes. Associated with the sieve–tube cells, are
companion cells and other supporting tissue. Organic materials including
sugars, amino acids and hormones are transported by the living
Part 5: Obtaining and transporting materials in plants
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sieve–tube cells of phloem tissue. This movement is called translocation.
Materials move both up and down through the plant in the phloem.
The movement is too fast to be caused only by diffusion. There are
several theories suggesting possible forces involved but the exact
mechanism remains unknown.
Cambium cells are capable of cell division. They divide to form cells
which become new xylem and phloem tissue. In older stems division of
the cambium cells results in a continuous ring of vascular tissue.
Roots
You have now read about how and why plants transport water. Have you
asked yourself where they get the water? Roots do not photosynthesise
but grow through the soil anchoring the plant and supplying the plant
with water and mineral ions. To do this roots have to have an extensive
surface area to be able to absorb water. The drawing below shows a
young root covered in root hairs. The root hairs greatly increase the
surface area of the root so that water can pass from the soil into the plant.
As well as root hairs plants have different types of roots. The two main
types of roots are fibrous roots and tap roots.
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fibrous
taproot
1
Outline the differences between taproots and fibrous root systems.
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
What structures of root systems increase surface area to improve
water uptake?
_____________________________________________________
3
Explain how the large surface area of roots assists in the survival of
plants in dry weather.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Complete Exercise 5.4.
Part 5: Obtaining and transporting materials in plants
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Transport systems in plants
You have already looked at some of the structures involved in the
transport system of plants. Answer these questions below for revision.
1
What is the role of the root, stem and leaf in flowering plants?
_____________________________________________________
_____________________________________________________
2
Plants have a system of vascular bundles to transport sugars, gases
and water within the plant. The term vascular bundle is used to
describe the conducting tissue in a stem.
What type of tissue is found in a vascular bundle?
______________________________________________________
______________________________________________________
Check your answers.
xylem tubes move water
up the plant from the roots
phloem tubes move sugars
dissolved in water throughout
the plant
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Like all multicellular organisms, plants need to transport materials from
one place to another. Sugars are produced by the process of
photosynthesis in the leaves. Every cell in a plant requires sugars for
respiration. So, sugars are transferred from where they are produced to
where they are needed.
The same is true for water and minerals. These are taken into the plant
through the root hairs and are needed by every cell in the plant.
This transport function is carried out by xylem tissues (for water and
minerals) and phloem (for sugars).
Cambium cells are capable of cell division. They divide to form cells,
which become new xylem and phloem tissue. In older stems, division of
the cambium cells results in a continuous ring of vascular tissue.
Xylem
Xylem forms long tubes up to one metre in length. They are made up of
dead cells, thickened with woody material (lignin), the cross walls have
broken down. They are known as xylem vessels. Xylem gives support,
strength and rigidity to the stem, and transports water and mineral ions
upwards from the roots to the leaves. Note: water and mineral ions travel
only in one direction in the xylem–upwards.
Movement of water and dissolved chemicals takes place in xylem vessels
which form part of the vascular bundles within roots, stems and leaves.
Detailed information on the processes involved in the movement of
substances through the xylem can be found in the Additional resources
section of this part.
Stems can be recognised because they have leaves and buds.
Most stems are above ground, forming part of the erect shoot system of
plants. Some plants have underground stems and their leaves may be
reduced to scales.
Stems are usually green when young and can carry out photosynthesis.
They may become woody when older. At the tip of each stem is a
terminal bud, a growing point for the plant. Stems also support the
flowers and fruit of the plant.
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In this activity you will be investigating the movement of water through the
plant.
Materials required:
•
stick of celery
•
glass of water with food colouring (red/blue works best)
•
knife, small kitchen type
•
hand lens or microscope with lamp
•
glass slides and cover slips if using microscope.
What you will do:
1
Place a stick of celery into the glass of coloured water for a few
hours.
Place the celery into the glass. (Photo: J West)
2
20
Remove celery from water and cut in half, carefully, cutting away from
fingers. Using the hand lens, examine the stem and the location of the
coloured water.
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Cut across the stem. (Photo: J West)
The coloured liquid is seen in the xylem. (Photo: J West)
4
Where in the stem is the coloured water located?
_____________________________________________________
5
Name the tissue in which the water is located.
_____________________________________________________
Part 5: Obtaining and transporting materials in plants
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6
Draw a sketch of your observations.
If you have access to a microscope, prepare a slide of a cross–section of
the celery stem. Observe this specimen under the microscope and look at
it. (A cross–section is produced by cutting across, not lengthways.).
Water enters the plant through the roots. The roots are covered by fine
root hairs which increase the surface area for absorption of water.
The root hairs are single celled extensions of the root epidermis (surface
or outer layer of the root).
Water enters the root hair by diffusion. The concentration of solutes in
the soil water is lower than inside the root hair cells. Water will move
from an area of high water concentration (in the soil) to an area of low
water concentration (within the root hair cells).
root hair
soil particles
water
Water moves into the plant from the soil through the root hairs.
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One of the main functions of stems is transport of substances around the
plant. Stems contain tubes of conducting tissue or vascular bundles,
which consist of the xylem and phloem, that carry materials between the
shoot and root systems. The conducting tissue is arranged in a ring or
scattered throughout the stem tissue.
Transpiration
When stomates are open gases including carbon dioxide can diffuse into
a plant. At the same time, however, water molecules can diffuse into the
air because of the higher water concentration inside the plant.
Water evaporates from the cell surfaces, diffuses through the intercellular
spaces and out through the stomates. This diffusion of water from a plant
is called transpiration.
Water loss by transpiration is unavoidable by a plant with the stomates
open. The water lost needs to be replaced by uptake through the roots.
There is a constant upward flow of water through a plant. This is known
as the transpiration stream.
If water loss exceeds water intake, the stomates close and cells lose their
turgidity. The stems and leaves wilt and the plant may die.
Transpiration is an important part of the mechanism by which water and
mineral ions are transported from the roots to the stems and leaves.
The evaporation of water has a cooling effect on the plant, particularly
the leaves.
Factors affecting transpiration
The structure of the plant has an effect on the transpiration rate.
Stomates may be open or closed. When they are closed the transpiration
rate drops and diffusion occurs at a much slower rate through the cuticle.
Normally, stomates are open during the day for the exchange of gases in
photosynthesis and closed at night.
Some plants have special features (adaptations) to reduce the
transpiration rate. Structural features may include a very thick cuticle,
sunken stomates, hairs on the leaf or a reduction in leaf surface area.
Physiological features may include the closure of the stomates or rolling
up of the leaf to reduce surface area, during the day when the temperature
is high.
There are a number of external (environmental) factors that affect
transpiration in a plant. These are temperature, humidity, wind, light
and soil.
Part 5: Obtaining and transporting materials in plants
23
•
In high temperatures, diffusion is more rapid (warm air holds more
water than cold air).
•
If the atmosphere is saturated with water vapour (conditions of high
humidity) transpiration is decreased.
•
Moving air increases the transpiration rate. Water vapour is carried
away from the leaf and a high diffusion gradient maintained.
•
Light intensity affects stomate opening and this in turn affects the
transpiration rate.
•
The water content of the soil and the solute concentration affect the
rate at which water can be taken up by a plant.
Measuring transpiration
A potometer is an instrument which can be used to measure the rate of
transpiration. There are several varieties of potometers. In a potometer,
water is run into the apparatus through a glass funnel.
A soft, fleshy twig or branch is pushed into the tubing which must be
completely filled with water (it may assist to colour the water so that the
movement can be easily seen).
This must be a tight fit, otherwise water will run out of the capillary tube
and the instrument will not function correctly. The area around the neck
of the tubing where the plant has been inserted needs to be sealed, using
Vaseline® or petroleum jelly.
soft fleshy plant
funnel or well
water
base
very thin gradated glass tubing
Experimental set up to measure the rate of transpiration.
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As time passes, the thin thread of water moving along the capillary tube
towards the plant will be visible. A scale fitted behind the capillary tube
helps in the measurement of the rate of transpiration. It is possible to
accelerate the process by changing the environmental conditions of the
plant. For example, by using a fan it is possible to simulate windy
conditions. Other variables (factors) such as temperature can be
investigated for their effect on the rate of transpiration.
The effect of the environment on transpiration
Optional activity
If you have access to the equipment below then carry out the experiment.
If not, answer the questions at the end of the experiment in the results
section.
The aim of this experiment is to compare the rate at which a leaf loses
water, that is, transpires, under different conditions.
Materials required:
•
thin glass tubing or clear plastic tubing (look at the diagram to see
what it could look like)
•
Vaseline® or petroleum jelly
•
soft fleshy plant stem eg. Impatiens
•
marker pen or sheet of graph paper
•
scissors
•
retort and clamp or similar.
What you will do:
1
Fill the funnel with water.
2
Insert the soft, fleshy branch into the tubing
3
Smear Vaseline®, paraffin or fat around the join between the stalk
and the tubing. This join must be airtight when removed from the
water.
4
Check that there is no water leaking from your potometer at any
point. Water is most likely to escape in the area where the stalk is
placed into the tubing.
5
Set up a second potometer in the same way, but this time omit the
leafy branch. The second potometer is used as the control.
What is the function of a control?
_____________________________________________________
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25
6
7
26
Expose the potometer and the control to the following conditions and
measure the time taken for the water to move 2 cm along the glass
tube. Conditions to which the experiment and control and to be
exposed are:
•
cool and shady (in a room away from a window or draught)
•
cool and windy (use a fan for creating ‘wind’)
•
hot and shady (use a radiator)
•
hot and windy (use a radiator plus a fan).
Draw up a table of results on your own paper and enter your
measurements.
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Results:
1
Outline the conditions where you would expect the transpiration rate
to be greatest.
_____________________________________________________
_____________________________________________________
2
Which type of conditions causes plants to wilt?
_____________________________________________________
_____________________________________________________
3
Why do people need to top up the water in vases with cut flowers?
_____________________________________________________
_____________________________________________________
4
Not all the water lost from vases of flowers is taken up by the plants.
Explain.
_____________________________________________________
_____________________________________________________
Conclusion
Transpiration rate is affected by different external conditions.
For each of the conditions below describe the rate of transpiration (fast,
medium, slow).
•
cool and shady ________________________________________
•
cool and windy ________________________________________
•
hot and shady _________________________________________
•
hot and windy ________________________________________
Explain briefly how nutrients are obtained and transported around plants.
Your answer should include the names of the main structures and processes
involved.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Part 5: Obtaining and transporting materials in plants
27
Summary of processes of water transport
Several processes appear to be involved in the upward movement of
water in plants.
•
Adhesion: forces of attraction between different particles are called
forces of adhesion. The cellulose cell walls in plants soak up water
by this process, in much the some way as a blotter soaks up water.
•
Capillarity: capillarity is the rise of water in thin tubes by forces of
adhesion and cohesion. The water rises up thin tubes because of
attraction between the particles of the plant and water particles
(adhesion) and because of the attraction between the water particles
themselves (cohesion).
•
Root pressure: this refers to the upward movement of water caused
by the pressure from water moving into the root as a result of
osmosis.
•
Transpiration–cohesion: the transpiration cohesion theory proposes
that the loss of water molecules from the leaves, that is,
transpiration, results in the upward movement of more water
molecules since these molecules are attracted to each other by forces
of cohesion.
•
Guttation: is the loss of water in the form of a liquid from openings
on the leaves.
Phloem
Phloem tissue like xylem tissue, consists of tube–like cells. In phloem
these cells are called sieve cells and they form long columns.
When these cells mature they lose their nuclei. There are perforations in
the cell walls at the end so that the cytoplasm of the cells can connect
along the tubes. These are called sieve plates. Associated with the sieve
tube cells are companion cells (which retain their nuclei and cytoplasm)
and other supporting tissue.
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Phloem tube.
Translocation
Organic materials including sugars, amino acids and hormones are
transported by the living sieve tube cells of the phloem tissue.
This movement is called translocation.
The material that flows through phloem is called sap. The approximate
composition is 30% plant sugars and 70% water.
Materials move both upwards and downwards through the plant.
The movement is too fast to be caused only by diffusion. There are
several theories suggesting possible forces involved however the exact
mechanism remains unknown. Probably the most widely accepted
explanation for the mechanism of phloem translocation is the pressure
flow hypothesis of Ernst Much, which was proposed in 1930.
Leaves and roots can be sources of nutrients; nutrients are unloaded into
stem apexes, flowers, fruits and roots.
Movement of sap through the phloem results in pressure within the cells.
When aphids stick a feeding tube into the phloem, the sap is forced
through the aphid’s body.
Complete Exercise 5.4.
Part 5: Obtaining and transporting materials in plants
29
Gas exchange in plants
Like multicellular animals, multicellular plants usually have specialised
tissues for gas exchange. You will look mainly at angiosperms
(flowering plants) and algae (seaweeds and their relatives) in this section.
Respiration is the process by which energy is released for use by the cell.
All plant cells respire. Plant cells respire aerobically (most of the time).
This means that they use oxygen gas in the process and release carbon
dioxide gas as a waste product.
Some plant cells produce glucose by the process of photosynthesis.
During photosynthesis carbon dioxide is used and oxygen is released.
During daylight a plant respires as well as carrying out photosynthesis.
In sunlight, plants:
•
release more oxygen from photosynthesis than their cells use in
respiration
•
use all the carbon dioxide released by their cells in respiration in
photosynthesis
•
take in additional carbon dioxide from the atmosphere to satisfy the
needs of photosynthesis.
At night, plants:
•
do not photosynthesise
•
take in oxygen gas from the atmosphere for respiration.
•
release carbon dioxide gas as a product of respiration.
Overall plant metabolism results in the release of more oxygen than
carbon dioxide.
If a plant was placed in a sealed container for several days and nights the
composition of air in the container would change. Even though plants do
not photosynthesise at night, they still release more oxygen during
photosynthesis than they absorb by respiration.
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So, what is the evolutionary significance of plants releasing more oxygen
than carbon dioxide?
Prior to the evolution of photosynthesising organisms, the Earth’s
atmosphere was significantly different as it contained no free oxygen gas.
It probably contained gases like methane as well as higher levels of
carbon dioxide than today. As free oxygen, produced by photosynthesis,
became available it could react with the methane to produce carbon
dioxide. That carbon dioxide was then available for photosynthesis.
Since photosynthesis produces a net amount of oxygen, the atmospheric
oxygen levels were able to gradually increase.
Photosynthesis has changed the composition of the atmosphere of the
planet. This must surely be one of the most significant change made by
living things on the planet.
1
Imagine that you have placed a plant into an airtight container.
There is air in the container, but it cannot escape from the container.
The container is placed outside and left in sunlight for eight hours.
What would you expect to happen to the composition of the air in
the container?
_____________________________________________________
_____________________________________________________
2
Return to the plant in the sealed container. What would happen to
the composition of the air in the container at night?
_____________________________________________________
_____________________________________________________
Check your answers.
Gas exchange in leaves
Green plants require gas exchange for two purposes:
•
provision of carbon dioxide gas for photosynthesis
•
provision of oxygen gas for respiration.
When these gases are not available within the cells of a plant then the
gases need to be brought in from the surrounding atmosphere.
The leaf is one of the most important gas exchange sites. Cells in leaves
respire and are also some of the most important cells involved in the
process of photosynthesis.
Part 5: Obtaining and transporting materials in plants
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You will already know that the outer surface of a leaf has a waxy
covering (cuticle). The cuticle is a most unsatisfactory surface for gas
exchange. However, it does prevent excessive water loss.
Gases enter the leaf through tiny holes called stomates. Cells in the
interior of a leaf do not have a waxy covering.
In most plants, there are more stomates on the underside of the leaf than
on the upper surface. This reduces the amount of water that can be lost
through the stomatal openings. Look at the diagram of a cross–section of
a leaf following.
cuticle
epidermal cell
palisade mesophyll
many chloroplasts
in cytoplasm of cell
xylem and phloem cells
in leaf vein
spongy mesophyllwith fewer
chloroplasts in cytoplasm
air space
epidermis
cuticle
The stomate is shown on the lower surface of the leaf. The two guard
cells of a stomate surround a pore. When the guard cells are turgid
(full of fluid) they are curved like a banana. The curve of the two turgid
guard cells creates the opening that allows gases into and out of the leaf.
When the guard cells are flaccid they collapse, sealing the stomate.
When stomates are closed, water vapour and gases cannot pass into or
out of the leaf.
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Leaf epidermis. Note the stomates are all open.
Many consider stomate closure to be an adaptation to prevent desiccation or
drying out. Explain.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Where does gas exchange occur?
Gas exchange in plants occurs on the surface of each cell in the leaf.
Moisture on the outside of the cells allows gases to dissolve into fluid.
Once dissolved, gases can diffuse into the cell.
Wastes are removed by a similar method. The waste gases diffuse to the
moisture on the outside of each cell and from there to the gases in the
cavities within the leaf.
For cells that are not immediately adjacent to a leaf cavity, gases are
passed by diffusion from cell to cell to deliver gases to cells deeper in the
leaf. Stomates open into cavities and there are considerable air spaces
within the leaf.
Because the cells within the leaf are so tiny, the surface area to volume
ratio is high for each cell adjacent to a leaf cavity. Just as in lungs, gills
and insect tracheoles a high surface area to volume ratio is important for
gas exchange in plants.
Part 5: Obtaining and transporting materials in plants
33
Gas exchange is required for all plant cells, even the ones that are not
photosynthesising.
Gas exchange in stems
As stems are often thick, a different structure is required to allow gases to
pass through outer coverings such as bark. This structure is the lenticel.
Lenticels are a loose association of cells with many intercellular spaces
between them. These spaces allow oxygen to pass from the atmosphere
to the respiring cells within the stem. Lenticels also allow waste carbon
dioxide to leave the plant.
lenticels
Lenticels can be found on a woody stem.
lenticel
Lenticels allow gas exchange to occur.
Root hairs are sufficiently moist, small and thin to allow adequate gas
exchange between the gases in the soil air and the roots.
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1
With the aid of examples, explain why gas exchange surfaces have a
high surface area to volume ratio.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Outline the role of stomates in:
a) gas exchange
_________________________________________________
b) prevention of desiccation.
_________________________________________________
3
A number of plants growing in arid parts of Australia close their
stomates during the heat of the day. Their stomates are only opened
during the evening, early morning and late afternoon.
Make a hypothesis to explain these observations.
_____________________________________________________
_____________________________________________________
Check your answers.
Complete Exercise 5.5.
Part 5: Obtaining and transporting materials in plants
35
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Suggested answers
Autotrophic cells and heterotrophic cells
Autotrophic cells include plant cells and some bacterial cells.
Heterotrophic cells include animal cells, fungal cells and some
bacterial cells.
Photosynthesis
1
Photosynthesis takes place in the green parts of plants, especially the
leaves.
2
Photosynthesis occurs in the chloroplasts.
3
Light provides the energy that drives the process, photosynthesis.
Chlorophyll (in the chloroplasts), carbon dioxide and water are
necessary materials.
The phases of photosynthesis
1
Light is absorbed by chlorophyll and water molecules are split into
hydrogen and oxygen.
2
Carbon fixation and glucose is synthesised in the dark phase of
photosynthesis.
Function of leaves
1
Leaves are mostly green, thin and flat. They have a network of
veins.
2
The major role of leaves is to provide the plant with food.
Photosynthesis occurs mainly in the leaves.
3
a) Photosynthesis occurs in mesophyll (spongy and palisade).
The mesophyll is located between the layers of epidermis.
Mesophyll has access to carbon dioxide that enters through the
stomata.
Part 5: Obtaining and transporting materials in plants
37
b) Xylem provides water for photosynthesis.
c) Phloem transports the products of photosynthesis to the rest of
the plant.
4
The flat shape increases the SA:V ratio. This means there is a large
surface area available for light absorption for photosynthesis.
Because leaves are thin, the mesophyll is close to the epidermis.
So, carbon dioxide can easily absorb to the mesophyll for the
process.
Roots
1
Tap roots have a main central root from which the root hairs grow.
Fibrous roots do not have a central root, but simply a collection of
fine roots spreading out.
2
Roots have root hairs on their surface that increase the surface area.
3
The increased surface area enables the plants to spread out within the
soil to gather available moisture and dissolved nutrients. This aids in
the survival of the plants, particularly in times/areas when there may
be a water shortage.
Transport systems in plants
1
Roots absorb water and dissolved nutrients from the soil.
The stem supports the plant and the leaf is the site of photosynthesis.
2
Each vascular bundle contains three types of tissue: xylem, phloem
and cambium. (Note: cambium is not a conducting tissue.)
The effect of the environment on transpiration
Nutrients are absorbed through the root hairs on the root system of a
plant. They are transported via the xylem by adhesive and cohesive
forces, to the leaves. The movement of water upward through the
plant is called transpiration. From the leaves, the products of
photosynthesis are transported via the phloem to the rest of the plant.
The movement of sugars around the plant is called translocation.
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Exercises – Part 5
Exercises 5.1 to 5.5
Name: _________________________________
Exercise 5.1
Outline the main difference between autotrophs and heterotrophs.
_________________________________________________________
_________________________________________________________
Exercise 5.2: The phases of photosynthesis
a)
Write down the general word equation for photosynthesis.
b) Explain why this equation can be thought of as a summary of a chain
of biochemical reactions.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Exercise 5.3: Photosynthesis
a)
List the materials required for photosynthesis.
_____________________________________________________
_____________________________________________________
b) What is the role of photosynthesis in the ecosystem?
_____________________________________________________
_____________________________________________________
Part 5: Obtaining and transporting materials in plants
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Exercise 5.4: Absorption of water and minerals in
plants
Plants obtain water and minerals through their root systems. Roots are
generally long and thin. Root hairs are found along the tips of growing
roots. How does the structure of the root system affect a plant’s ability to
obtain water and minerals?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Exercise 5.5: Transport systems in plants
a)
Multicellular plants and animals have transport systems. Why is this
necessary?
______________________________________________________
______________________________________________________
b) Describe the movement of water through a plant starting with root
hairs and finishing with the water leaving the plant.
______________________________________________________
______________________________________________________
______________________________________________________
c)
You have learned that materials move upwards in xylem.
Substances move upward and downward in the phloem.
How do we know this? Briefly describe the evidence for movement
of substances in the xylem or the phloem.
You may need to consult a biology textbook and another source such
as the Internet.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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