Transport in plants:
on evolutionary leaf adaptations to aid
photosynthesis, gas exchanges and environmental factors affecting rate of water loss - rate of
Doc Brown's Biology exam study revision notes
Sub-index of biology notes on transport
systems in plants
on leaf adaptations to aid
photosynthesis and gas exchange
See also detailed notes on
explained, limiting factors affecting rate
Gas exchange in plants
In plants carbon dioxide
enters leaves by diffusion and then diffuses into cells where photosynthesis
Oxygen will diffuse out from the leaf surface.
Reminder: Photosynthesis takes place inside the
subcellular structures called chloroplasts in the palisade cells.
carbon dioxide + water ==
==> glucose + oxygen
+ 6CO2(g) == sunlight/chlorophyll ==> C6H12O6(aq)
In daylight more carbon dioxide will be taken
in for photosynthesis in than
given out from respiration and more oxygen given out than taken in - the effect of more
photosynthesis than respiration - the surplus glucose is converted into
At night-time the opposite will happen, more carbon
dioxide from respiration will be given out than taken in, and more oxygen taken in than given
out - respiration increases and food stored as starch becomes the source of
energy in the dark.
Beneath the apparently flat
surface of a leaf is quite a porous layer of air spaces between the outer
layers of cells - particularly on the underside of leaves - quite often the
lower surface of a leaves feel rougher and 'roughness' means a more
disrupted surface of a larger gas exchange surface area.
Photosynthesis and diffusion
(with reference to the above diagram of leaf structure)
Plants have stomata
(tiny pores or holes), mainly on the underside of leaves in the spongy
mesophyll, to obtain
carbon dioxide gas from the atmosphere for photosynthesis and to give out the
'waste' oxygen gas produced
as a by-product in
Carbon dioxide is absorbed from
air and water from the roots for
Carbon dioxide diffuses into the
leaves through the stomata and is depleted through photosynthesis.
Therefore as photosynthesis
proceeds, the internal carbon dioxide concentration in the leaf is
much lower than in the surrounding air, so carbon dioxide will
diffuse into the leaf down this concentration gradient.
The rate of diffusion of the
carbon dioxide (and any other gas) is increased by:
Increasing the surface
area of the leaf - always the broadest part of any plant.
The smaller the distance
the molecules have to travel as they diffuse - thin leaves
with an even thinner mesophyll layer.
An increase in the carbon
dioxide concentration gradient - always be there while
photosynthesis is taking place.
As the CO2 is
absorbed, wind blows by fresh supplies of carbon dioxide to
maintain a high inward concentration gradient.
Oxygen from photosynthesis diffuses out through the
stomata, and most water is lost in the same way (transpiration).
The air spaces in the leaf
structure create a larger surface area to allow this diffusion to
take place efficiently.
Leaves are also thin, so distance
and diffusion times are short, further increasing the efficiency of
Carbon dioxide can diffuse in
through the stomata and oxygen can diffuse out and stomata also allow water
vapour to escape as part of the process of transpiration (details in
Since carbon dioxide is being
used up in photosynthesis, the concentration gradient enables more carbon
dioxide to diffuse in through the stomata.
The size of the stomata are
controlled by guard cells (more on this in the next two sections on
The flattened shape of leaves
increases the surface area over which efficient gas exchange can
take place - greater chance of carbon dioxide to diffuse into the
leaves (see photographs below).
Inside the leaf the cell walls
form another exchange surface and the air spaces between these cells
further increase the surface are for gas exchange - carbon dioxide
in, oxygen and water vapour out.
Water vapour evaporates from the
surfaces of the leaf cells.
The higher concentration of water vapour
in the leaves means there is a natural diffusion gradient to the
outside air so the it can exit the leaves in the process of
A high humidity reduces the
concentration gradient of water vapour between the interior and
exterior of the leaf, slowing down the diffusion of water vapour
- slowing down transpiration.
Conversely, very dry air (low
humidity) will increase the concentration gradient and increase
the rate of water loss from leaves.
leaf surface area
If a surface has a regular shape like a
square or rectangle it is easy to measure and calculate the surface area
(length x breadth).
However, in the case of a leaf, you have
an irregular shaped surface, even if there is a line of symmetry down the
If the leaf is laid out on a marked
out grid, like the ones illustrated above, you can count the squares to
arrive at an estimate of the leaf's surface area. Make sure the leaf is
fully flattened out.
If a square is mainly filled with
leaf (over ½ filled) you count it towards the total.
If a square is not well filled (less
than ½ filled), it should not be counted in the total area.
Using this method I estimate that the
has an area of 52 cm2.
(±1, do you agree?)
In this case I found it easier to
count the blank squares.
On the right, I estimate the
smaller leaf has
an area of 32 cm2.
(± do you agree?)
In this case I found it easier to
count the green squares.
side of a leaf is smoother and greener - richer in chloroplasts to
capture the sunlight
side of a leaf is rougher - more 'porous' for efficient gas exchange
and the veins more prominent
A summary of adaptations
for the effective functions of leaves - some very important for
transparent waxy cuticle
light through and protect leaf from excess water loss
shaped cells arranged in end-on, near the upper surface,
maximises chloroplasts at the top of the cells
maximum amount of light to be absorbed
shaped cells creating air spaces
the surface are for gas exchange - CO2 in, O2
out - increases efficiency
stomata (pores) for gas exchange surrounded by pairs of guard
cells open and close each stoma (pore) to control the diffusion
of the gas exchange
xylem and phloem tubes in the veins
substances around the plant including to, and from, the leaves
Plant adaptations and controlling water
More on the environmental
factors (ambient conditions) affecting the rate of water loss -
the rate of
1. Air flow
The more air that flows over
the leaves, e.g. stronger wind, the greater the rate of
transpiration. Conversely, the lower the wind speed, the slower
the rate of transpiration.
The more quickly the water
vapour is removed and swept away by a greater air flow, the steeper the
water diffusion gradient out of the leaf is - the concentration of water vapour is
much greater in the stomata than in the air in and surrounding the leaf
- which is much lower because the water vapour is being
constantly carried away in the air current across the surface of
If the air is quite still,
the water vapour accumulates around the leaf, considerably
reducing the diffusion gradient because the water vapour
concentrations become similar. The concentration of water vapour
is high in the stomata and BUT only a bit less in the
A particle model of diffusion in gases
Reminder that the net flow of a substance in
diffusion is from a higher concentration to a lower
concentration e.g. the movement of the 'green' water
particles in the diagram sequence below.
The less humid (more drier)
the air surrounding the leaves, the greater the rate of
The lower the water
concentration on the outside of leaves, the steeper the water
diffusion gradient from the leaves to the external air, the faster the rate of water
loss by transpiration.
When the air is very humid
with a high water concentration, there is a smaller difference
in the higher (in leaf) and lower (outside leaf) concentrations
- so a smaller water diffusion gradient resulting in smaller
rate of water loss by transpiration.
3. Light intensity
The greater the intensity of
light (e.g. the brighter the sunlight) the greater the rate of
transpiration because the rate of photosynthesis increases. This
stimulates the guard cells to open up the
stomata more to let more carbon dioxide in and water vapour
and oxygen out.
As it gets darker,
photosynthesis rate decreases and the stomata begin to close up
- they don't need to be open to allow carbon dioxide to diffuse
in. When the stomata are closed, little water can escape - I
presume enough oxygen can get in for the plant's respiration at
The warmer the surrounding
air the greater the rate of transpiration.
This is because the water
molecules have more kinetic energy to escape the intermolecular
forces at the surface of the liquid water in the stomata. So,
the water molecules can evaporate more quickly and diffuse out
of the stomata.
Also, the rate of
photosynthesis increases with increase in temperature - just
like any other chemical reaction - therefore more water is
required to be drawn up through the roots.
The daily cycle of the rate of
The graphs above show how the
rate of transpiration is likely to vary through the day for two
The rates of both photosynthesis
and transpiration increase and decrease with change in light
intensity over a 24 hour daily cycle.
On average, at midday (noon) the
sun is at its maximum height, the sunlight intensity is at a
maximum, so photosynthesis can be at a maximum, but only if the
transpiration rate maximises too, to supply the water for
The peak heights will vary
depending on the effect of the factors that control
transpiration (discussed above) and
The light intensity has a greater
effect on the rate of transpiration of plant B compared to plant A, even though it starts from a lower
base at midnight.
In the night time, when
photosynthesis is at a minimum, the water uptake through the roots
is greater than the rate of transpiration.
Through the day, and peaking at
midday, the transpiration rate exceeds the rate of water uptake.
In daylight the rate of
transpiration cannot be the same as the rate of water uptake because
some of the water is used in photosynthesis and the rest of the
plant's metabolic processes and the rate of evaporation increases
That rate of transpiration
exceeds the rate of water uptake as the rate of photosynthesis
As the light intensity
increases, the stomata open to allow in more carbon dioxide for
BUT, this also allows more
water vapour to evaporate.
Plant adaptations and controlling water
(5B) More on the factors affecting the rate of transpiration and function of the stomata and guard cells
Transpiration is defined as the
loss of water vapour from plant leaves by evaporation of water at the
surfaces of the mesophyll cells, followed by diffusion of water vapour
through the stomata.
Plants are constantly losing water,
but cannot be healthy without a balancing water intake. The water is
needed for transportation and photosynthesis - in fact most of the water
is used in the transport of materials through the plant, only a few% is
used in photosynthesis.
Transpiration is result of the way
leaves have become adapted to facilitate photosynthesis - the
stomata aiding the transport system by allowing gas exchange - carbon
dioxide, oxygen and water vapour.
The process of water movement from the roots
through the xylem and out of the leaves is called transpiration and
is essential for a plant's transport system.
Water is absorbed through the root
hairs, passes up the root, continues up the stem and spreads into all
Transpiration is caused by the
evaporation and diffusion of water from a plant's surface - mostly from
Most of the loss of water vapour takes place
through the stomata on the surface of leaves.
Water on the spongy surface of the
mesophyll evaporates and diffuses out of the leaves.
Plants continually lose
water because the concentration of water in the plant fluids is greater than
the concentration of water in the air outside - the concentration gradient
is in the 'outward' direction.
The cell surface of a leaf is large area
punctuated by the interconnecting air spaces and stomata.
Since plants need water all the
time, water is continually transported through the xylem in
The loss of water from leaves by
a small shortage of water in the leaves and so a column of water
molecules is drawn up by cohesion in the xylem, from the rest of the plant through the xylem tubes to replace the water
Therefore, this causes in turn,
more water to be absorbed and
drawn up from the roots.
So, there is a constant flow of water
up through the plant - the transpiration stream - which carries the
mineral ions dissolved from the soil up into the whole of the plant.
So, important functions of
Water is needed for
Water carries dissolved
substances around the plant.
Evaporation from leaves cools
Cells filled with water give
the plant physical support.
Evaporation is more rapid in hot, dry and windy
If plants lose water faster than it is replaced by the roots,
the stomata can close to prevent wilting.
If too much water is lost through
the stomata, plants will wilt ('flop') and die.
As we have said, plants are
constantly losing water by evaporation, but cannot be healthy
without a balancing water intake. If plants lose water too fast they
will wilt - the leaves droop and hang down. This reduces the surface
area available for evaporation through the stomata. The stomata
close and photosynthesis stops to prevent water loss. There is a
danger the plant will overheat. Plants will stay wilted until they
can absorb water and the temperature falls and no longer in
The size of stomata is
controlled by pairs guard cells, which surround them.
Therefore stomata and guard cells control the
rate of evaporation from leaves.
(Note: stoma is singular, stomata is
Two guard cells surround each
The size of the opening of the
stomata (diagram on left) must be controlled by the guard cells or a plant might lose too much
water and wilt.
It is the guard cells that
regulate the rate of transpiration.
It is the guard cells that control
the rates of water loss and gain AND the rate of gas exchanges.
The 'kidney shaped' guard cells can change shape to control the size
of the pore.
Water diffuses out of the spongy
mesophyll producing a film of water on the surface of the cells. Water
evaporates into air spaces between the cells and the water vapour
diffuses down the concentration gradient to the stomata and escapes from
the leaf into the surrounding air.
Water will diffuse out and
evaporate away much faster in less humid-drier, hotter or windier weather
Stomata close automatically if
the water supply begins to 'dry up' to reduce water loss.
The guard cells will respond to
the ambient conditions ie close up the stomata if the rate of water loss is
to great for water to be replenished from the roots.
When the plant has lots of
water, the guard cells become swollen with water (turgid) and the
stomata are open to increase the rate of water loss, but also
increase the intake of carbon dioxide for photosynthesis (and oxygen
When the plant is short of water,
guard cells lose water (flaccid, 'limp') and
the stomata are closed to decrease the rate of water loss.
If the plant is very short of
water the cytoplasm inside the cells shrinks and the cell
membrane comes away from the rigid cell wall. This process is called
plasmolysis and the cell is said to plasmolysed.
Three more points
Adaptations of guard
Apart from their shape, guard
cells have other adaptations which help them in their function
to aid in controlling gas exchange and water loss.
They have thin outer walls
and thickened inner walls which allow the opening and closing
mechanism to work efficiently.
The guard cells also
respond to light levels - they close at night to save water -
conserved for photosynthesis and open up again when daylight returns
to allow the exchange of gases.
(ii) You usually find more
stomata on the underside of leaves compared to the top.
The lower leaf surface is
more shaded and cooler, this reduces water loss, compared to the
water loss that would happen on the upper surface.
Plants growing hot climates
need to conserve water and so they have fewer and smaller
stomata on the underside of the leaves and no stomata on the
upper epidermal surface.
plant adaptations - examples in
(iii) It is changes in the concentration of ions inside the guard
cells that facilitate the opening and closing of stomata.
When guard cells lose
water, it causes the cells to become flaccid and the stomata
openings to close - reducing water loss. This occurs when plants
has lost an excessive amount of water OR if light levels drop
and the use of carbon dioxide in photosynthesis decreases.
cells respond to light, if light levels increase, potassium
are pumped into them by active transport. (diagram below)
This increases the
concentration of dissolved particles in the guard cells fluid
and decreases the
concentration of water molecules (decreases the cell's water
Therefore water diffuses into the
guard cells by spontaneous osmosis making the guard cells turgid
(diagram above) and the
stoma opens allowing carbon dioxide to enter for photosynthesis.
The reverse happens light
levels or water levels are low.
When potassium ions exit
the guard cells, the concentration of water molecules
increases (increasing the cell's water potential).
Water will then move out
of the guard cells by osmosis, they become flaccid and the
stoma closes reducing the loss of water, and not as much
carbon dioxide is available for photosynthesis - not needed
at all at night.
Extra note on plant cells and
(i) When you water a plant it
increases the water potential of the soil around it.
Therefore the plant cells
will draw water in by osmosis until they become turgid -
fatter and swollen.
The cell fluids (contents of
the cell) will push against the cell wall, known as turgor
pressure, and this helps support the plant tissues
(therefore the plant as a whole).
(ii) If the soil is very dry,
lacking in water, the plant starts to wilt and the water potential
of the plant is greater than the surrounding soil.
The result is the plant cells
become flaccid and begin to lose water.
The plant doesn't droop
(flop) completely and retains much of its shape because the
strong cellulose cell wall is relatively inelastic and helps the
plant retain its shape.
to measure the rate of transpiration
Place some damp soil in a plastic bag.
Plant a small plant into the soil and tie the bag
tight around the stem, ensuring the leaves stick out of the bag -
otherwise transpiration can't happen!
Weigh the 'bagged' plant and record the mass in
You then leave the plant in a well-lit place for 24
hours - you set up it in the lab and leave a lit table lamp in front
of the plant at night - but the light intensity will vary from
daylight time to nighttime..
Ideally the plant is placed in a dark room, at
the same temperature, and illuminated artificially for the 24
hours, to standardise experimental conditions.
After 24 hours, re-weigh the bag and plant and
record the mass in grams.
The mass should have decreased and the mass loss
equals the amount of water lost by transpiration.
You can then do some
Suppose the initial bag and plant weighed 400 g.
After 24 hours it weighed 350 g.
mass loss = 400 - 350 =
50 g of water.
rate of transpiration = 50 / 24 = 2.1
g/hour (water loss rate to 2 s.f.).
If you weighed the plant before placing it in
the plastic bag, you can then calculate the percentage change in
the mass of the plant.
e.g. suppose the initial plant weighed 380 g
before planting, and using the 50 g loss from above.
the plant weighed 380 - 50 = 330 g after
the % loss = 100 x mass loss of water /
initial mass of plant
% loss = 100 x 50 / 380 =
13% (to 2
Sources of error
The plant will lose a tiny amount of mass as
oxygen is produced by photosynthesis, but their is small gain in
weight as carbon dioxide is absorbed for photosynthesis.
Variations on the experiment
You can vary the light intensity level, from
intense to dark.
Keywords, phrases and learning objectives for this part on transport systems in
Be able to understand, describe and explain the
environmental factors e.g. ambient conditions, that affect rate of
water loss by transpiration.
Be able to describe the evolutionary leaf
adaptations that aid photosynthesis and gas exchanges.
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