(5A)
More
on leaf adaptations to aid
photosynthesis and gas exchange
See also detailed notes on
Photosynthesis,
importance
explained, limiting factors affecting rate
Gas exchange in plants
In plants carbon dioxide
enters leaves by diffusion and then diffuses into cells where photosynthesis
takes place.
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 ==
light +
chlorophyll
==> glucose + oxygen
6H2O(l)
+ 6CO2(g) == sunlight/chlorophyll ==> C6H12O6(aq)
+ 6O2(g)
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
starch.
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
photosynthesis.
Carbon dioxide is absorbed from
air and water from the roots for
photosynthesis.
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
gas exchange.
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
later section).
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
transpiration).
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
transpiration.
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.
Estimating
leaf surface area
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
the leaves
Transpiration is caused by the
evaporation and diffusion of water from a plant's surface - mostly from
the leaves.
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 veins.
The loss of water from leaves by
evaporation, creates
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
loss.
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
transpiration
Water is needed for
photosynthesis.
Water carries dissolved
substances around the plant.
Evaporation from leaves cools
the plant.
Cells filled with water give
the plant physical support.
Evaporation is more rapid in hot, dry and windy
conditions.
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
sunlight.
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
plural).
Two guard cells surround each
stoma.
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
conditions.
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
diffusing out).
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
(i) Adaptations of guard
cells:
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.
See also
plant adaptations - examples in
extreme environments
(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.
Guard
cells respond to light, if light levels increase, potassium
ions (K+)
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
potential).
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
water potential
(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.
An experiment
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
grams.
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
calculations.
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
transpiration.
the % loss = 100 x mass loss of water /
initial mass of plant
% loss = 100 x 50 / 380 =
13% (to 2
s.f.)
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.
