Key words needed in you paragraph:
 Carbon dioxide
 Stomata
 Closed in hot sunny conditions
 Water loss
 Photosynthesis (slow down)
 Guard cells
 Diffusion
 Open = turgid = bulges
 Closed = flaccid = collapse
 Include a diagram
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understand gas exchange (of carbon dioxide and
oxygen) in relation to respiration and
photosynthesis
understand that respiration continues during the
day and night, but that the net exchange of
carbon dioxide and oxygen depends on the
intensity of light
explain how the structure of the leaf is adapted
for gas exchange
describe the role of stomata in gas exchange
describe simple controlled experiments to
investigate the effect of light on net gas
exchange from a leaf, using hydrogen-carbonate
indicator

Diffusion = the random movement of particles from a high
concentration to a low concentration, down the
concentration gradient. It is a passive process (no energy
needed).

CO2 diffuses into leaves during the day, the concentration
gradient is maintained as the CO2 is being used up in
photosynthesis. This allows CO2 to always diffuse down its
concentration gradient.

Plants must respire all the time to stay alive. During the
day they can get some of the O2 from photosynthesis to
respire. However at night, photosynthesis has stopped. To
keep respiring O2 must enter the leaf. O2 moves down its
concentration gradient from a high concentration outside
the leaf to a low concentration inside the leaf. The
gradient is maintained as the O2 is being used up in
respiration.
 Gases
diffuse through the stomata, through
the air spaces and into the spongy and
palisade cells for photosynthesis.
 During
the day, the rate of photosynthesis is
faster than respiration, so the net movement
of carbon dioxide will be greater into the
leaf and oxygen out.
 At
night carbon dioxide is always released, as
respiration is the only process being carried
out.
 During the day photosynthesis usually
happens at a faster rate than respiration and
so more oxygen is made than is used up.
 However, on a really dull/ cloudy day – there
might be very little oxygen produced as
photosynthesis slows down and the small
amount being made is used in respiration.
a) In which month would you expect the rate of photosynthesis in the oak
trees to be greatest?
(1)
b) There are plants living on the ground in the wood. In which month would
you expect their rate of growth to be fastest? Explain your answer.
(3)
c) Name two factors, other than light intensity, that would affect the rate of
photosynthesis in the oak trees.
(2)
a) In which month would you expect the rate of photosynthesis in
the oak trees to be greatest?
(1)
June/ July as there is a lot of light available.
b) There are plants living on the ground in the wood. In which
month would you expect their rate of growth to be fastest?
Explain your answer.
(3)
April  there is more light reaching the ground in the wood.
 The oak trees lost their leaves during winter and they have not
grown back yet so the light can get through to the plants on the
ground
c) Name two factors, other than light intensity, that would affect
the rate of photosynthesis in the oak trees.
(2)

Carbon dioxide levels and temperature
 Palisade
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Lots of chloroplasts containing chlorophyll to
trap light for photosynthesis.
 Guard
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cells
cells
Open and close stomata for gas exchange.
They will close the stomata when it is too dry
and hot to stop water loss by evaporation

Sunlight and plenty of water
Stomata open.
Guard cells are turgid and this pulls the stomata
open.
 Carbon dioxide can diffuse in for photosynthesis.
 When the guard cells are turgid one side bulges out
more than the other which opens the stomata.
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Hot dry day and lack of water
Stomata close.
 Guard cells are flaccid and this closes the stomata.
 Carbon dioxide can no longer diffuse in and
photosynthesis stops.
 When guard cells are flaccid they “collapse” and this
closes the stomata.
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 Hydrogen
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Turns yellow in CO2
 Simple
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Carbonate Indicator
experiments
Plants use CO2 during photosynthesis.
At the start of an experiment with plenty of CO2,
hydrogen carbonate indicator would be yellow.
At the end of an experiment, as the plant uses up
CO2, hydrogen carbonate indicator would be
purple.