2.64 Describe the role of stomata in gas exchange

1. 2.64 Describe the role of stomata in gas exchange 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

2. 2g – Plant gas exchange • 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 netexchange 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 gasexchange from a leaf, using hydrogen-carbonate indicator

3. Diffusion • 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.

4. Diffusion • 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.

5. Light intensity • 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.

6. 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)

7. 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

8. Leaf adaptations

9. Gas exchange • Palisade cells • Lots of chloroplasts containing chlorophyll to trap light for photosynthesis. • Guard 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

10. Stomata • 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. • 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.

11. stomata

12. Gas exchange experiments • Hydrogen Carbonate Indicator • Turns yellow in CO2 • Simple 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.