Gas Exchange in a Leaf
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of plant cell structures and diffusion to understand how gases move in and out for photosynthesis and respiration.
Which part of the plant cell maintains the shape of the cell?
Cell wall (vacuole maintains the pressure inside the cell)
Which part of the plant cell carries out photosynthesis?
Chloroplast
How does distance affect the rate of diffusion?
The larger the distance the slower the rate of diffusion
Topic Explainer Video
Check out this @JoeDoesBiology video that explains gas exchange in a leaf, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Gas Exchange in a Leaf
What Gases Are Exchanged?
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Carbon dioxide (CO₂) enters the leaf for photosynthesis.
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Oxygen (O₂), a by-product of photosynthesis, diffuses out (some may be used by cells for respiration).
During respiration, O₂ is used and CO₂ is released.
Where Gas Exchange Happens
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Gases diffuse in and out of leaves through stomata – tiny pores mainly on the underside of the leaf.
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Guard cells control the opening of stomata to regulate gas exchange and prevent water loss. They open to allow gas exchange to take place but can close to reduce water loss via evaporation.
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The spongy mesophyll layer inside the leaf has air spaces that allow gases to quickly diffuse between the stomata and the palisade mesophyll cells. For example, carbon dioxide can quickly diffuse through the air spaces to the palisade mesophyll cells for photosynthesis. (Palisade mesophyll cells are the main site of photosynthesis and are locted towards the upper surface of the leaf).
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Adaptations of Leaves for Gas Exchange
|
Adaptation |
Explanation |
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Thin leaf |
Short diffusion distance for gases. |
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Stomata |
Allow gases to enter/leave the leaf; opening is controlled by guard cells. |
|
Air spaces in spongy mesophyll |
Let gases diffuse quickly between cells. |
|
Large surface area |
Maximises area for diffusion and light absorption for photosynthesis. |
Key Terms
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Stomata – pores that control gas exchange.
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Guard cells – open/close stomata based on water and light conditions.
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Spongy mesophyll – leaf layer with air spaces for diffusion of gases.
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Exam Tip
- Link the adaptations of the leaf to the factors that affect the rate of diffusion.
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Practice Question
Describe how the structure of a leaf is adapted for gas exchange. (4 marks)
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Leaves have many stomata to allow for gas exchange.
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The leaf is thin, providing a short diffusion distance.
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Air spaces in the spongy mesophyll allow gases to diffuse quickly.
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Guard cells control the opening of stomata to regulate gas exchange and prevent water loss.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!