Gas Exchange in Plants: Stomata, Mesophyll and Air Spaces
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Plants exchange gases through stomata in the leaf epidermis. During the day, leaves take in carbon dioxide for photosynthesis and release oxygen. At all times, cells respire and use oxygen while releasing carbon dioxide. The spongy mesophyll air spaces provide a large internal surface area for gas exchange.
Plant gas exchange is tested alongside leaf structure from Topic 6. The key concept is that both photosynthesis and respiration occur in plants: they are not alternatives. In daylight, net gas exchange reflects the faster rate of photosynthesis.
Gas exchange through stomata
Gases enter and leave the leaf through stomata (tiny pores mostly on the lower epidermis). Guard cells control stomatal opening: they open during the day to allow carbon dioxide to enter for photosynthesis, and close at night to reduce water loss by transpiration.
When stomata are open, carbon dioxide diffuses in from the surrounding air because its concentration is higher outside than inside the photosynthesising leaf. Oxygen produced by photosynthesis diffuses out through the same stomata.
Photosynthesis and respiration: the balance
During the day, the rate of photosynthesis exceeds the rate of respiration. The leaf produces more oxygen than it uses and consumes more carbon dioxide than it produces. Net gas exchange: CO₂ in, O₂ out.
At night, only respiration occurs (no light for photosynthesis). The plant uses oxygen and releases carbon dioxide. Net gas exchange: O₂ in, CO₂ out.
At low light levels, there is a compensation point where the rate of photosynthesis exactly equals the rate of respiration. At this point, no net gas exchange with the atmosphere occurs.
Leaf structure for gas exchange
The spongy mesophyll layer has loosely packed cells with large air spaces between them. These air spaces connect to the stomata and provide a large internal surface area for gas exchange. Gases diffuse through the air spaces to reach all mesophyll cells.
The moist cell surfaces of mesophyll cells allow carbon dioxide to dissolve before entering the cells for photosynthesis. The thin cell walls and the short distances between air spaces and cells ensure efficient diffusion throughout the leaf interior.
Try a focused question (4 marks)
Explain why a plant takes in carbon dioxide during the day but releases it at night.
Show the answer and marking guidance
During the day, photosynthesis uses carbon dioxide faster than respiration produces it.
The net result is that the plant absorbs carbon dioxide from the atmosphere.
At night, photosynthesis stops because there is no light.
Only respiration occurs, which produces carbon dioxide, so the plant releases carbon dioxide.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying plants only photosynthesise and do not respire: plants respire continuously.
Claiming plants respire only at night: respiration happens all the time, day and night.
Confusing gas exchange with transpiration: gas exchange involves O₂ and CO₂, transpiration involves water vapour.
Not mentioning the compensation point when comparing rates of the two processes.