Cells and organisation · GCSE Biology
Diffusion in plants
Explain gas exchange in leaves for GCSE Biology: diffusion of carbon dioxide and oxygen, stomata, guard cells and links to photosynthesis and respiration.
CO₂ diffuses into leaf through stomata for photosynthesis (day). O₂ diffuses out in light; at night O₂ diffuses in for respiration. Guard cells open stomata. Mesophyll air spaces shorten the path.
The important bits
What you need to know
- 1
Gas exchange in plants occurs mainly in leaves by diffusion through stomata — pores in the epidermis surrounded by guard cells.
- 2
During the day, photosynthesis in mesophyll cells uses carbon dioxide and produces oxygen. CO₂ diffuses from the air (higher concentration outside when stomata are open) into the leaf down its concentration gradient.
- 3
Oxygen produced by photosynthesis diffuses out through stomata when its concentration inside the leaf is higher than in the air.
- 4
At night, photosynthesis stops but respiration continues in all living cells. Oxygen diffuses into the leaf and carbon dioxide diffuses out.
- 5
Guard cells control stomatal opening. When turgid, they bow apart and open the pore; when flaccid, the stoma closes, reducing gas exchange and transpiration.
- 6
Spongy mesophyll has air spaces that increase the surface area exposed to gases and shorten the diffusion distance to chloroplasts in palisade cells.
- 7
Diffusion does not require energy from respiration. Rate depends on concentration gradient, temperature, surface area and distance — the same factors as in animals.
- 8
Stomata are usually more numerous on the lower epidermis to reduce water loss while still allowing gas exchange; waxy cuticle on the upper surface limits evaporation.
Quotations worth analysing
Short evidence. Real method.
“Carbon dioxide diffuses into leaves through stomata for photosynthesis.”
Name CO₂, stomata and photosynthesis. Diffusion is down a concentration gradient, not active transport.
“Guard cells control the opening and closing of stomata.”
Link turgor in guard cells to pore size. Opening allows both CO₂ in and water vapour out.
“Air spaces in spongy mesophyll increase the surface area for gas exchange.”
Structure–function again: mesophyll layout supports diffusion to chloroplasts.
Go deeper
Day versus night — direction of diffusion flips
In daylight, photosynthesis rate often exceeds respiration in green cells: net CO₂ uptake, net O₂ release. Stomata open in light (usually) to supply CO₂. At night, only respiration runs: O₂ enters, CO₂ leaves. Students write “plants only take in CO₂” without the time condition. A graph of gas exchange over 24 hours crosses zero at compensation point — extension on some specs. For GCSE, state “during the day” or “when photosynthesising” whenever you describe CO₂ influx.
Go deeper
Leaf anatomy as an exchange surface
Upper waxy cuticle reduces water loss. Palisade mesophyll — dense chloroplasts, main photosynthesis. Spongy mesophyll — air spaces for gas diffusion. Lower epidermis with stomata and guard cells. Xylem brings water; phloem removes sucrose. The leaf is a system of tissues optimised for light capture and gas diffusion. Compare to alveoli: large internal surface, thin cells, maintained gradients (ventilation in animals; stomatal opening in plants).
Go deeper
Stomata trade-offs
Open stomata: good CO₂ supply for photosynthesis, but faster transpiration and wilting risk. Closed stomata: conserve water, limit CO₂ and slow photosynthesis. Guard cell ion uptake and water follow osmosis — link to transport topic. Xerophyte adaptations reduce water loss while keeping enough gas exchange for survival. In explain questions, never say plants “breathe”; use diffuse, concentration gradient, stomata.
See the idea in action
Explain how a leaf obtains carbon dioxide for photosynthesis during the day (3 marks). Carbon dioxide in the air is at a higher concentration outside the leaf when stomata are open. CO₂ diffuses down its concentration gradient through the stomata into air spaces in the spongy mesophyll. It then diffuses into palisade mesophyll cells to reach chloroplasts for photosynthesis. No energy from respiration is required for this diffusion.
Exam technique
Turn knowledge into marks
Specify day vs night and which gas moves which way. Always mention stomata and concentration gradient. Do not confuse gas exchange with transpiration stream in xylem.
Common mistakes
Do not give these marks away
- 01
Saying plants take in CO₂ at night for photosynthesis, or only respire at night.
- 02
Calling CO₂ movement osmosis or active transport.
- 03
Ignoring stomata and writing that gases cross the waxy cuticle (it is largely impermeable).
During the day, how does carbon dioxide enter a leaf for photosynthesis?
AActive transport through root hairs
BDiffusion through open stomata down a concentration gradient
COsmosis through the waxy cuticle
DTranslocation in phloem vessels
Show the answer
Diffusion through open stomata down a concentration gradient. CO₂ diffuses from the air into the leaf when stomata are open. Root hairs take up water; phloem carries sucrose; the cuticle limits water loss.
Quick questions
If this is the bit you searched
How do gases enter and leave a leaf?
Mainly by diffusion through stomata. CO₂ enters for photosynthesis in the day; O₂ enters at night for respiration.
What is the role of guard cells?
They control stomatal opening and closing, balancing gas exchange for photosynthesis against water loss.
Why does spongy mesophyll have air spaces?
They increase surface area and allow gases to diffuse quickly to photosynthesising cells.
Do plants respire?
Yes, all living plant cells respire all the time. At night, respiration is the main gas exchange process in leaves.