Gas exchange in plants: diffusion and leaf structure
Diffusion does all the work
Plants have no lungs, no heart and no blood. Every gas that enters or leaves a leaf moves by diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process: no energy from respiration is used.
Diffusion is fast enough only over short distances, so the leaf is built to keep the diffusion distance tiny and the gradient steep.
Which gas goes which way
This is where marks are lost most often, so learn it as a table and never mix the two processes up.
| Gas taken in | Gas given out | Needs light? | |
|---|---|---|---|
| Photosynthesis | carbon dioxide | oxygen | yes |
| Respiration | oxygen | carbon dioxide | no — happens all the time |
In bright light photosynthesis is faster than respiration, so:
- carbon dioxide concentration inside the leaf falls below that of the air → CO₂ diffuses in through the stomata
- oxygen concentration inside the leaf rises above that of the air → O₂ diffuses out
Common mistake: examiners repeatedly report candidates writing that "oxygen enters the leaf for photosynthesis" or that "more photosynthesis means less oxygen". Photosynthesis uses carbon dioxide and produces oxygen. Respiration is the opposite. Also, respiration does not need light — only photosynthesis absorbs light energy.
How the leaf is adapted for gas exchange
| Feature | How it helps gas exchange |
|---|---|
| Broad and flat | large surface area for diffusion |
| Thin | short diffusion distance to the mesophyll cells |
| Stomata (mainly on the lower epidermis) | pores that let CO₂ diffuse in and O₂ and water vapour diffuse out |
| Air spaces in the spongy mesophyll | gases move freely inside the leaf and reach every cell; also increase the internal surface area |
| Moist cell surfaces | gases dissolve before diffusing into the cell |
| Photosynthesising palisade cells close to the surface | they use up CO₂ and make O₂, keeping the concentration gradients steep |
A steep gradient is the key idea: because the mesophyll cells constantly use CO₂ (in the light) and constantly produce CO₂ (all the time), the concentration inside the air spaces is always different from the air outside, so diffusion keeps going.
Examiner tip. Never say a leaf "needs oxygen for photosynthesis" — a reported examiner error. Photosynthesis takes in carbon dioxide and releases oxygen; respiration takes in oxygen and releases carbon dioxide, day and night, with no light needed.