Leaf structure and adaptationsSpec 2.21, 2.42B
A leaf is adapted to absorb light for photosynthesis and to allow gas exchange of carbon dioxide and oxygen.
| Structure | Adaptation | How it helps |
|---|---|---|
| Whole leaf | Broad, flat and thin | Large surface area to absorb light, and a short distance for gases to diffuse |
| Waxy cuticle | A waxy, waterproof layer on the surface | Reduces water loss by evaporation |
| Upper epidermis | A thin layer of transparent cells | Lets light through to the cells underneath |
| Palisade mesophyll | Tightly packed, tall cells near the top of the leaf, with many chloroplasts | Most photosynthesis takes place here, close to the light |
| Spongy mesophyll | Loosely arranged cells with air spaces between them | Carbon dioxide can diffuse through the leaf to the cells, and oxygen can diffuse out |
| Stomata and guard cells | Pores, mostly in the lower epidermis, opened and closed by guard cells | Allow gases to enter and leave, and control water loss |
| Xylem and phloem (veins) | Vascular bundles running through the leaf | Xylem brings water for photosynthesis. Phloem carries sucrose away. |
(opens full size in a new tab)Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanHow the leaf is adapted for gas exchange
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanCarbon dioxide and oxygen move into and out of a leaf by diffusion, down their concentration gradients. The structure of the leaf makes this diffusion fast.
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels mean- Broad, flat shape: a large surface area for gases to diffuse in and out.
- Thin leaf: a short diffusion distance, so gases reach the cells quickly.
- Many stomata, mostly in the lower epidermis: pores that let carbon dioxide diffuse in and oxygen diffuse out. Guard cells open and close them.
- Air spaces in the spongy mesophyll: gases diffuse quickly between the stomata and the cells, and a large surface area of cells is exposed to the air.
- Moist cell surfaces: gases dissolve in the thin film of water on the mesophyll cells, then diffuse into or out of the cells.
Exam tip:
Always give an adaptation and the reason it helps. For example, 'the air spaces in the spongy mesophyll allow carbon dioxide to diffuse to the photosynthesising cells'.