The leaf as a photosynthesis organ
Every part of a leaf is built to do one job efficiently — photosynthesis.
Photosynthesis happens mainly in the leaf. To make lots of glucose, a leaf must:
- Absorb as much light as possible.
- Get carbon dioxide (CO₂) to the photosynthesising cells and let oxygen out.
- Have water delivered to the cells, and carry sugars away.
The exam command is almost always "explain how the leaf is adapted for photosynthesis". The trick is to link each structure to its function — never just name a part, always say what it does and why that helps.
| Part of leaf | What it does (function) |
|---|---|
| Waxy cuticle | Reduces water loss; transparent, lets light in |
| Upper epidermis | Thin and clear — lets light through to cells below |
| Palisade mesophyll | Main photosynthesis — packed with chloroplasts |
| Spongy mesophyll | Air spaces for gas exchange (CO₂ in, O₂ out) |
| Stomata + guard cells | Pores controlling gas exchange and water loss |
| Xylem (in veins) | Brings water (and minerals) to the leaf |
| Phloem (in veins) | Carries away sugars (sucrose) made in the leaf |
Exam tip. A common mark scheme reads "structure (1) + linked function (1)". Saying "palisade cells have chloroplasts" gets the structure; you only get the second mark by adding "so they absorb light for photosynthesis".
- A leaf must absorb light, exchange gases, get water and remove sugars.
- Always link STRUCTURE to FUNCTION to score the second mark.
- Xylem brings water IN; phloem carries sugars AWAY.
See the full worked example for leaf: structure & adaptations →