The leaf as an organ for photosynthesis
A leaf is a plant organ adapted to capture light energy and exchange gases. Its broad, flat lamina gives a large surface area to absorb sunlight, while being thin means gases diffuse only a short distance to the photosynthesising cells.
Key idea
Photosynthesis mainly occurs in the palisade mesophyll, whose tall column-shaped cells are packed with chloroplasts and lie just beneath the transparent upper epidermis.
Tissues and their functions
- Waxy cuticle — a waterproof layer that reduces water loss by evaporation.
- Upper epidermis — transparent, with no chloroplasts, so light passes through to the mesophyll.
- Palisade mesophyll — main site of photosynthesis; many chloroplasts.
- Spongy mesophyll — loosely packed cells with large air spaces for the diffusion of carbon dioxide and oxygen.
- Stomata and guard cells — pores, mostly on the lower surface, that control gas exchange and water loss.
- Vascular bundle (vein) — xylem carries water and mineral ions to the leaf; phloem carries away sucrose and amino acids.
Mineral requirements
Plants absorb mineral ions dissolved in water from the soil. Two are especially important:
| Nitrate ions | Needed to make amino acids and therefore proteins. Deficiency causes stunted growth and older leaves turning yellow. |
| Magnesium ions | Needed to make chlorophyll. Deficiency causes yellowing between leaf veins (chlorosis). |
Remember
- Palisade cells are near the top to receive the most light.
- Air spaces in the spongy layer speed up gas diffusion.
- No nitrate → poor growth; no magnesium → no chlorophyll → yellow leaves.