6.1.9Plant Nutrition
Leaf Adaptations — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 11
Part 1 of 4Why leaves are the photosynthesis organs
Main site of food production
- Main site of food production Leaves contain most of a plant's chloroplasts, so most photosynthesis takes place there.
- Glucose made in the leaves is moved to the rest of the plant to be used or stored.
What the leaf must obtain
- What the leaf must obtain A leaf needs a supply of light, carbon dioxide and water to photosynthesise at a high rate.
- It must also be able to get rid of the oxygen that is produced.
Adaptation
- Adaptation An adaptation is a feature that helps an organism survive or carry out a process more effectively.
- Every part of a leaf's structure can be explained in this way.
Part 2 of 4Adaptations of the whole leaf
Leaf shape and surface
Large surface area
- Large surface area A broad, flat blade exposes as many chloroplast-containing cells as possible to sunlight, so more light is absorbed.
Thin blade
- Thin blade Gases only have a very short distance to diffuse between the air and the photosynthesising cells,
- so carbon dioxide arrives quickly and oxygen leaves quickly.
Waxy cuticle
- Waxy cuticle A transparent waxy layer on the upper surface reduces water loss by evaporation while still letting light pass through.
Stomata
- Stomata Tiny pores, mostly on the underside, let carbon dioxide in and oxygen and water vapour out.
- They open and close to control gas exchange.
Part 3 of 4Adaptations of the leaf's internal layers
| Layer or structure | Adaptation | Benefit for photosynthesis |
|---|---|---|
| Upper epidermis | Thin and transparent, with no chloroplasts | Allows light to pass straight through to the cells below |
| Palisade mesophyll | Tall, tightly packed cells near the top, full of chloroplasts | Most light is absorbed here, where it is strongest |
| Spongy mesophyll | Rounded cells with large air spaces between them | Gases diffuse easily through the leaf to reach every cell |
| Guard cells | Pairs of cells that change shape around each stoma | Open the pore in daylight to take in carbon dioxide and close it to save water |
| Vascular bundles | Xylem and phloem running through the leaf | Xylem brings water to the cells; phloem carries away glucose |
Part 4 of 4Following a molecule of carbon dioxide
1Enters through a stoma
- Enters through a stoma Carbon dioxide from the air diffuses in through an open stoma on the lower surface.
2Moves through air spaces
- Moves through air spaces It spreads through the air spaces in the spongy mesophyll, moving down its concentration gradient.
3Dissolves and enters cells
- Dissolves and enters cells It dissolves in the moisture on cell walls and diffuses into the palisade cells.
4Used in chloroplasts
- Used in chloroplasts Inside the chloroplasts it is combined with water to make glucose.
Key terms
| Term | Meaning |
|---|---|
| Adaptation | A feature of an organism that helps it to survive or to carry out a process efficiently. |
| Cuticle | A waxy, waterproof layer on the leaf surface that reduces water loss. |
| Stomata | Pores in the leaf surface through which gases enter and leave (singular: stoma). |
| Guard cells | Paired cells that open and close a stoma. |
| Mesophyll | The tissue in the middle of the leaf where most photosynthesis takes place. |
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration. |
Avoid these mistakes
1
Saying leaves are thin 'to let light through' only. Thinness mainly gives a short diffusion distance for gases, although it also lets light reach every layer.
2
Stating that the cuticle lets gases in. The cuticle is waterproof; gases enter through the stomata.
3
Describing a large surface area as helping 'photosynthesis' without saying why. State that more light can be absorbed.
4
Forgetting that stomata also let water vapour escape, which is the trade-off the plant must manage.
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