Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using light energy trapped by chlorophyll. Oxygen is released as a by-product. The leaf is the main organ for photosynthesis because its structure is adapted to gather the raw materials and light efficiently.
Key idea
carbon dioxide + water —(light energy, chlorophyll)→ glucose + oxygen. Light is trapped by chlorophyll inside chloroplasts.
Where photosynthesis happens
Photosynthesis takes place in the chloroplasts, which contain the green pigment chlorophyll. Chloroplasts are most numerous in the palisade mesophyll near the top of the leaf, where light is strongest; the spongy mesophyll also has some.
Supplying the raw materials
- Light passes through the transparent cuticle and upper epidermis to reach the palisade cells.
- Carbon dioxide diffuses in through the stomata and travels through the air spaces to the mesophyll cells.
- Water is absorbed by the roots and carried up to the leaf in the xylem.
Removing the products
- Glucose may be used at once, stored as starch, or converted to sucrose and carried away in the phloem.
- Oxygen diffuses out of the leaf through the stomata (or is used in respiration).
Example
Testing a leaf for starch (decolourise in alcohol, then add iodine) shows blue-black only in the green parts that contain chloroplasts. A variegated leaf stays brown where there is no chlorophyll, proving chlorophyll is needed for photosynthesis.
Limiting factors
The rate of photosynthesis is controlled by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. Raising a limiting factor increases the rate until another factor becomes limiting.
Remember
The leaf is adapted for photosynthesis: broad and thin for light and gases, packed with chloroplasts in the palisade layer, with veins to bring water and remove sugar.