Plants produce their own food using light energy. This process is called photosynthesis.
The leaf is the plant organ where photosynthesis occurs. The leaf is well adapted for trapping light used for photosynthesis.
- The lamina is the large flat surface of the leaf. It gives leaves a large surface area to trap light.
- The vein network is a branched transportation system. It transports water around the leaf and manufactured food to the rest of the plant. It also provides a scaffold that stretches the lamina out.
- The petiole is the stalk that connects the leaf to the stem. It allows leaves to cover a larger area and minimises overlap between leaves.
The leaf is made up of three layers of tissue:
The upper epidermis consists of a single layer of transparent cells. These cells are transparent because they lack chloroplasts.
Upper epidermal cells are coated by a waxy covering called the cuticle. This reduces evaporation and protects the leaf from damage and infections.
Plants in hot environments have thick cuticles to reduce water loss.
The mesophyll is where photosynthesis occurs and the food products are stored. It contains large air spaces that facilitate gas exchange.
The lower epidermis forms a protective layer at the bottom of the leaf. It is punctured with small gaps in its surface called stomata.
The mesophyll layer contains two cell types that are involved in photosynthesis.
|Palisade cell||Large, long cells packed vertically below the upper epidermis||Maximum light absorption for photosynthesis|
|Spongy cell||Small, round cells with large air spaces in-between them||Large surface area for gas exchange between cells and air spaces|
|Cells next to air spaces are covered by a thin film of water||Gases from air spaces dissolve in water and then into cells|
Palisade and spongy cells both contain chloroplasts, the organelle required for photosynthesis. Palisade cells have more chloroplasts because they are on the upper side of the leaf and so absorb most of the light.
Stomata (singular: stoma) are small gaps in the lower epidermis that allow the leaf to exchange gases with the environment.
The stomata provide a connection between the air spaces in the mesophyll, and the outside atmosphere. This connection allows the:
- Entry of carbon dioxide needed for photosynthesis into leaf
- Excretion of oxygen as a by-product of photosynthesis from leaf
- Diffusion of water vapour from leaf
Stomata are formed by two specialised cells, called guard cells. They bend in a bean shape to form an opening (the stoma) between the cells.
The stoma is the gap between the guard cells, not the cells themselves.
Guard cells are the only cells in the lower epidermis that contain chloroplasts.
Stomata can open and close depending on the water content of the guard cells. This changes depending on the time of day.
In most plants, the opening of the stomata is related to whether or not the leaf is photosynthesising, as shown in the table.
|Time of day||Stomata||Function|
|Day|| Open |
Turgid (stiff, filled with water)
|Allow gas exchange for photosynthesis|
|Night|| Closed |
Flaccid (loose, lacking water)
|Limit water loss by evaporation|