4.2 Leaves
Summary: Leaves come in all shapes and sizes. Most are green. However, have you ever looked carefully at a leaf? Some, like stinging nettles, are covered in tiny hairs. Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe the main adaptations of a leaf
- describe the role of stomata
- describe how water travels through a plant.
Think back
- What are the main organs of a plant?
- Why are plants called producers?
- What do plants need to photosynthesize?
Key idea Leaves are complex structures. The chlorophyll in the chloroplasts makes a leaf green and absorbs sunlight for Photosynthesis. The stomata allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out of the leaf.
Key words photosynthesis, chlorophyll, chloroplasts, stomata, guard Cells
Leaves come in all shapes and sizes. Most are green. However, have you ever looked carefully at a leaf? Some, like stinging nettles, are covered in tiny hairs.
Figure 1Leaves come in all shapes and sizes.
Structure of a leaf
Leaves are adapted for photosynthesis. Each part of a leaf has a special function that helps it carry out photosynthesis. Most leaves:
- are green – they contain chlorophyll, which absorbs (takes in) sunlight
- are thin – this allows gases to diffuse (move) into and out of the leaf easily
- have a large surface area – to absorb as much Light as possible
- have veins – they contain vessels (with tubes to carry water, and other tubes to carry sugars).
A Describe two adaptations of leaves for absorbing sunlight.
The lower surface of a green leaf is lighter than the top. This is because the cells in the bottom of the leaf contain fewer chloroplasts, which means there is less chlorophyll. Most sunlight hits the top of the leaf. Therefore, this is where the chloroplasts need to be to absorb as much sunlight as possible.
B State which part of the leaf contains the most chloroplasts.
The top of the leaf feels waxy, but the bottom is normally much drier. This waxy layer reduces the amount of water evaporating out of the leaf. The Sun heats up the top of the leaf. Without the waxy layer, a lot of water would escape from the leaf’s top surface. You have to boil leaves before you can test them for starch to destroy the waxy layer. This then allows the iodine to reach the cells.
C State why the top surface of the leaf is covered in a waxy layer.
How do gases get into and out of the leaf?
On the lower surface of the leaf, there are tiny holes called stomata (singular: stoma). Stomata allow gases to diffuse into and out of the leaf.
- Carbon dioxide diffuses in. Carbon dioxide is a reactant in photosynthesis. It takes part in and changes during the reaction.
- Oxygen diffuses out. Oxygen is a product of photosynthesis.
Figure 2 shows guard cells. These cells open the stomata during the day, and close them at night.
Figure 2Top – open stomata. Bottom – closed stomata.
D Describe the movement of gases through stomata during the day.
Maths skills A scientist calculated that one type of plant leaf had 9200 stomata per cm2 on its lower surface. The leaf had a surface area of 10 cm2. Estimate the number of stomata on the lower surface of the leaf.
How does water get into a plant?
Water diffuses into the root hair cells. It then moves through the plant in water vessels, which you can see in Figure 3.
Figure 3As the water evaporates from the leaves, more water is taken up through the plant. It is a bit like sucking on a straw! Water vapour is lost from the leaf through the stomata.
Figure 4 summarizes how the Products and Reactants of photosynthesis enter and leave a plant.
Figure 4How products and reactants move in photosynthesis. Full text alternative to the content
Stretch zone
- Match the parts of a leaf to their function. stomata reduces amount of water evaporating waxy layer allow gases to diffuse into and out of leaf guard cells transport water to cells in leaf veins open and close stomata
- Describe how water travels through a plant.
Stretch zone
- Explain why leaves are thin and have a large surface area.