Adaptations of the leaf for photosynthesis and gaseous exchange

Leaves are adapted for photosynthesis and gaseous exchange.

They are adapted for photosynthesis by having a large surface area, and contain openings, called stomata to allow carbon dioxide into the leaf and oxygen out. Although these design features are good for photosynthesis, they can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf.

When water evaporates from the leaves, resulting in more water being drawn up from the roots, it is called transpiration.

To reduce water loss the leaf is coated in a waxy cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss.

Leaves enable photosynthesis to occur. Photosynthesis is the process by which leaves absorb light and carbon dioxide to produce glucose (food) for plants to grow. Leaves are adapted to perform their function, eg they have a large surface area to absorb sunlight.

Plants have two different types of 'transport' tissue, xylem and phloem. These specialised tissues move substances in and around the plant.

Functions of leaves

The function of a leaf is photosynthesis - to absorb light and carbon dioxide to produce glucose (food). The equation for photosynthesis is:

\text{carbon dioxide and water} \rightarrow \text{glucose and oxygen}

Diagram of a plant leaf

Leaves are also involved in gas exchange. Carbon dioxide enters the leaf and oxygen and water vapour leave the plant through the stomata. Leaves are adapted in several ways to help them perform their functions.

Features of leaves

Large surface areaTo absorb more light
ThinShort distance for carbon dioxide to diffuse into leaf cells
ChlorophyllAbsorbs sunlight to transfer energy into chemicals
Network of veinsTo support the leaf and transport water, mineral ions and sucrose (sugar)
StomataAllow carbon dioxide to diffuse into the leaf and oxygen to diffuse out

Structure of a leaf

Diagram showing how a leaf is structured and what happens during photosynthesis

The internal structure of the leaf is also adapted to promote efficient photosynthesis.

Functions of tissues of the leaf

Epidermis is thin and transparentTo allow more light to reach the palisade cells
Thin cuticle made of waxTo protect the leaf from infection and prevent water loss without blocking out light
Palisade cell layer at top of leafTo absorb more light and increase the rate of photosynthesis
Spongy layerAir spaces allow gases to diffuse through the leaf
Palisade cells contain many chloroplastsTo absorb all the available light

Gas exchange

When a plant is carrying out photosynthesis carbon dioxide needs to move from the air into the leaf. It does this by diffusing through small pores called stomata.

At the same time oxygen moves out of the leaf through the stomata. This movement of gases in opposite directions is called gas exchange.

Water vapour also diffuses out of the stomata. The stomata are surrounded by guard cells, which control their opening and closing. Cells in the leaf are loosely packed.

Absorbing light energy

Light absorption happens in the palisade mesophyll tissue of the leaf. Palisade cells are column shaped and packed with many chloroplasts. They are arranged closely together so that a lot of light energy can be absorbed.