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:
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
Adaption
Purpose
Large surface area
To absorb more light
Thin
Short distance for carbon dioxide to diffuse into leaf cells
Chlorophyll
Absorbs sunlight to transfer energy into chemicals
Network of veins
To support the leaf and transport water, mineral ions and sucrose (sugar)
Adaption
Stomata
Purpose
Allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out
Structure of a leaf
The internal structure of the leaf is also adapted to promote efficient photosynthesis.
Functions of tissues of the leaf
Adaption
Purpose
Epidermis is thin and
transparent
To allow more light to reach the palisade cells
Thin cuticle made of wax
To protect the leaf from infection and prevent water loss
without blocking out light
Palisade cell layer at top of leaf
To absorb more light and increase the rate of photosynthesis
Spongy layer
Air spaces allow gases to diffuse through the leaf
Palisade cells contain many
chloroplasts
To 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.