Photosynthesis

Plants, unlike animals, can make their own food. They do this using a process called photosynthesis.

During photosynthesis, plants produce glucose from simple inorganic molecules - carbon dioxide and water - using light.

Photosynthesis requires energy to drive the chemical reaction. Photosynthesis is an endothermic reaction. This means it cannot occur without energy (from the Sun).

Diagram illustrating the word and symbol equations for photosynthesis

The light required is absorbed by a green pigment called chlorophyll in the leaves. Chlorophyll is found in chloroplasts in plant cells, particularly the palisade and spongy mesophyll cells.

Plant leaves are the main photosynthetic organ, but other parts of the plant exposed to the light can develop chlorophyll and photosynthesise.

The carbon dioxide required for photosynthesis comes from the air. It enters leaves through the stomata. Water enters the plant through the roots, and is transported to the leaves in the xylem.

Oxygen is formed as the waste product. Some is used for respiration by the plant. The excess is released from the leaves, making it available for respiration to animals and many microorganisms. During the light, provided the rate of photosynthesis is sufficiently high, plants, give out oxygen.

Photosynthesis in detail

The overall reaction for photosynthesis as given above is a simplification. Photosynthesis involves several different chemical reactions, but these can be summarised in two main stages. In the first reaction, energy is used to split water molecules into hydrogen and oxygen. The hydrogen is needed for the second stage of reactions and the oxygen is released by the plant as a waste product. In the reactions of the second stage, the hydrogen is combined with carbon dioxide to make glucose.

Biosynthesis

Some of the glucose produced by photosynthesis is used for respiration.

Glucose is the starting point for the biosynthesis of materials that plants need to live.

The glucose not used for respiration is used in the following ways:

An image showing the cycle of gluecose.