How do plants make sugar?

All green plants make sugars.

They take up water and minerals from the soil through their roots and carbon dioxide through their leaves.

Leaves contain a green substance called chlorophyll (this gives plants their characteristic colour).

The chlorophyll uses light energy from the sun to combine carbon dioxide and water to produce sugar.

The by-product of this process is oxygen.

The term used to describe the process by which plants make sugar is photosynthesis.

It comes from the Greek word ‘photo’ (light) and ‘synthesis’ (putting together), so in simple terms plants use light to join water and carbon dioxide.

As a result of photosynthesis the sugars glucose, fructose and sucrose are produced. These are then stored in the plant. Sucrose is the sugar most commonly extracted from plants by man.

Only sugar cane and sugar beet make and store enough sucrose to make it worthwhile for us to grow, harvest and extract sugar from them.

The family of sugars

There are a whole range of substances which make up the family of sugars. These include the sugars made by plants during photosynthesis, milk sugar and honey.

Our bodies use all sugars in basically the same way, whatever the source, to give us energy for life.

Sugars and their sources

Glucose: fruit, vegetables, honey
Maltose: barley
Sucrose: sugar beet, sugar cane, fruits
Lactose: milk
Fructose: fruits, honey

Sugars - the building blocks for plants

The sugars produced by photosynthesis provide an immediate source of energy for plants to live and grow. Excess sugars can be stored as sucrose. This provides an energy reserve at night, when plants cannot photosynthesise.

Sugars are also used as building blocks for making all the other substances which plants need for growth and repair. For example, sugars can be used to make complex substances (dietary fibre) which form plant cell walls and provide the plant with structure and support.

Glucose can be used by plants to make starch which is made up of long, branched chains of glucose. Starch is found in large quantities in potatoes, rice and cereal grains (eg wheat).