Photosynthesis: how plants make food

Animals have to find their food. Green plants do something no animal can do: they make their own food. The process is called photosynthesis. The word comes from "photo", meaning light, and "synthesis", meaning putting together. So photosynthesis means "putting together with the help of light".

What a plant needs

• Sunlight, which gives the energy.
• Water, which the roots take in from the soil and the stem carries up to the leaves.
• Carbon dioxide, a gas in the air that enters the leaf through tiny openings called stomata.
• Chlorophyll, the green substance in the leaf that traps sunlight.

What happens in the leaf

Inside the leaf, chlorophyll captures the energy of sunlight. The plant uses this energy to join water and carbon dioxide together to make a kind of sugar called glucose. This sugar is the plant's food. During the process, oxygen is released into the air through the stomata.

Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen

What happens to the food

The plant uses some of the glucose at once for energy and growth. The rest is changed into starch and stored in leaves, roots, stems, fruits and seeds. When we eat a potato, rice or a mango, we are eating food that a plant stored. Even when we eat an egg or drink milk, the hen and the cow got their food from plants. In this way almost all food on earth can be traced back to photosynthesis.

Why photosynthesis matters to us

• It makes food for plants, and through them for all animals and people.
• It gives out oxygen, the gas that we and other animals need to breathe.
• It uses up carbon dioxide, which helps to keep the air balanced.

This is why cutting down forests harms everyone, and why planting trees helps.

Common doubts

Do plants make food at night? No. Photosynthesis needs light, so it stops in the dark. Do plants breathe? Yes. Plants respire all day and night, like animals, using a little oxygen. But during the day they give out far more oxygen than they use. What about plants with red or purple leaves? They also have chlorophyll; other colours hide the green.