How plants make food — Chemistry, 7–10 years

Plants do not eat sandwiches or take food from the soil. Using light, water and carbon dioxide from the air, their green parts make sugar and release oxygen.

Light helps leaves make sugar

A green leaf is like a tiny food-making workshop. It uses light energy to join water from the roots with carbon dioxide from the air, making sugar for the plant and releasing oxygen.

Why does a plant need light?

People noticed that a plant can grow in soil and water, but those alone do not explain all its new leaves. The puzzle was where the plant’s food came from, so scientists compared plants kept in light with plants kept in darkness.

A simple leaf test

1. Cover part of a leaf with dark paper and leave the plant in sunlight for a few hours. 2. Remove the paper and test the leaf for starch with iodine: the lit part turns blue-black, while the covered part stays yellow-brown. This shows that light helped make stored food.

The plant does not eat soil

It is reasonable to think that a plant’s food comes from the soil, because roots are in the soil and fertiliser helps plants grow. But roots mainly take in water and minerals. Most of the plant’s food mass is made from carbon dioxide in the air, using light energy.

Why plants matter to us

Photosynthesis feeds plants, and plants feed many food chains: wheat, fruit, vegetables and the animals that eat plants all depend on it. It also adds oxygen to the air. Forests, gardens and crops therefore change both the food around us and the air we breathe.

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