How plants make food — Science, 7–10 years
Green plants use light energy to make sugar from water and carbon dioxide. This process, called photosynthesis, also releases oxygen into the air.
Leaves as food makers
A green leaf is more than a flat decoration: it is a tiny food-making place. Light energy helps the leaf join water from the roots with carbon dioxide from the air to make sugar. The plant uses the sugar for energy and growth, and stores some of it as starch.
Why plants need light
Plants cannot walk to a meal, so they need a way to build their own food from things around them. The mystery was why a leafy plant grows better in light than in a dark cupboard. Studying this problem led scientists to connect sunlight, air and water with plant growth and oxygen.
A simple leaf test
Cover half a leaf with dark paper and leave the plant in sunlight for a few hours. Remove the paper, soak the leaf in hot water, then use iodine with an adult's help. The uncovered half turns blue-black because it made starch; the covered half stays brownish because it received no light.
Plants do not eat soil
It is easy to think that soil is the plant's food because roots grow in it and fertiliser helps plants. In fact, roots take in water and minerals, while most of the plant's sugar is made in green parts using carbon dioxide and light. Soil is important, but it is not a plate of ready-made food.
Why this matters to us
Crops such as wheat, tomatoes and beans depend on photosynthesis to grow, so farmers consider light, water and healthy leaves. The oxygen released by plants becomes part of the air that animals breathe. Plants also remove some carbon dioxide from the air while they make sugar, although this does not solve every climate problem.
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