Photosynthesis — Science, 11–13 years
Green plants use light to make sugar from water and carbon dioxide, releasing oxygen. This explains how a plant builds much of its own food and why light, leaves and air matter to it.
The idea
Photosynthesis is how green plants make sugar inside their cells. Light supplies the energy for joining water from the roots with carbon dioxide from the air. The plant uses the sugar to grow or stores it, while oxygen is released through the leaves.
Why plants need this
A plant cannot walk to find food, so it needs a way to build energy-rich material where it grows. Photosynthesis solves that problem using sunlight and simple substances from the air and soil. It also explains why plants are the starting source of food for many living things.
A leaf in sunlight
Place a healthy plant in sunlight and cover part of one leaf with opaque paper for a day. After testing the leaf for starch, the uncovered part turns blue-black, but the covered part does not. The leaf made and stored sugar where light reached it; the test gives evidence, not a direct view of the process.
The common mistake
It is easy to think plants take food straight from the soil because roots absorb water and minerals. Those materials are important, but most of the plant’s dry food is made from carbon dioxide in the air. Roots mainly supply water and minerals; leaves use light to build sugar.
Where it appears
Photosynthesis supports crops, forests and gardens by turning sunlight into plant material. It also adds oxygen to the air and removes carbon dioxide while light is available. Farmers use this knowledge when choosing light, water and spacing, but plants still need minerals and suitable temperatures too.
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