Photosynthesis stores light energy in food — Science, 14–17 years
Green plants use light to build sugar from carbon dioxide and water. This links sunlight, plant growth and much of the food energy used by living things.
What photosynthesis does
A green plant captures light with chlorophyll and uses that energy to join carbon dioxide with water. The main product is glucose, a sugar that stores energy; oxygen is released as a by-product.
Why plants need photosynthesis
Plants cannot eat to obtain the carbon needed to grow, so they must build their own organic molecules. Photosynthesis solves this problem by turning simple materials and light into chemical energy that can be stored and used later.
Worked example: tracing the atoms
For one glucose molecule, start with 6 carbon dioxide molecules and 6 water molecules. The plant uses light energy to form C₆H₁₂O₆, glucose, and releases 6 oxygen molecules. The atoms are rearranged, not created.
The oxygen trap
It is tempting to say that plants make food from sunlight, because light is essential. But light is energy, not a material ingredient. The carbon in glucose comes mainly from carbon dioxide, while water supplies hydrogen and some oxygen.
Photosynthesis beyond the classroom
Crop growth, forests and greenhouse farming all depend on photosynthesis. Measuring light, carbon dioxide or leaf area helps growers predict how quickly plants may produce biomass, although temperature and water also limit the result.
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