Photosynthesis: how plants make food — Biology, 11–13 years
Plants do not take food from the soil in the way animals eat it. In photosynthesis, light energy helps green plant cells turn water and carbon dioxide into sugar, releasing oxygen as a by-product.
Light becomes stored food
Photosynthesis is the process in which green plant cells use light energy to make sugar. They take in water through the roots and carbon dioxide through tiny openings in leaves. The sugar stores energy and can be used to build the plant; oxygen is released into the air.
Why plants need this process
An animal can obtain energy by eating food made by another organism, but a rooted plant cannot go looking for a meal. Scientists needed to explain how a plant gains mass and grows from air, water and light. Experiments with covered leaves and changing gases showed that light and carbon dioxide are essential for making new plant material.
Testing a leaf for starch
1. Keep part of a geranium leaf in darkness and leave the rest in light for a few hours. 2. Remove the green colour safely in a school laboratory, then add iodine solution. 3. The lit part turns blue-black because it made starch from sugar; the covered part stays yellow-brown. This is evidence of light's role, not a direct picture of photosynthesis itself.
Plants do not eat soil
Because roots take in water and minerals, it is reasonable to imagine that soil is the plant's food. Minerals are important, but they are not the energy-rich sugar that makes most of the plant's dry mass. The carbon in that mass mainly comes from carbon dioxide in the air, while roots provide water and mineral nutrients.
Crops and the atmosphere
Farmers manage light, water, carbon dioxide and mineral nutrients because these affect photosynthesis and crop growth. Greenhouses may control temperature and light, but adding more of one factor cannot fix a shortage of another. Photosynthesis also matters globally: plants remove carbon dioxide from air while making food and releasing oxygen.
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