Lessons · Teaching · how a plant feeds itself
A plant builds its own food out of air and water
In photosynthesis a plant uses light energy to turn carbon dioxide and water into glucose, releasing oxygen. It happens in the chloroplasts, mostly in the leaves. The plant is not eating soil: the mass of a tree comes overwhelmingly from carbon taken out of the air.
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What it is for
Almost every adult believes a plant's bulk comes from the soil, and it is a beautiful thing to be corrected about. A tree is mostly air and water rearranged by sunlight. Once a pupil has that, the carbon cycle and the reason forests matter follow without a separate lesson.
How to think about it
Track the four things in and the two things out. In: light, carbon dioxide, water, and the chlorophyll to catch the light. Out: glucose for the plant to use or store, and oxygen as waste. Minerals from the soil matter, but they are a supplement rather than the food.
Worked example
Carbon dioxide and water go in, glucose and oxygen come out, with light driving itThe whole equation in one sentence.
It happens in the chloroplasts, which is why leaves are the main siteBroad, flat and full of chloroplasts: a leaf is a solar panel.
The oxygen is a waste product of the plant, and the reason we can breatheOne organism's waste is another's requirement.
A tree's mass is mostly carbon taken from the air, not soil taken from the groundThe single most surprising fact in elementary biology.
Your turn
Name the gas a plant takes in for photosynthesis.
A plant takes in dioxide
Solve one, graded on the server
The trap
Saying plants breathe in carbon dioxide and breathe out oxygen, full stop. Plants respire too, all the time, taking in oxygen and giving out carbon dioxide. In daylight photosynthesis is much the larger of the two, which is why the net flow looks one way.