Science
What is photosynthesis?
Quick answer
Photosynthesis is how green plants make their own food. They use energy from sunlight to turn carbon dioxide from the air and water from the soil into sugar, giving out oxygen.
In one sentence
Photosynthesis is the way plants use sunlight to make their own food from air and water, releasing oxygen as they do it.
Explained
Animals have to eat to get energy. Plants are different: they make their own food. The word photosynthesis means "putting together with light".
A leaf is like a tiny solar-powered kitchen. Inside its cells are green parts called chloroplasts, packed with a green substance called chlorophyll. Chlorophyll soaks up energy from sunlight. The plant uses that energy to join together two simple ingredients:
- water, pulled up from the soil through the roots and stem
- carbon dioxide, a gas that enters through tiny holes on the underside of leaves
The result is glucose, a sugar the plant uses for energy and to build new leaves, stems and roots. Oxygen is left over and goes out into the air.
carbon dioxide + water → (light energy) → glucose + oxygen
Here is the surprising part: a huge tree is built mostly from air. The carbon in its wood came from carbon dioxide. The soil gives the plant water and small amounts of minerals, but not most of its mass.
Plants also respire, just as animals do. They break down some of the sugar they make to release energy, day and night.
Examples
- A sunflower growing towards the light to catch as much energy as possible.
- Pondweed giving off small bubbles of oxygen on a sunny day.
- Algae in the ocean, which produce a large share of the world's oxygen.
- A plant left in a dark cupboard turning pale and weak because it cannot photosynthesise.
Common mistakes
- "Plants get their food from the soil." Soil gives water and minerals, a bit like vitamins. The food (sugar) is made in the leaves.
- "Plants breathe in carbon dioxide and animals breathe in oxygen." Plants take in carbon dioxide for photosynthesis, but they also use oxygen to respire, all the time.
- "Only big green plants photosynthesise." Seaweed, algae and tiny floating plankton in the ocean do it too. Leaves look green because chlorophyll absorbs red and blue light and reflects green.
Quick quiz
Photosynthesis
Check what you know with 4 quick questions.
Question 1 of 4
Which gas do plants take in for photosynthesis?
Practise it
Try it
Learn which parts of a plant do which jobs, then try a quick experiment: cover part of a leaf on a houseplant with foil for a few days and see what changes.
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For grown-ups
Related topics
Parts of a plant
The main parts of a flowering plant are the roots, stem, leaves and flowers. Each part has a job, from soaking up water to making food and producing seeds.
Food chains
A food chain shows who eats whom in nature, starting with a plant that makes its own food and following the energy as it passes from one living thing to the next.
Cells
A cell is the smallest living unit. Every living thing is made of one or more cells, each with a membrane, cytoplasm and, in plants and animals, a nucleus that controls it.
Light
Light is a form of energy that travels in straight lines, very fast. We see things when light from them enters our eyes, either because they make light or because they reflect it.
Electric circuits
An electric circuit is a complete loop that electricity flows around. A simple circuit has a battery, wires and a component like a bulb. If the loop is broken, the electricity stops.
Digestive system
The digestive system is the long tube and organs that break food down into tiny pieces your body can absorb for energy and growth, then get rid of what is left as waste.