Photosynthesis · Grade 7
What is the difference between photosynthesis and cellular respiration?
Photosynthesis builds sugar from carbon dioxide and water using light; cellular respiration breaks that sugar down using oxygen to release energy. They are opposites in their raw materials and products. A plant does both — photosynthesis only in light, respiration all the time.
Learning objectives
- Compare photosynthesis and cellular respiration by their inputs, their outputs and when each occurs
- Explain why a plant gives off oxygen during the day and carbon dioxide at night
- Trace the energy in a meal back to sunlight along a food chain
- Explain the part plants play in keeping carbon dioxide and oxygen balanced in the air
Two opposite processes
Photosynthesis takes carbon dioxide and water and makes glucose and oxygen, and it needs light. Respiration takes glucose and oxygen and makes carbon dioxide and water, releasing energy.
What comes out of one goes into the other. But they are not one process running in both directions — they happen in different organelles and by entirely different steps.
And one difference is decisive: respiration happens in every living cell, all the time, in the plant as much as in us. Photosynthesis happens only in cells with chloroplasts, and only when there is light.
Day and night
By day both processes run in the plant, but photosynthesis is far faster. So the balance is positive: more carbon dioxide goes in than comes out, and more oxygen comes out than is used.
At night there is no light, photosynthesis stops and respiration continues. So at night a plant consumes oxygen and releases carbon dioxide — just as we do.
There is no contradiction in that. The plant does not make oxygen instead of breathing; it makes more than it uses, and the surplus is what reaches us.
Every meal starts with light
The plant is the only link in the chain that makes organic matter from inorganic. Everything that comes after it — the grazer, the predator that eats the grazer — gets its energy from the link before.
So you can trace any meal backwards to sunlight captured in a leaf. Even a steak is, in the end, light that passed through grass and through a cow.
Worked examples
Compare: what goes in and what comes out of each process?
- Photosynthesis: carbon dioxide and water in, glucose and oxygen out
- Respiration: glucose and oxygen in, carbon dioxide and water out
- One requires light, the other releases energy
Answer: Exact opposites in raw materials and products
Does a plant respire at night?
- Cellular respiration happens in every living cell, always
- At night there is no light and so no photosynthesis
- Only the oxygen-consuming process remains
Answer: Yes. At night it consumes oxygen and releases carbon dioxide
Trace the energy in a cheese sandwich back to its source
- The cheese came from a cow
- The cow ate grass
- The grass built itself by photosynthesis
Answer: From sunlight, through grass and through a cow
Common mistakes
- Thinking plants do not respire
- Every living cell respires, constantly. By day photosynthesis is simply the larger of the two, which makes it look as though only that is happening.
- Confusing cellular respiration with breathing
- Cellular respiration is the breakdown of sugar inside a cell. Moving air into lungs is the transport of gases to and from it — two different things sharing a name.
- Thinking a plant in a bedroom is dangerous
- The oxygen a houseplant consumes at night is negligible against the volume of the room and against what a person in that room uses themselves.
What to remember
- The processes are opposites in inputs and outputs.
- Respiration always; photosynthesis only in light.
- By day the balance is positive, at night negative.
- Every food chain begins with photosynthesis.