Curriculum fit: GCSE Biology and Combined Science (photosynthesis). US: middle and high school life science (matter and energy in organisms and ecosystems); also AP Biology.
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What Is It?
Photosynthesis is the process by which plants, algae and some bacteria make food using light energy. It happens in the chloroplasts, which contain the green pigment chlorophyll that absorbs light.
The plant takes in carbon dioxide from the air and water from the soil. It makes glucose, a sugar, and releases oxygen as a by-product.
Key Formulas and Ideas
- Word equation: carbon dioxide + water → glucose + oxygen (using light energy).
- Chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Photosynthesis is endothermic: it takes in energy from light.
- Limiting factors: light intensity, carbon dioxide concentration and temperature.
Worked Examples
Check the Equation Is Balanced
Show that 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is balanced.
- Carbon: 6 on the left, 6 on the right.
- Hydrogen: 12 on the left (6 × 2), 12 on the right.
- Oxygen: 12 + 6 = 18 on the left, 6 + 12 = 18 on the right.
✓ Answer Each element has the same number of atoms on both sides, so it is balanced
A Limiting Factor
At night, a plant with plenty of carbon dioxide and a warm temperature makes no glucose. Why?
- Photosynthesis needs light energy.
- With no light, light is the limiting factor, so the rate falls to zero.
✓ Answer Light is the limiting factor
Uses of Glucose
State three uses of the glucose a plant makes.
- It is used for respiration to release energy.
- It is stored as starch.
- It is used to make cellulose for cell walls, and with nitrates to make proteins.
✓ Answer Respiration, storage as starch, making cellulose and proteins
Common Mistakes to Avoid
- Saying plants get their food from the soil. They make it by photosynthesis.
- Writing 'carbon dioxide + oxygen' on the reactants side.
- Thinking plants do not respire. They respire all the time.
- Describing a limiting factor as something that 'stops' the reaction rather than slows it.
Practice Questions
Frequently Asked Questions
Why are leaves green?
Chlorophyll reflects green light while absorbing mostly red and blue light.
Do plants also respire?
Yes. Plants respire all the time, and photosynthesise only when there is light.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.