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BiologyGCSEAQA & EdexcelYears 10–11

GCSE Biology Photosynthesis Notes

Photosynthesis converts carbon dioxide and water into glucose using light energy. Rate depends on light, CO₂ and temperature — the three limiting factors.

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Key concepts

Photosynthesis equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Endothermic — takes in energy from light.
Chlorophyll
Green pigment in chloroplasts that absorbs light energy (especially red and blue).
Limiting factor
The factor whose shortage caps the rate. Increasing it raises the rate; other factors do not until this one is met.
  • Glucose is used for respiration, converted to starch for storage, cellulose for cell walls, and lipids/proteins for growth.
  • In greenhouses, growers optimise light, CO₂ and warmth — often at a cost that must be justified by extra yield.
  • Inverse-square law: doubling distance from a lamp quarters the light intensity.

Key formulas

Photosynthesis (symbol)

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Light intensity (inverse square)

intensity ∝ 1 ÷ distance²

Required practicals

  • Investigate the effect of light intensity on the rate of photosynthesis in pondweed.

Worked example

In a photosynthesis practical, moving the lamp from 20 cm to 10 cm changes bubble count from 5 to 20 per minute. Explain.

  1. 1Halving distance quadruples light intensity (inverse square law).
  2. 2Light was the limiting factor at 20 cm.
  3. 3More light → more photosynthesis → more oxygen bubbles released.
  4. 4Rate roughly quadruples, from 5 to 20 bubbles per minute.

Answer: Light was limiting; quadrupling intensity roughly quadrupled the rate.

Common mistakes

✗ Ignoring the inverse-square law in distance-of-lamp questions.

✓ Halving distance quadruples intensity, not doubles it.

✗ Saying plants only respire at night.

✓ Plants respire 24 hours a day; they photosynthesise only in light.

✗ Writing an unbalanced photosynthesis equation.

✓ Check both sides have 6 C, 12 H (from 6 H₂O), 18 O.

Quick check

Try these, then reveal the answers.

What two products does photosynthesis make?Reveal answer

Glucose and oxygen.

Why is chlorophyll green?Reveal answer

It reflects green light and absorbs red and blue light.

Give one use plants have for glucose.Reveal answer

Respiration, storage as starch, cellulose for cell walls, or making amino acids/lipids.

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Exam tips

  • State whether a reaction is endothermic (photosynthesis) or exothermic (respiration).
  • Refer to the inverse-square law in any lamp-and-pondweed question.
  • When analysing rate graphs, name the limiting factor in each region.

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Related topics

BioenergeticsOrganisation

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is photosynthesis?Full glossary

Frequently asked questions

Why is photosynthesis important?+

It produces glucose (food for the plant and every organism that eats plants) and oxygen for aerobic respiration.

How do commercial growers boost yield?+

By controlling temperature, CO₂ and light intensity in greenhouses so none of these factors limit the rate — as long as the extra cost is repaid in extra yield.

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