MathsGradeUp logoMathsGradeUp
BiologyGCSEAQA & EdexcelYears 10–11

GCSE Biology Bioenergetics Notes

Photosynthesis transfers light energy into chemical energy in glucose; respiration releases that energy for the cell to use.

Practise this topic free Ask Study Coach

Key concepts

Photosynthesis
Endothermic reaction in chloroplasts: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy).
Limiting factor
The factor in shortest supply that limits the rate — light intensity, CO₂ concentration or temperature.
Aerobic respiration
Exothermic reaction in mitochondria: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Anaerobic respiration
Without oxygen. In muscles → lactic acid (oxygen debt). In yeast → ethanol + CO₂ (fermentation).
  • Photosynthesis uses light energy absorbed by chlorophyll.
  • Aerobic respiration releases much more energy per glucose than anaerobic.
  • During exercise, breathing and heart rate rise to supply more oxygen and glucose and remove more CO₂.

Key formulas

Photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Aerobic respiration

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration (muscle)

glucose → lactic acid

Worked example

Explain why the rate of photosynthesis rises then plateaus as light intensity increases.

  1. 1At low light, light is the limiting factor — rate rises with light intensity.
  2. 2As light increases, another factor (CO₂ or temperature) becomes limiting.
  3. 3Increasing light further has no effect — rate plateaus.

Answer: Light is the limiting factor at low intensity; the plateau shows CO₂ or temperature has become the new limiting factor.

Common mistakes

✗ Saying photosynthesis 'makes energy'.

✓ It transfers light energy to chemical energy (glucose) — energy is conserved, not made.

✗ Forgetting mitochondria as the site of aerobic respiration.

✓ State 'in the mitochondria' for the location mark.

✗ Confusing lactic acid with ethanol.

✓ Human muscle → lactic acid; yeast → ethanol and CO₂.

Quick check

Try these, then reveal the answers.

Word equation for aerobic respiration?Reveal answer

Glucose + oxygen → carbon dioxide + water (+ energy).

Name three limiting factors of photosynthesis.Reveal answer

Light intensity, CO₂ concentration and temperature.

What is oxygen debt?Reveal answer

Extra oxygen needed after exercise to break down lactic acid built up in muscles.

Practise more questions with Study Coach feedback

Exam tips

  • Learn both symbol and word equations for photosynthesis and respiration.
  • In limiting-factor graphs, name the factor that limits at each region of the curve.
  • Compare energy yield: aerobic releases far more per glucose than anaerobic.

Related practice

Practise this topic

Exam-style questions with instant Study Coach feedback.

Flashcards

Quick-fire recall to lock in the key facts.

Worksheets

Printable practice you can work through offline.

Past papers

Full papers with examiner-grade marking.

Study Coach

Ask for a step-by-step explanation any time.

All Biology

Every Biology topic across every exam board.

Related topics

PhotosynthesisOrganisation

Key terms explained

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

What is photosynthesis?What is respiration?Full glossary

Frequently asked questions

Where does photosynthesis occur?+

In the chloroplasts of plant cells — mostly in leaf mesophyll cells.

Why do we breathe faster during exercise?+

To supply more oxygen for aerobic respiration and remove the extra CO₂ produced, meeting the higher energy demand of muscles.

Create a free account to practise bioenergetics with Study Coach feedback

Try 5 questions now, get instant examiner-grade marking and ask the Study Coach to explain anything. Free during our early launch period — no credit card required.

Create free account Try the Study Coach

Related on MathsGradeUp

Keep studying

Practise it