GCSE Biology Bioenergetics Notes
Photosynthesis transfers light energy into chemical energy in glucose; respiration releases that energy for the cell to use.
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.
- 1At low light, light is the limiting factor — rate rises with light intensity.
- 2As light increases, another factor (CO₂ or temperature) becomes limiting.
- 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.
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.
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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.
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