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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Know both equations (aerobic and anaerobic in animals AND yeast). Understand why anaerobic releases less energy. Explain oxygen debt after exercise.
Topic 12 is often confused with gas exchange (Topic 11). Respiration is a chemical reaction in cells: not breathing. Know both equations (aerobic and anaerobic), where each occurs, and the concept of oxygen debt.
Priority 1: Aerobic respiration
glucose + oxygen → carbon dioxide + water (+ energy). Occurs in the mitochondria. Releases a large amount of energy. Takes place continuously in all living cells.
Priority 2: Anaerobic respiration
In animals: glucose → lactic acid (+ some energy). In yeast: glucose → ethanol + carbon dioxide (+ some energy). Occurs in the cytoplasm only. Releases much less energy because glucose is only partially broken down.
Priority 3: Oxygen debt
After intense exercise, lactic acid must be broken down using oxygen. The extra oxygen needed is the oxygen debt. This is why you continue breathing heavily after exercise: to supply oxygen for lactic acid breakdown in the liver.
Frequently asked questions
Do plants respire?
Yes: all living organisms respire continuously, including plants. Plants respire 24/7; they only photosynthesise in the light.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.