Aerobic vs anaerobic respiration: equations and differences
TutorAura guide · Updated
Respiration is the chemical reaction in every living cell that releases energy from glucose. It's exothermic. Aerobic respiration uses oxygen and releases much more energy; anaerobic respiration happens without oxygen and releases much less.
Key takeaways
- Aerobic: glucose + oxygen → carbon dioxide + water.
- Anaerobic in muscles: glucose → lactic acid.
- Anaerobic in yeast and plants: glucose → ethanol + carbon dioxide.
- Aerobic respiration releases much more energy per glucose molecule.
Aerobic respiration
It happens continuously in plants and animals, mostly in the mitochondria.
Anaerobic respiration
In muscles, during hard exercise when not enough oxygen reaches them:
In plant cells and yeast (also called fermentation):
Fermentation is used to make bread rise (the carbon dioxide) and to make alcoholic drinks (the ethanol).
Side by side
| Aerobic | Anaerobic | |
|---|---|---|
| Oxygen | Needed | Not needed |
| Products (animals) | Carbon dioxide and water | Lactic acid |
| Products (yeast, plants) | Carbon dioxide and water | Ethanol and carbon dioxide |
| Energy released | Much more | Much less (glucose is only partly broken down) |
Exercise and oxygen debt
During exercise, heart rate, breathing rate and breath volume increase to deliver more oxygen and remove carbon dioxide faster. If muscles still don't get enough oxygen, they respire anaerobically and lactic acid builds up, causing fatigue. Afterwards, extra oxygen is needed to deal with the lactic acid: this is the oxygen debt. In the AQA specification, lactic acid is carried to the liver and converted back to glucose.
Respiration isn't breathing
Breathing (ventilation) moves air in and out of the lungs. Respiration is a chemical reaction inside cells. Gas exchange in the lungs supplies the oxygen respiration needs and removes its carbon dioxide. Respiration is also roughly the reverse of photosynthesis.
Common mistakes
- Writing that respiration "produces energy" (energy is released, not made).
- Saying plants only respire at night: they respire all the time.
- Putting carbon dioxide in the anaerobic equation for muscles.
Metabolism
Metabolism is the sum of all the reactions in a cell or the body. The energy released by respiration is used for many of them, including:
- turning glucose into starch, glycogen and cellulose;
- making lipids from glycerol and fatty acids;
- making amino acids and then proteins;
- breaking down excess proteins to form urea for excretion.
Respiration also releases energy for muscle contraction and, in mammals and birds, keeping body temperature steady.
Practice questions
- Write the word equation for aerobic respiration.
- Which type of respiration happens in yeast when bread rises, and what gas makes the dough rise?
- Why do muscles feel tired after a sprint?
Answers: glucose + oxygen → carbon dioxide + water; anaerobic respiration (fermentation), producing carbon dioxide; lactic acid builds up from anaerobic respiration.
Frequently asked questions
Do plants respire?
Yes, all the time, day and night, like all living cells.
Is respiration exothermic or endothermic?
Exothermic: it releases energy, some of which is transferred to the surroundings as heat.
What happens to lactic acid after exercise?
Blood carries it to the liver, where it is converted back into glucose. The extra oxygen needed for this is the oxygen debt.
Is respiration the same as breathing?
No. Breathing moves air in and out of the lungs; respiration is a chemical reaction in cells that releases energy from glucose.
Why does anaerobic respiration release less energy?
Glucose is only partly broken down. Lactic acid (or ethanol) still contains energy that hasn't been released.
Where does aerobic respiration happen?
Mostly in the mitochondria of cells.
What is the equation for aerobic respiration?
Glucose + oxygen → carbon dioxide + water; C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
What does anaerobic respiration produce in humans?
Lactic acid, and much less energy than aerobic respiration.
What is oxygen debt?
The extra oxygen needed after exercise to break down the lactic acid built up during anaerobic respiration.
Sources
- GCSE Biology (8461) specification: subject content. AQA.
- Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028. Cambridge International.
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