Aerobic vs Anaerobic Respiration

Aerobic respiration uses oxygen and transfers more energy from glucose. Anaerobic respiration happens without oxygen and transfers much less energy.

Aerobic respiration

Aerobic means with oxygen. It occurs continuously in living cells when enough oxygen is available.

The word equation is:

$$glucose+oxygen\to carbon\; dioxide+water$$

Glucose is fully oxidised, so a relatively large amount of energy is transferred. Organisms use this energy for movement, keeping warm and building larger molecules.

Anaerobic respiration

Anaerobic means without oxygen. It occurs when cells cannot obtain enough oxygen, such as in human muscles during vigorous exercise.

In muscle cells:

$$glucose\to lactic\; acid$$

Glucose is only partly oxidised, so much less energy is transferred than during aerobic respiration. Lactic acid builds up and contributes to muscle fatigue.

In yeast and plant cells, the products are different:

$$glucose\to ethanol+carbon\; dioxide$$

Anaerobic respiration in yeast is called fermentation. It is used in making bread and alcoholic drinks.

Worked example

A runner is sprinting and their muscles are not receiving enough oxygen. Which type of respiration increases?

Answer: Anaerobic respiration increases because it does not require oxygen. It transfers some energy from glucose but produces lactic acid and transfers much less energy than aerobic respiration.

Common exam mistake

Do not say that anaerobic respiration produces carbon dioxide in human muscles. Human muscle cells produce lactic acid. Carbon dioxide is produced during anaerobic respiration in yeast and plant cells.

Also, respiration is not the same as breathing. Respiration is a chemical process in cells that transfers energy, whereas breathing moves air into and out of the lungs.

This answer assumes Higher tier for AQA GCSE Biology, but the comparison above is required at both Foundation and Higher tiers and is not biology-only content.