Anaerobic respiration

Unlike aerobic respiration, anaerobic respiration does not need oxygen. It is the release of a relatively small amount of energy in cells by the breakdown of food substances in the absence of oxygen.

Anaerobic respiration in muscles

Anaerobic respiration happens in muscles during hard exercise:

glucose → lactic acid

C6H12O6 → 2C3H6O3

Glucose is not completely broken down, so much less energy is released than during aerobic respiration.

There is a build-up of lactic acid in the muscles during vigorous exercise. The lactic acid needs to be oxidised to carbon dioxide and water later.

This causes an oxygen debt - known as excess post-exercise oxygen consumption (EPOC) - that needs to be ‘repaid’ after the exercise stops. This is why we keep on breathing deeply for a few minutes after we have finished exercising.

Anaerobic respiration in plants and yeast

Anaerobic respiration also happens in plant cells and some microorganisms. Anaerobic respiration in yeast is used during brewing and bread-making:

glucose → ethanol + carbon dioxide

C6H12O6 → 2C2H5OH + 2C02

Ethanol is the alcohol found in alcoholic drinks like beer and wine. In bread-making, bubbles of carbon dioxide gas expand the dough and help the bread rise.

Two bread tins with dough. One has risen more than the other
Anaerobic respiration by yeast helps bread dough rise

Aerobic respiration vs anaerobic respiration

The table summarises some differences between the two types of respiration.

OxygenNeededNot needed
Glucose breakdownCompleteIncomplete
End product(s)Carbon dioxide and waterAnimal cells: lactic acid. Plant cells and yeast: carbon dioxide and ethanol
Energy releasedRelatively large amountRelatively small amount

Aerobic respiration releases 19 times more energy than anaerobic respiration from the same amount of glucose.