Anaerobic respiration

Anaerobic respiration does not need oxygen (unlike aerobic respiration). 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.

\text{glucose} \rightarrow \text{lactic acid + energy in the form of ATP}

Glucose is not completely broken down, so 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.

The creation of lactic acid (which needs oxygen to be broken down) generates an oxygen debt 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.

Graph showing oxygen consumption over time during exercise. Labelled on the x axis are Start exercise, End exercise and End recovery. Labelled on the curve are Oxygen deficit, debt and requirement.

A short intense burst of exercise such as sprinting will generate energy anaerobically so an oxygen debt will be generated. This is because your body will have released energy without the oxygen it would normally have used performing low intensity exercise like slow, steady running.

The difference between the oxygen the body needs during the sudden sprint and what it actually managed to take in is called oxygen deficit.

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 amount from each glucose moleculeRelatively small amount from each glucose molecule

Aerobic respiration produces 38 molecules of ATP, but anaerobic respiration produces only two molecules of ATP. In comparison to aerobic respiration, what percentage of ATP is produced by anaerobic respiration?

2 ÷ 38 × 100 = 5%