Energy is needed for life processes such as:

  • growth and repair
  • movement
  • control of body temperature in mammals

Respiration is a chemical reaction that happens in all living cells, including plant cells and animal cells. It is the way that energy is released from glucose so that all the other chemical processes needed for life can happen. Do not confuse respiration with breathing (which is properly called ventilation).

Aerobic respiration

Glucose and oxygen react together in cells to produce carbon dioxide and water and releases energy. The reaction is called aerobic respiration because oxygen from the air is needed for it to work.

Here is the word equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water

Energy is released in the reaction. The mitochondria, found in the cell cytoplasm, are where most respiration happens.

Jon Chase explains aerobic respiration

Anaerobic respiration

During hard exercise, not enough oxygen can reach your muscle cells. So, aerobic respiration is replaced with anaerobic respiration. This does not need oxygen for it to happen.

Here is the word equation for anaerobic respiration in humans:

glucose → lactic acid

Anaerobic respiration produces much less energy than aerobic respiration. The waste product, lactic acid, builds up in the muscles causing pain and tiredness . This leads to cramp. Lactic acid is only broken down when you start aerobic respiration again.

Anaerobic respiration happens in microorganisms such as bacteria because they need to release energy from glucose too. Yeast, which are unicellular fungi, can carry out an anaerobic process called fermentation. Here is the word equation for fermentation:

glucose → ethanol + carbon dioxide

The ethanol (alcohol) is useful for brewers and wine-makers, and the carbon dioxide is useful to bakers because it helps their bread rise.

Bang Goes the Theory presenters Dallas and Liz undergo endurance tests to determine how fast they run during aerobic and anaerobic respiration

Aerobic and anaerobic respiration comparison

The table compares the main features of aerobic and anaerobic respiration:

Needs oxygen?YesNo
Needs glucose?YesYes
Product(s) formedCarbon dioxide and waterLactic acid

In addition, aerobic respiration releases much more energy per glucose molecule than anaerobic respiration does.

Sport and Exercise Science Professor Lars McNaughton talks about the different products of aerobic and anaerobic exercise.