Aerobic and anaerobic respiration

Aerobic respiration

Respiration using oxygen to break down food molecules is called aerobic respiration. Glucose is the molecule normally used for respiration - it is the main respiratory substrate. Glucose is oxidised to release its energy.

The word equation for aerobic respiration is:

 \text{glucose} + \text{oxygen} \rightarrow \text{carbon~dioxide} + \text{water}   \text{~(+energy~released)}

You need to be able to recognise the chemical symbols:

 C_{6}H_{12}O_{6} +	6O_{2} \rightarrow 6CO_{2} + 6H_{2}O  \text{~(+energy~released)}

Respiration is a series of reactions but this summarises the overall process.

The first stages of respiration occur in the cytoplasm of cells, but most of the energy released is in the mitochondria.

Anaerobic respiration

Most organisms cannot respire without oxygen but some organisms and tissues can continue to respire if the oxygen runs out. Oxygen may be in short supply because during vigorous exercise the muscles need additional energy. So:

  • the breathing rate and volume of each breath increases to bring more oxygen into the body and remove the carbon dioxide produced
  • the heart rate increases to supply the muscles with extra oxygen and remove the carbon dioxide produced

If insufficient oxygen is available to the muscles the organisms use the process of anaerobic respiration.

In anaerobic respiration the glucose in muscle is converted to lactic acid:

 \text{glucose} \rightarrow \text{lactic~acid} \text{~(+energy released)}

Anaerobic respiration occurs only in the cytoplasm of cells.

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

The lactic acid that builds up needs to be oxidised to carbon dioxide and water.

The creation of lactic acid - which needs oxygen to be broken down - generates an oxygen debt, known as excess post-exercise oxygen consumption (EPOC). This needs to be 'repaid' after the exercise stops, and is why we keep on breathing deeply for a few minutes after we have finished exercising.

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 produced energy without the oxygen it would normally have used performing low intensity exercise like slow steady running (aerobic exercise).

The difference between the oxygen the body needs during the sudden sprint and what was actually taken in is oxygen deficit.

Yeast also respire anaerobically. The glucose is broken down and ethanol and carbon dioxide are produced.

Aerobic and anaerobic respiration compared:

Aerobic respirationAnaerobic respiration
Presence of oxygenPresent.Absent or in short supply.
Oxidation of glucoseComplete.Incomplete. The products of respiration still contain energy.
Products of respirationCarbon dioxide and water. The products do not contain stored chemical energy.Mammalian muscle: lactic acid. Yeast: ethanol and carbon dioxide. Some plants: ethanol and carbon dioxide. The products still contain stored chemical energy.
Amount of energy releasedRelatively large amount.Small amount, but released quickly.
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