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Anaerobic respiration in animals

Most organisms cannot respire without oxygen but some organisms and tissues, including muscles, can continue to respire if the oxygen runs out. These organisms and tissues use the process of anaerobic respiration.

During anaerobic respiration, the oxidation of glucose is incomplete. The reaction releases much less energy – around 1/19th of the energy released during aerobic respiration.

Human muscle can respire anaerobically for short periods of time – even though the process is relatively inefficient, it's better to continue respiring and be able to run away from danger – or run a race.

The glucose in muscle is converted to lactic acid:

glucose → lactic acid + energy released

The reactant in this reaction is glucose. The product is lactic acid.

During long periods of vigorous activity:

• lactic acid levels build up
• glycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver

As body stores of glycogen become low, the person suffers from muscle fatigue.

Oxygen debt

When a period of exercise is over, lactic acid must be removed from the body. The body's tolerance of lactic acid is limited.

Lactic acid is taken to the liver by the blood, and either:

• oxidised to carbon dioxide and water, or
• converted to glucose, then glycogen - glycogen levels in the liver and muscles can then be restored

These processes require oxygen. This is why, when the period of activity is over, a person’s breathing rate and heart rate do not return to normal straightaway.

The amount of oxygen required to remove the lactic acid, and replace the body's reserves of oxygen, is called the oxygen debt.

When someone who has been exercising pays back an oxygen debt, it can take between a few hours for normal exercise, to several days after a marathon.

Anaerobic respiration in fungi and plants

Some plants and fungi such as yeast can respire anaerobically – it's preferable to release less energy but remain alive.

Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply as 'alcohol':

glucose → ethanol + carbon dioxide + energy released

The reactant in this reaction is glucose. The products are ethanol and carbon dioxide.

Anaerobic respiration is economically important – many foods are produced by microorganisms respiring anaerobically.

Yeast is used to make alcoholic drinks. When yeast cells are reproducing rapidly during beer or wine production, the oxygen runs out. The yeast switches to anaerobic respiration. Ethanol and carbon dioxide are produced.

Yeast can also be used to produce bread. Yeast respires using sugar added to the dough. Bubbles of carbon dioxide make the bread rise. The alcohol that is produced evaporates as the bread is baked.

Aerobic and anaerobic respiration compared