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Anaerobic Respiration. All organisms need energy to survive. Animals obtain their energy from the food they eat, but plants can make their own food by photosynthesis.

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All organisms need energy to survive.

Animals obtain their energy from the food they eat, but plants can make their own food by photosynthesis.

In both cases, however, energy must first be converted into a form that can easily be used by cells. This process is called respiration.


Aerobic respiration uses oxygen to break down glucose. It releases a lot of energy from each glucose molecule by breaking it down completely into carbon dioxide and water.

Anaerobic respiration takes place without oxygen.

Less energy is released per glucose molecule than in aerobic respiration because glucose is only partially broken down.

respiration in animals
Respiration in animals

carbon dioxide







( energy)


( energy)


lactic acid

When an animal cell is getting enough oxygen and glucose, it carries out aerobic respiration.

When the animal cell does not have the necessary oxygen to break down glucose molecules, it has to carry out anaerobic respiration. This can occur in muscle cells during strenuous exercise.


 carbon dioxide + ethanol (+ energy)


Plants and some micro-organisms, such as yeast, will also carry out anaerobic respiration if necessary.

Anaerobic respiration might take place in waterlogged root cells, or in bacteria infecting deep puncture wounds.

The products of anaerobic respiration are different in plants than in animals.

lactic acid
Lactic acid

The incomplete breakdown of glucose during anaerobic respiration produces lactic acid.

Lactic acid builds up in muscle cells and prevents the muscles from contracting efficiently. The build-up of lactic acid can cause fatigue, pain and cramping.

After vigorous exercise, the body needs to remove lactic acid before it damages cells. Some lactic acid is broken down in the muscles. Some passes into the bloodstream and is taken to the liver to be broken down there.

oxygen debt
Oxygen debt

Oxygen is needed to break lactic acid into water and carbon dioxide.

Following strenuous exercise, an individual breathes heavily and maintains an elevated heart rate. This moves lactic acid to the liver and supplies liver and muscle cells with the necessary oxygen to break down lactic acid.

The amount of oxygen needed to break down lactic acid remaining after exercise is called excess post-exercise oxygen consumption (EPOC), oran oxygen debt.


Respiring micro-organisms are used in the commercial production of many different products, including:

  • bread
  • alcohol
  • biofuels.

These micro-organisms break substances down using aerobic or anaerobic respiration. The effect of anaerobic respiration is often called fermentation.


Yeast is a type of fungus used in bread production. Yeast digests the carbohydrates in flour, producing carbon dioxide.

This carbon dioxide causes bread to rise. It also creates gas pockets in the dough, giving baked bread a spongy texture.

At first the yeast respires aerobically, but once it uses up the available oxygen it begins to respire anaerobically.


Yeast is used to make alcoholic beverages such as beer, wine, rum, whiskey and vodka.

During fermentation, anaerobic respiration in yeast cells converts glucose into ethanol.

The amount of ethanol produced is limited as alcohol becomes toxic to microbes at certain concentrations.

Glucose comes from different sources in the production of different alcohols. For example, in winemaking, natural sugars in the grapes form the energy source for yeast.