Anaerobic respiration is the release of energy from a foodstuff in form cells can use in the absence of oxygen. It differs from aerobic respiration as it does not require oxygen. The word and symbol equations for anaerobic respiration in humans are as follows:

  • Glucose --> Lactic Acid
  • C6H12O6 --> 2C3H6O3

And in Yeast:

Anaerobic Respiration in Humans:[edit | edit source]

In humans, it is generally muscle tissue that respires anaerobically usually during exercise, when the body cannot take enough oxygen to the cells for respiration. This means not enough energy is made, and the muscles need more. Thus they do it without the presence of oxygen. However, when they have stopped exercising, often an oxygen debt has been created, due to the large amounts of lactic acid in the muscles. Therefore, humans often need to breathe heavily after exercising in order to remove the oxygen debt.

Anaerobic Respiration in Yeast:[edit | edit source]

This process is used in baking and brewing

Brewing – the production of alcoholic drinks by fermentation:[edit | edit source]

In brewing, anaerobic respiration is very important as it turns the glucose into a beer using yeast. Yeast is used in brewing in order to make carbon dioxide and ethanol. These are made by anaerobic respiration even if there is a good supply of oxygen. This process is called fermentation and is defined as the breakdown of sugars by bacteria and yeasts by respiration without oxygen.

The first step of fermentation in brewing is where complex sugars or polysaccharides are broken down into monosaccharides, for example, glucose by yeast. The glucose is then broken down into ethanol and carbon dioxide in a decomposition reaction, where energy is also produced – although there is less energy produced than in aerobic respiration. This step is called ethanol fermentation, whereas lactic acid fermentation is another form of anaerobic respiration that happens in animal cells in the absence of oxygen.

The chemical reaction for ethanol fermentation is:

C6H12O6 → 2CH3CH2OH + 2CO2

Word equation: glucose --> ethanol + carbon dioxide + energy

The glucose in beer brewing is from grain starches turned into sugars by amylase – an enzyme. The products are then the alcohol (ethanol) in beer and wine, and the bubbles (carbon dioxide) in champagne and beer.

Bread making:[edit | edit source]

This employs exactly the same process as brewing, except in this it is the carbon dioxide that makes the bread rise – as the bubbles of CO2 get caught in the bread, while the ethanol evaporates from the dough when it is baked. The yeast ‘gets’ the glucose from the dough.

Unleavened bread, such as pitta bread, does not use yeast and therefore they do not rise, as there is no carbon dioxide.

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