Fermentation

What is fermentation?

Fermentation is a type of anaerobic respiration: it releases energy from glucose without oxygen. Because glucose is only partly broken down, fermentation releases much less energy than aerobic respiration — only about 2 ATP per glucose molecule.

Two kinds of fermentation

• Alcoholic fermentation (in yeast and some plant cells): glucose → ethanol + carbon dioxide + a little energy.
• Lactic acid fermentation (in animal muscle cells): glucose → lactic acid + a little energy.

In the human body

During very hard exercise, muscles may not get enough oxygen. They then carry out lactic acid fermentation to keep making some ATP. The lactic acid that builds up causes muscle fatigue and cramps. After exercise, extra oxygen (the "oxygen debt") is taken in to break down the lactic acid.

Uses in industry

Fermentation by yeast is used in baking (the carbon dioxide makes bread rise) and in brewing and wine-making (the ethanol produces alcohol). Bacteria are used to make yoghurt and other fermented foods.

Why fermentation gives so little energy

In fermentation, glucose is only partly broken down. Without oxygen, the later energy-rich stages that take place in the mitochondria cannot run, so most of the chemical energy stays locked inside the ethanol or lactic acid produced. That is why fermentation yields only about 2 ATP per glucose, compared with about 38 ATP in aerobic respiration. Fermentation is therefore a useful emergency way of getting energy quickly when oxygen is short, but it cannot supply enough ATP to keep an active animal going for long.