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Living without Oxygen
Some fungi and bacteria are able to respire without oxygen.
Certain types of fungi (singular: fungus), especially those called yeasts, respire like this. This process is called fermentation or anaerobic respiration. Anaerobic means "without oxygen". You might have guessed that respiration which requires oxygen is known as aerobic respiration.
Only microbes, such as yeasts and bacteria, can live for long periods without oxygen. Fermenting yeasts still produce carbon dioxide, but they also produce something else - alcohol.
Yeasts are used to make the alcohol that we find in wine, beer or cider. They are also used to make bread and certain cakes 'rise' because when they ferment, they produce a lot of carbon dioxide which forms bubbles of gas in the dough. When the bread is baked the yeasts are killed but the gas bubbles are left, giving the bread its spongy texture.
Alcohol and carbon dioxide are produced by yeasts which are fermenting but other microscopic organisms, called bacteria (singular: bacterium) make lactic acid when they ferment. These bacteria are used when we make cheese and yoghurt from milk.
Anaerobic Respiration in Humans
A human cannot live without oxygen for long. Many parts of our bodies, such as the brain and the heart, are very sensitive and they need a constant supply of oxygen. It is true, however, that some muscles in our bodies can survive without oxygen for a short time. When you do a lot of exercise your muscles may not receive enough oxygen from the blood. The muscles start to respire anaerobically and they produce lactic acid. You cannot tolerate this for long. Your muscles will develop cramps and you will have to stop for a while so that the oxygen can return.