The two forms of respiration - aerobic and anaerobic - are both looked at in GCSE Biology. This quiz focusses on anaerobic respiration, in which glucose is broken down in the absence of oxygen, releasing energy - but also lactic acid.
Respiration is one of the key features of all living creatures. Aerobic respiration occurs where there is a plentiful supply of oxygen. Where the oxygen supply is more limited, anaerobic respiration takes place. During anaerobic respiration, considerably less energy is released by the cell. It involves the incomplete breakdown of glucose which produces lactic acid in the muscles of an animal, and alcohol plus carbon dioxide when it occurs in yeasts and bacteria.
During exercise, breathing rate increases to get more oxygen into the body which is transferred to the blood in the lungs. Heart rate increases to pump more blood round the body but, during hard exercise, the body is unable to supply sufficient oxygen to the muscle cells. This results in anaerobic respiration.
During this form of respiration, lactic acid is formed; only a small fraction of the energy is released from the glucose and so the muscles become more and more fatigued. Large quantities of lactic acid in muscles will cause them to contract suddenly - we call this cramp. Heart muscles don't fatigue as easily as the other muscles in your body as their cells contain more mitochondria than normal muscle cells and can continue to release energy through aerobic respiration even when the other muscles can't. After exercise, the heart and breathing rate remain high for a period of time as the body oxidises the lactic acid that has built up in the muscles. The amount of oxygen equired to do this is referred to as the oxygen debt
But anaerobic respiration isn't all bad - we rely on it to make bread, yoghurt and alcoholic drinks. Yeasts, and some bacteria, release energy by anaerobic respiration to produce carbon dioxide and ethanol. This is called fermentation and the carbon dioxide can be used to make bread 'rise' and to create the 'sparkle' in beers and fizzy wines. On a large scale, fermentation can be used to produce bioethanol, which is used as a sustainable fuel, or to release methane in a biodigester to produce biogas. Biogas is another sustainable fuel that is most commonly used in developing countries - although there are farmers in the UK who use the waste from their animals to provide this form of renewable energy to their farms.