The respiration which takes place without oxygen or without air is called anaerobic respiration. It is called anaerobic respiration because it takes place without oxygen or without air which contains oxygen (‘anaerobic’ means ‘without air’). The microscopic (which cannot see with naked eye) organisms like yeast and some bacteria obtain energy by anaerobic respiration (which is called fermentation). In anaerobic respiration, the micro-organisms like yeast break down glucose (food) into ethanol and carbon dioxide, and release energy. This energy is then used by the micro-organisms. This respiration produces much less energy which gets stored in the ATP molecules. The breaking down of glucose (food) during this respiration carried out by yeast (plants) can be represented as follows :
Glucose ————> Pyruvate —————-> 2C2H5OH + 2CO2 + 2ATP
please note that during anaerobic respiration (shown above), 1 molecule of glucose (food) produces only 2 energy-rich ATP molecules. A few organisms such as yeast plant and certain bacteria (called anaerobic bacteria) can obtain energy from food in the absence of oxygen by the process of anaerobic respiration. Those organisms which obtain energy by anaerobic respiration can live without oxygen (of air). For example, the single-celled, non-green plant called ‘yeast’ can live without oxygen because it obtain energy by the process of anaerobic respiration. From this discussion we conclude that all the cells do not use oxygen to produce energy. Energy can be produced in cells even without oxygen. The whole process of anaerobic respiration takes place in the cytoplasm of cells.
We can carry out the fermentation of sugar by using the anaerobic respiration of yeast as follows : Take some sugar solution (or fruit juice) in a test-tube and add a little of yeast to it and close the cork of the tube. Now, open the cock and smell. A characteristic smell of ethanol (ethyl alcohol) is obtained from the test-tube. A gas is also evolved during this process. When this gas is passed through lime-water (dilute solution of calcium hydroxide), the lime-water turns milky showing that it is carbon dioxide gas. This experiment tells us that the products of fermentation of sugar brought about by yeast are ethanol and carbon dioxide.
We (the human beings) obtain energy by aeroblic respiration or we need air for alive. But this respiration can sometimes take place in our muscles (or the muscles of other animals). For example, anaerobic respiration takes place in our muscles during vigorous physical exercise when oxygen gets used up faster in the muscle cells then can be supplied by the blood. When anaerobic respiration takes place in human muscles (or animals muscles), then glucose (food) is converted into lactic acid with the release of a small amount of energy. The breaking down of glucose (food) during this respiration in muscles can be represented as follows :
Glucose ————> Pyruvate ———————> 2 Lactic Acid + 2ATP
The sudden build up of lactic acid in our muscle during vigorous physical activity can cause muscular ‘cramps’. (The painful contractions of muscles are called cramps). Let us discuss this in a little more detail. During heavy physical exercise (or any other heavy physical activity), most of the energy in our muscles is produced by aerobic respiration. The anaerobic respiration by muscles brings about Partial breakdown (or small part) of glucose (food) to form lactic acid. This lactic acid accumulates in the the muscles. The accumulation of lactic acid in the muscles causes muscle cramps (pain in muscles). Thus, muscle cramps occur due to the accumulation of lactic acid in muscle when the muscles respire anaerobically (without oxygen) while doing hard physical exercise. We can get relief from cramps in muscles caused by heavy exercise by taking a hot water bath or a massage. When we bath from hot water (or massage) improves the circulation of blood in the muscles. Due to improved blood flow, the supply of oxygen to the muscles increases. This oxygen breakdown lactic acid accumulated in muscles into carbon dioxide and water, and hence gives up relief from cramps. This respiration does not take place only in the muscles of human beings, it also takes place in the muscles of the other animals such as lion, tiger, cheetah, deer, and many other animals who run fast when they Run very fast and require much more energy than normal. This means that even the animals like lion, tiger, cheetah and deer, etc., can get leg cramps due to the accumulation of lactic acid in leg muscle if they run very fast for a considerable time. Please not that :
(i) The anaerobic respiration in plants (like yeast) produces ethanol and carbon dioxide as end products.
(ii) The anaerobic respiration in animals muscles tissue produce lactic acid as the end product.
The similarity between aerobic and anaerobic respiration is that in both the cases, energy is produced by the breakdown of food like glucose.