Oxygen, Energy and Wastefulness
The question arises why living organisms use atmospheric oxygen if energy can be obtained by mere fermentation. There are many important reasons for this. Fermentation never results in the complete oxidation of a substance and, therefore, little energy is released. If one gram-molecule of glucose is completely oxidized to carbon dioxide and water, 673 large calories will be obtained. But with fermentation, which results in the formation of ethyl alcohol and carbon dioxide, only as little as 25 large calories will be released, i. e. almost 27 times less. This means that anaerobes have to use 27 times as much glucose as aerobes to obtain the same amount of energy. The difference is, of course, appreciable and nature cannot tolerate such wastefulness.
Another important reason is that substances such as ethyl and butyl alcohol, lactic and butyric acid, acetone, etc., which are bad for the organism, are formed as a result of fermentation. It is not easy to dispose of these harmful substances.
Respiration frequently produces combustible gases. Microorganisms often release hydrogen. This is how microbes living in the intestine of termites breathe. Of the multicellular creatures, the larvae of some flies, in particular, release a great deal of hydrogen. Some organisms liberate not only hydrogen, but also methane and other gases, some of which are still not known, including spontaneously inflammable gases. It is a particularly beautiful sight when the gases, which have collected in the silt at the bottom of a pool, rise to the surface of the water and burn with a mysterious bluish flame.
How then have animals managed to change their way of breathing to such an extent and adapt themselves to an absence of oxygen? This did not prove difficult. At the dawn of life on the Earth there was little free oxygen and the earliest living creatures had to become anaerobes. It was not until the atmosphere became rich in oxygen that animals learned to burn energy-forming products completely. At the same time, the anaerobic method of breathing did not disappear but was passed on and finally came down to us.
As has been mentioned at the beginning, in all animals without exception the first stages of energy release proceed without oxygen. When aerobic animals felt like returning to the places where no oxygen could be obtained, they again had to restrict themselves to partial utilization of the energy contained in nutrient substances. To do this they had to remember how to render partially oxidized products harmless.
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