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Many people today are justly concerned about the environmental consequences of human activities. But in the past other organisms have caused revolutionary changes in the earth's environment (although very much more slowly). Nowhere is this more apparent than in the composition of the earth's atmosphere, which through oxygen-releasing photosynthesis was transformed from a mixture containing practically no molecular oxygen to one in which oxygen constitutes 21% of the total (Figure 1-17).
Since oxygen is an extremely reactive chemical that can interact with most cytoplasmic constituents, it must have been toxic to many early organisms, just as it is to many present-day anaerobic bacteria. However, this reactivity also provides a source of chemical energy, and, not surprisingly, this has been exploited by organisms during the course of evolution. By using oxygen, organisms are able to oxidize more completely the molecules they ingest. For example, in the absence of oxygen glucose can be broken down only to lactic acid or ethanol, the end products of anaerobic glycolysis. But in the presence of oxygen glucose can be completely degraded to CO2 and H2O. In this way much more energy can be derived from each gram of glucose. The energy released in respiration - the aerobic oxidation of food molecules - is used to drive the synthesis of ATP in much the same way that photosynthetic organisms produce ATP from the energy of sunlight. In both processes there is a series of electron-transfer reactions that generates an H+ gradient between the outside and inside of a membrane-bounded compartment; the H+ gradient then serves to drive the synthesis of the ATP. Today, respiration is used by the great majority of organisms, including most procaryotes.