Lactic acid, ethyl alcohol, or other compounds
Many of the reactions in cellular respiration are redox reactions. Recall that in a redox reaction, one reactant is oxidized (loses electrons) while another is reduced (gains electrons). Although many kinds of organic compounds can be oxidized in cellular respiration, it is customary to focus on the simple sugar called glucose (C6H12O6). The following equation summarizes cellular respiration:
This equation, however, does not explain how cellular respiration occurs. It is useful to examine each of the two stages, summarized in Figure 7-2a. (Figure 7-2b illustrates the differences between cellular respiration and fermentation.) The first stage of cellular respiration is glycolysis.
Glycolysis is a biochemical pathway in which one six-carbon molecule of glucose is oxidized to produce two three-carbon molecules of pyruvic acid. Like other biochemical pathways, glycolysis is a series of chemical reactions catalyzed by specific enzymes. All of the reactions of glycolysis take place in the cytosol and occur in four main steps, as illustrated in Figure 7-3 on the next page.
In step O, two phosphate groups are attached to one molecule of glucose, forming a new six-carbon compound that has two phosphate groups. The phosphate groups are supplied by two molecules of ATP, which are converted into two molecules of ADP in the process.
In step ©, the six-carbon compound formed in step Q is split into two three-carbon molecules of glyceraldehyde 3-phosphate (G3P). Recall that G3P is also produced by the Calvin cycle in photosynthesis.
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