Up to this point, we have followed a single metabolic path: the breakdown of glucose in the presence of oxygen to produce carbon dioxide and water. However, metabolic pathways more often resemble intersecting roads rather than a single, linear path. We saw this earlier in the discussion of the citric acid cycle, where intermediates in the cycle often feed into other metabolic pathways.

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One of the major forks in the metabolic road occurs at pyruvate, the end product of glycolysis (section 10.2). When oxygen is present, it is converted to acetyl-CoA, which then enters the citric acid cycle, resulting in the production of ATP and reduced electron carriers to fuel the electron transport chain, as we saw. When oxygen is not present, however, pyruvate is metabolized along a number of different pathways. These pathways occur in many living organisms today and played an important role in the early evolution of life on Earth.