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Chapter: Biochemistry: The Citric Acid Cycle

The Central Role of the Citric Acid Cycle in Metabolism

The Central Role of the Citric Acid Cycle in Metabolism
The evolution of aerobic metabolism, by which nutrients are oxidized to carbon dioxide and water, was an important step in the history of life on the Earth.

The Central Role of the Citric Acid Cyclein Metabolism

The evolution of aerobic metabolism, by which nutrients are oxidized to carbon dioxide and water, was an important step in the history of life on the Earth. Organisms can obtain far more energy from nutrients by aerobic oxidation than by anaerobic oxidation. (Even yeast—which is usually thought of in terms of the anaerobic reactions of alcoholic fermentation and is responsible for producing bread, beer, and wine—uses the citric acid cycle and aerobically degrades glucose to carbon dioxide and water.) We saw that glycolysis produces only two molecules of ATP for each molecule of glucose metabolized. In this Three processes play roles in aerobic metabolism: the citric acid cycle, which we discuss, and electron transport and oxidative phosphorylation, both of which we shall discuss (Figure19.1).


Metabolism consists of catabolism, which is the oxidative breakdown of nutrients, and anabolism, which is reductive synthesis of biomolecules. The citric acid cycle is amphibolic, meaning that it plays a role in both catabolism and anabolism. Although the citric acid cycle is a part of the pathway of aero-bic oxidation of nutrients, some of the molecules that are included in this cycle are the starting points of biosynthetic (anabolic) pathways. Metabolic pathways operate simultane-ously, even though we talk about them separately. We should always keep this point in mind.

The citric acid cycle has two other common names. One is the Krebs cycle, after Sir Hans Krebs, who first investigated the pathway (work for which he received a Nobel Prize in 1953). The other name is the tricarboxylic acid cycle(or TCA cycle), from the fact that some of the molecules involved are acids withthree carboxyl groups. We shall start our discussion with a general overview of the pathway and then go on to discuss specific reactions.

Summary

The citric acid cycle is amphibolic. It plays a role in both catabolism and anabolism. It is the central metabolic pathway.


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