Concept27 - Concept 9.3 The citric acid cycle completes the...

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Concept 9.3 The citric acid cycle completes the energy-yielding oxidation of organic molecules More than three-quarters of the original energy in glucose is still present in the two  molecules of pyruvate. If oxygen is present, pyruvate enters the mitochondrion where enzymes of the citric acid  cycle complete the oxidation of the organic fuel to carbon dioxide. After pyruvate enters the mitochondrion via active transport, it is converted to a compound  called acetyl coenzyme A or  acetyl CoA. This step is accomplished by a multienzyme complex that catalyzes three reactions: A carboxyl group is removed as CO2. The remaining two-carbon fragment is oxidized to form acetate. An enzyme transfers  the pair of electrons to NAD+ to form NADH. Acetate combines with coenzyme A to form the very reactive molecule acetyl CoA. Acetyl CoA is now ready to feed its acetyl group into the citric acid cycle for further 
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