recap

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9.2 recap

The oxidation of glucose in the presence of O2 involves glycolysis, pyruvate oxidation, and the citric acid cycle. In glycolysis, glucose is converted to pyruvate with some energy capture. Pyruvate is oxidized first to acetyl CoA by pyruvate dehydrogenase, then completely to CO2 by the citric acid cycle, releasing energy that is captured in the form of reduced electron carriers.

learning outcomes

You should be able to:

  • Describe the energy inputs and outputs of glycolysis.

  • Explain how pyruvate oxidation connects the transformations of glycolysis and the citric acid cycle.

  • Compare and contrast glycolysis and the citric acid cycle.

Question 1

What is the net energy yield of glycolysis in terms of energy in ATP and reduced coenzymes invested and energy harvested?

In glycolysis, the net yield is 2 ATP per glucose and 1 NADH per glucose.

Question 2

What role does pyruvate oxidation play in relation to the citric acid cycle?

Pyruvate oxidation produces the two-carbon molecule acetyl CoA, which is activated and then participates as a first substrate in the citric acid cycle.

Question 3

How is the citric acid cycle a “cycle” but glycolysis is not?

The citric acid cycle regenerates the four-carbon acceptor molecule for the next acetyl CoA from pyruvate. Glycolysis does not regenerate a starting material.

Pyruvate oxidation and the citric acid cycle cannot continue operating unless O2 is available to receive electrons during the reoxidation of reduced electron carriers. However, these electrons are not passed directly to O2, as you will learn next.