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The Krebs Cycle

Pyruvic acid must now enter a cyclic series of reaction, the Krebs cycle, also known as the citric acid cycle or the TCA (tricarboxylic acid) cycle. The reactions of the Krebs cycle take place in the mitochondrial matrix, and are summarized in Figure 4.3. Again, it is inadvisable to attempt to memorize all of these reactions. Before pyruvic acid can enter the cycle, it must undergo some initial preparatory steps:
  • Pyruvic acid is oxidized, releasing one molecule of CO2 and a two-carbon acetyl group.
  • The acetyl group is attached to a molecule called Coenzyme A, to form acetyl CoA.
  • In the process, a molecule of NAD+ is reduced to NADH.
It is important to note that carbon atoms from pyruvic acid can enter the Krebs cycle only in the form of acetyl CoA.
Next, the acetyl group is donated to the four-carbon molecule oxaloacetic acid to form citric acid, a six-carbon compound. The citric acid is subsequently broken down, in a series of steps, back to oxaloacetic acid. As with glycolysis, there are three main points to keep in mind regarding the Krebs cycle. Specifically, for each turn of the cycle: 
  • Two carbons enter the cycle as an acetyl group, and two carbons are released as carbon dioxide.
  • One molecule of ATP is harvested.
  • Three molecules of NAD+ are reduced to NADH, and one molecule of another coenzyme, FAD, is reduced to FADH2
Highlights of the Krebs cycle
It is important to remember that for every glucose molecule we started with, two molecules of pyruvic acid were created. Since we have only been keeping track of one pyruvic acid molecule as it travels through the Krebs cycle, we must multiply our totals by two if wish to know the yield per molecule of glucose. If we consider the preparatory events with those of the Krebs cycle itself, we can summarize as follows:
For each three-carbon molecule of pyruvic acid that enter the mitochondria, three molecules of carbon dioxide are released, for a total of six carbon dioxides. This is the source of the CO2 we exhale.
A total of two molecules of ATP are harvested.
Overall, eight molecules of NAD+ are reduced to NADH, and two FAD molecules are reduced to FADH2.

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