The Krebs Cycle

The Krebs cycle is a biochemical pathway that breaks down acetyl CoA, producing CO2, hydrogen atoms, and ATP. The reactions that make up the cycle were identified by Hans Krebs (1900-1981), a German biochemist. The Krebs cycle has five main steps. In eukaryotic cells, all five steps occur in the mitochondrial matrix. Examine Figure 7-10 as you read about the steps in the Krebs cycle.

In step Q, a two-carbon molecule of acetyl CoA combines with a four-carbon compound, oxaloacetic (AHKS-uh-loh-uh-SEET-ik) acid, to produce a six-carbon compound, citric (SI-trik) acid. Notice that this reaction regenerates coenzyme A.

In step ©, citric acid releases a CO2 molecule and a hydrogen atom to form a five-carbon compound. By losing a hydrogen atom with its electron, citric acid is oxidized. The electron in the hydrogen atom is transferred to NAD+, reducing it to NADH.

figure 7-10

The Krebs cycle takes place in the mitochondrial matrix and involves five main steps.

Mitochondrial matrix

Acetyl CoA C C

Oxaloacetic acid

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