2 (Phosphoenolpyruvic acid)

2 (Pyruvic acid)

Net reaction per molecule of glucose:

Glucose + 2ADP + 2PO4- ->-2 Pyruvic acid + 2ATP + 4H

Figure 67-5

Sequence of chemical reactions responsible for glycolysis.

From this reaction, it can be seen that two carbon dioxide molecules and four hydrogen atoms are released, while the remaining portions of the two pyruvic acid molecules combine with coenzyme A, a derivative of the vitamin pantothenic acid, to form two molecules of acetyl-CoA. In this conversion, no ATP is formed, but up to six molecules of ATP are formed when the four released hydrogen atoms are later oxidized, as discussed later.

Citric Acid Cycle (Krebs Cycle)

The next stage in the degradation of the glucose molecule is called the citric acid cycle (also called the tricarboxylic acid cycle or Krebs cycle). This is a sequence of chemical reactions in which the acetyl portion of acetyl-CoA is degraded to carbon dioxide and hydrogen atoms. These reactions all occur in the matrix of the mitochondrion. The released hydrogen atoms add to the number of these atoms that will subsequently be oxidized (as discussed later), releasing tremendous amounts of energy to form ATP.

Figure 67-6 shows the different stages of the chemical reactions in the citric acid cycle. The substances to the left are added during the chemical reactions, and the products of the chemical reactions are shown to the right. Note at the top of the column that the cycle begins with oxaloacetic acid, and at the bottom of the chain of reactions, oxaloacetic acid is formed again. Thus, the cycle can continue over and over.

In the initial stage of the citric acid cycle, acetyl-CoA combines with oxaloacetic acid to form citric acid. The coenzyme A portion of the acetyl-CoA is released and can be used again and again for the formation of still more quantities of acetyl-CoA from pyruvic acid. The

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