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Animation: The Citric Acid Cycle

by Pearson
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The oxidation of glucose continues in the citric acid cycle. Pyruvate molecules formed during glycolysis are transported from the cytosol into the mitochondrion, but pyruvate itself does not enter the citric acid cycle. A reaction occurs that removes a carbon atom, releasing it in carbon dioxide. Electrons are transferred to an NADH molecule, storing energy. Coenzyme A, or CoA, joins with the 2-carbon fragment, forming acetyl CoA. One molecule of acetyl CoA enters the citric acid cycle. The 2-carbon fragment of acetyl CoA attaches to the 4-carbon molecule oxaloacetate in the first reaction of the cycle. This forms citrate. In a series of steps, bonds break and reform. Two carbon atoms are released, one at a time, in molecules of carbon dioxide. Electrons are carried off by molecules of NADH and FADH2. One step produces an ATP molecule by substrate-level phosphorylation. A 4-carbon oxaloacetate molecule is regenerated. Since two acetyl CoA molecules are produced for each glucose molecule broken down, a second acetyl CoA enters the citric acid cycle. The same series of reactions occurs, releasing carbon dioxide and producing more NADH, FADH2, and ATP. The cell has gained two ATPs that can be used directly. However, most of the energy originally contained in the bonds of glucose is now carried by the NADH and FADH2 molecules.
The oxidation of glucose continues in the citric acid cycle. Pyruvate molecules formed during glycolysis are transported from the cytosol into the mitochondrion, but pyruvate itself does not enter the citric acid cycle. A reaction occurs that removes a carbon atom, releasing it in carbon dioxide. Electrons are transferred to an NADH molecule, storing energy. Coenzyme A, or CoA, joins with the 2-carbon fragment, forming acetyl CoA. One molecule of acetyl CoA enters the citric acid cycle. The 2-carbon fragment of acetyl CoA attaches to the 4-carbon molecule oxaloacetate in the first reaction of the cycle. This forms citrate. In a series of steps, bonds break and reform. Two carbon atoms are released, one at a time, in molecules of carbon dioxide. Electrons are carried off by molecules of NADH and FADH2. One step produces an ATP molecule by substrate-level phosphorylation. A 4-carbon oxaloacetate molecule is regenerated. Since two acetyl CoA molecules are produced for each glucose molecule broken down, a second acetyl CoA enters the citric acid cycle. The same series of reactions occurs, releasing carbon dioxide and producing more NADH, FADH2, and ATP. The cell has gained two ATPs that can be used directly. However, most of the energy originally contained in the bonds of glucose is now carried by the NADH and FADH2 molecules.