Final answer:
Molecules of three-carbon pyruvate are oxidized into the two-carbon compound acetyl Coenzyme A (acetyl CoA) through the action of the enzyme pyruvate dehydrogenase during the bridge reaction.
Step-by-step explanation:
During the bridge reaction, molecules of pyruvate, which have three carbons, are oxidized to form a two-carbon compound. This compound is acetyl Coenzyme A (acetyl CoA). The enzyme pyruvate dehydrogenase converts pyruvate into acetyl CoA through an oxidative decarboxylation reaction, where a carbon is removed from pyruvate as carbon dioxide. Furthermore, during this process, two high-energy electrons are transferred to NAD+ to form NADH. Acetyl CoA is then ready to enter the Krebs cycle, where it combines with oxaloacetate to continue cellular respiration.
The conversion of three-carbon pyruvate molecules into two-carbon acetyl groups that can enter the Krebs cycle is known as the bridge reaction or the transition reaction. During this reaction, pyruvate is oxidized and decarboxylated, resulting in the formation of acetyl coenzyme A (acetyl CoA). This process takes place in the mitochondrial matrix, where the enzyme pyruvate dehydrogenase converts pyruvate into acetyl CoA. This reaction releases carbon dioxide and transfers two electrons that combine with NAD+ to form NADH.