Citric Acid Cycle Explanation.

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a series of biochemical reactions that occur in the mitochondria of eukaryotic cells and the cytosol of prokaryotic cells. The cycle plays a crucial role in the cellular respiration process, which generates energy in the form of ATP.

 

The cycle starts with the conversion of pyruvate, which is produced by glycolysis, into acetyl-CoA. This reaction is catalyzed by a complex of enzymes called the pyruvate dehydrogenase complex. During this reaction, one molecule of CO2 and two electrons are removed from pyruvate, which is then bound to Coenzyme A (CoA) to form acetyl-CoA.

 

Once acetyl-CoA enters the citric acid cycle, it combines with a four-carbon molecule called oxaloacetate to form a six-carbon molecule called citrate. This reaction is catalyzed by the enzyme citrate synthase. Citrate is then converted to isocitrate through a series of reactions catalyzed by aconitase and isocitrate dehydrogenase. During these reactions, two molecules of CO2 and four electrons are released, which are captured by the electron carrier molecules NAD+ and FAD to form NADH and FADH2, respectively.

 

Next, isocitrate is oxidized to alpha-ketoglutarate by isocitrate dehydrogenase. This reaction produces another molecule of CO2 and another NADH. Alpha-ketoglutarate is then converted to succinyl-CoA through a reaction catalyzed by alpha-ketoglutarate dehydrogenase. During this reaction, another molecule of CO2 and another molecule of NADH are produced.

 

Succinyl-CoA is then converted to succinate through a reaction catalyzed by succinyl-CoA synthetase. This reaction produces a molecule of GTP, which can be converted to ATP, and another molecule of CoA. Succinate is then oxidized to fumarate by succinate dehydrogenase, which is a component of the electron transport chain. This reaction produces another molecule of FADH2.

 

Finally, fumarate is converted to malate by fumarase, and malate is then oxidized to oxaloacetate by malate dehydrogenase. This reaction produces another molecule of NADH, and oxaloacetate is then available to react with acetyl-CoA to start the cycle again.

 

In summary, the citric acid cycle is a series of biochemical reactions that converts acetyl-CoA into CO2 and produces energy in the form of ATP, NADH, and FADH2. The cycle is essential for cellular respiration, and it plays a critical role in generating energy for the cell.

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