Unlocking the Power of Cellular Respiration: The Science Behind the Citric Acid Cycle

Citric acid cycle

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a vital metabolic pathway that occurs in the mitochondria of eukaryotic cells

The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a vital metabolic pathway that occurs in the mitochondria of eukaryotic cells. It serves as the central hub for the oxidation of carbohydrates, fats, and proteins, producing energy-rich molecules necessary for cellular functions.

The cycle begins with the entry of acetyl-CoA, which is derived from the breakdown of carbohydrates or fatty acids. Acetyl-CoA combines with a molecule called oxaloacetate to form citrate, catalyzed by an enzyme called citrate synthase. This reaction also releases coenzyme A, which is then recycled to pick up another acetyl group.

Next, citrate undergoes a series of enzyme-catalyzed reactions, which involve a number of intermediate molecules. These reactions result in the regeneration of oxaloacetate, along with the production of high-energy compounds such as ATP, NADH, and FADH2.

The key reactions in the citric acid cycle include the following:

1. Citrate is converted to isocitrate by aconitase, involving the rearrangement of a hydroxyl group.
2. Isocitrate is oxidized by isocitrate dehydrogenase, resulting in the release of CO2 and the production of NADH and alpha-ketoglutarate.
3. Alpha-ketoglutarate is further oxidized by alpha-ketoglutarate dehydrogenase, releasing another CO2 molecule, generating NADH and producing succinyl-CoA.
4. Succinyl-CoA reacts with a molecule of GDP and inorganic phosphate to produce GTP and succinate, catalyzed by succinyl-CoA synthetase.
5. Succinate is oxidized by succinate dehydrogenase, generating FADH2 and fumarate.
6. Fumarate is hydrated by fumarase to form malate.
7. Malate is oxidized by malate dehydrogenase, producing NADH and regenerating oxaloacetate.

The net result of one cycle of the citric acid cycle is the production of three NADH, one FADH2, one GTP/ATP, and the regeneration of one molecule of oxaloacetate. The NADH and FADH2 generated in the cycle serve as electron carriers that donate their electrons to the electron transport chain. This generates a proton gradient across the inner mitochondrial membrane, which is used to produce ATP through oxidative phosphorylation.

The citric acid cycle plays a crucial role in cellular respiration, generating energy for various metabolic processes. It is also the source of several important intermediates that are used in other metabolic pathways, including amino acid synthesis and the production of various biomolecules.

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