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Citric Acid Cycle

Citric Acid Cycle, or Krebs cycle, a sequence of biochemical reactions which leads to the production of the energy essential to life.

There are several groups of compounds which can be metabolized to produce energy, the most important being carbohydrates, specifically glucose. The first stage of glucose metabolism is called Glycolsysis; this stage produces pyruvic acid as an end-product. At this point the citric acid cycle comes into play.

Pyruvic acid, a compound with three carbon atoms, is first converted to Acetyl Coenzyme A (two carbon atoms) by decarbolysation. The reaction involves several cofactors, including vitamin B, liporic acid and Nicotinamide Adenine Dinucleotide (NAD). The reaction is oxidative, and works by the removal of two hydrogen atoms to give NADH2. Carbon dioxide is expelled in the process.

The essential feature of this cycle is the combination of acetyl coenzyme A and oxaloacetic acid (four carbon atoms) to give citric acid. This is then broken down in stages, giving a-oxoglutaric acid and then succinic acid (five and four carbon atoms respectively). From this, oxaloacetic acid is regenerated and the process recommences. During the cycle various co-factors become reduced and one molecule of Adenosine Triphosphate (ATP) is produced.

The real source of energy in the body is not glucose, but ATP. This compound contains a high-energy bond, which, on breaking, will release its energy for muscle contraction, protein synthesis, steroid formation... in fact, all the processes of life. In addition to ATP, the cycle produces a considerable amount of reduced flavoproteins and nicotinamide adenine dinucleotides.

These can be oxidized by the Electron Transport Chain in a process called oxidative phosphorylation. During this process considerable amounts of ATP are produced. Thus the cycle not only produces ATP directly, but the essential precursors for ATP production. Taking the cycle together with oxidative phosphorylation, one molecule of pyruvic acid produces 15 molecules of ATP. But one molecule of glucose, when being converted to pyruvic acid by glycolysis, yields eight molecules of ATP plus two molecules of pyruvic acid. Hence one molecule of glucose can give 38 molecules of energy-rich ATP.

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