First stage of cellular respiration

Simple food molecules such as glucose, amino acid, and fatty acid can all be used as fuels for generation of energy in first stage of cellular respiration, but glucose is the principal fuel. Therefore, the principal process of first stage of cellular respiration is glycolysis.

In glycolysis, enzymatic breakdown of glucose occurs in the cytoplasm of a cell by ten step process and produce energy in the form of ATP. There are two types of glycolysis present, one is aerobic glycolysis and another is anaerobic glycolysis.

In aerobic glycolytic pathway of cellular respiration, glucose becomes oxidized in the presence of oxygen, and generates energy in the form of ATP. By this pathway, one molecule of glucose yields two molecules of ATP, two molecules of pyruvate (also called pyruvic acid), and two molecules of NADH. Pyruvate converted into Acetyl CoA by oxidative decarboxylation then enters into mitochondria of a cell for second stage of cellular respiration. NADH gives rise to ATP generation in the electron transport chain oxidation.

In anaerobic glycolytic pathway of cellular respiration, glucose becomes oxidized without participation of oxygen, and each molecule of glucose provides two molecules of ATP and two molecules of lactate. Anaerobic glycolysis is a valuable source of ATP under some situations, including (a) when oxygen supply is limited; and (b) for tissues with no mitochondria, such as mature red blood cells, leukocytes, lens and cornea of the eyes, retina, renal medulla and testes.

Metabolic fuels fatty acid and amino acid both are not produced any ATP in first stage of cellular respiration. In first stage of cellular respiration, fatty acids are oxidized by the process of β-oxidation producing acetyl CoA, NADH, and FADH2 with no ATP. Acetyl CoA then enters into second stage of cellular respiration (citric acid cycle ) for ATP generation, and NADH and FADH2 gives rise to ATP generation via third stage of cellular respiration (electron transport chain).

In first stage of cellular respiration, amino acids are converted to ammonia and alpha-ketoglutarate with no ATP by the process of transamination and subsequent deamination. A small portion of ammonia is excreted through the urine, but large portion is used in the formation of urea that is the most important route for removing of nitrogen from the body. Alpha- ketoglutarate can be enters into second stage of cellular respiration (citric acid cycle) for ATP generation.

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