Physiology of Finfish and Shellfish (2+1)

4.1.3. Pathways of cellular metabolism

The enzymatic degradation of each of the major energy – yielding nutrients of cells (CHO, fats and prots) produceds in a steperise marner thro’ a number of consecutive enzymatic reactions. The large biomolscules during digestion have been reduced to their building blocks viz. hexoses/pentosese, amino acids and fatty acids of glycerol. During the first phase of catabolism, these substances (building of blocks) are converted into a smaller number of yet simpler moleculer. Thus the hexoses, pentoses, amino acids and glycerol are degraded to a single 3-carbon intermediate pyrovate which is then converted into a single 2-carbon unit, the acetyl group of acetyl co-enzyme. A some a.a.s and Fas are directly – acetylcoa. In the next phase, the acetyl gr. of A CO A is fed into the citric acid cycle, the final common pathway by which most nuts are ultimately oxidized to CO₂. Water and NH₃ are the other end productions.

During the initial place of catabolism one molecules of glucose (6C) is degraded by 10 successive reactions to yield 2 moles of pyruvic acid (3C). This part of the metabolism is called Glycolysis. Glycolysis is a central pathway in the catabolism. Of glucose in all animals, these reactions occur in anaerobic condition. Whereas, oxidation of pyruvic acid (and further reactions) required O₂. When some muscles work vigorously the pyruvic a cannot oxidized for lack of O₂ under such conditions, the pyruvic a in converted into lactic acid. In the anaerobic micro organisms and som fungi, the pyruvate is converted into of ethyl alcohol. This process is called (alcoholic) fermentation. The amount of energy released from glycolysis will be far less than that released from complete oxidation of the glucose molecule. Thus glycosis yields only >10% of total energy contained in the nutrient.

The pyruvic acid formed is oxidized to a form the acetyl group of acetyl CoA. The acetyl group is then oxidized completely to CO₂ and water by the citric acid cycle or TCA cycle or krebs cycle with the intervention of molecular oxygen. Therefore, the actual utilization of assiminted O₂ thro’ breathing begins after glycolysis.

While glycolysis occurs in the cytoplasm of the UV, see page after the next krebs cycle and electron tr. Syst. Occur in the mitochondria.