Physiology of Finfish and Shellfish (2+1)

4.3.2.7 Corbohydrate Metabolism

4.3.2.7 Carbohydrate Metabolism

Carbohydrates made up of carbon, hydrogen and oxygen atoms are classified as mono-, di- and polysaccharides, depending on the number of sugar units they contain. Following digestion, carbohydrates are absorbed through the wall of the digestive tract. They are absorbed and enter the bloodstream as monosaccharides (glucose, galactose and fructose). In vertebrates, the absorbed nutrients are transported to the liver, where the initial processes of metabolism occur. Of the monosaccharides, glucose is the most important acting as a major metabolic energy source circulating in all vertebrates. They may be utilized directly for energy by all tissues; temporarily stored as glycogen in the liver or in muscle; or converted to fat, aminoacids and other biological compounds. Some tissues (brain) use only glucose as an energy source and so maintenance of blood glucose levels is a very important process. Glucose is the primary energy source for the brain, skeletal muscle and red blood cells. Deficiency can impair the brain and nervous system.

Four reaction pathways exist to do this: Carbohydrate metabolism begins with glycolysis , which releases energy from breakdown of glucose or glycogen from carbohydrates to form two molecules of pyruvate, which enter the Krebs cycle (orcitric acid cycle), an oxygen-requiring process, through which they are completely oxidized. Glycolysis occurs in the cytosol (fluid portion) of a cell and can proceed regardless of the presence of oxygen. In the first stage of glycolysis, energy is actually used to phosphorylate the 6-carbon glucose molecule. This means that a phosphate is taken from ATP (which becomes ADP) and added to the glucose molecule.

Before the Krebs cycle can begin, pyruvate loses a carbon dioxide group to form acetyl coenzyme A (acetyl-CoA). This reaction is irreversible and has important metabolic consequences. The conversion of pyruvate to acetyl-CoA requires the B vitamins and then proceeds through the Krebs cycle to produce ATP, CO₂ and water. The hydrogenin carbohydrate is carried to the electron transport chain, where the energy is conserved in ATP molecules. Metabolism of one molecule of glucose yields thirty-one molecules of ATP. The energy released from ATP through hydrolysis (a chemical reaction with water) can then be used for biological work. Only a few cells, such as liver and kidney cells can produce their own glucose from amino acids, and only liver and muscle cells store glucose in the form of glycogen. Other body cells must obtain glucose from the bloodstream. Glycolysis has a dual role. It degrades monosaccharides to generate energy and it provides glycerol for triglyceride synthesis. The Krebs cycle and the electron transport chain occur in the mitochondria. Most of the energy derived from carbohydrate, protein and fat is produced via the Krebs cycle and the electron transport system. Krebs cycle or the citric acid cycle is part of a metabolic pathway involved in the chemical conversion of carbohydrates , fats and proteins into carbondioxide water to generate a form of usable energy.