Three Main Metabolisms (Reference: FAQS.org)
Metabolism refers to the physical and chemical processes that occur inside the cells of the body and that maintain life. Metabolism consists of anabolism (the constructive phase) and catabolism (the destructive phase, in which complex materials are broken down). The transformation of the macronutrients carbohydrates, fats, and proteins in food to energy, and other physiological processes are parts of the metabolic process. ATP (adinosene triphosphate) is the major form of energy used for cellular metabolism.
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. The monosaccharides—glucose, galactose, and fructose—obtained from the digestion of food are transported from the intestinal mucosa via the portal vein to the liver. They may be utilized directly for energy by all tissues; temporarily stored as glycogen in the liver or in muscle; or converted to fat, amino acids, and other biological compounds.
Carbohydrate metabolism plays an important role in both types of diabetes mellitus. The entry of glucose into most tissues—including heart, muscle, and adipose tissue—is dependent upon the presence of the hormone insulin. Insulin controls the uptake and metabolism of glucose in these cells and plays a major role in regulating the blood glucose concentration. The reactions of carbohydrate metabolism cannot take place without the presence of the B vitamins, which function as coenzymes. Phosphorous, magnesium, iron, copper, manganese, zinc and chromium are also necessary as cofactors.
Carbohydrate metabolism begins with glycolysis, which releases energy from glucose or glycogen to form two molecules of pyruvate, which enter the Krebs cycle (or citric acid cycle), an oxygen-requiring process, through which they are completely oxidized. 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.
The hydrogen in 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.
Under anaerobic conditions, lactate is formed from pyruvate. This reaction is important in the muscle when energy demands exceed oxygen supply. Glycolysis occurs in the cytosol (fluid portion) of a cell and 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.
Glycogenesis is the conversion of excess glucose to glycogen. Glycogenolysis is the conversion of glycogen to glucose (which could occur several hours after a meal or overnight) in the liver or, in the absence of glucose-6-phosphate in the muscle, to lactate. Gluconeogenesis is the formation of glucose from noncarbohydrate sources, such as certain amino acids and the glycerol fraction of fats when carbohydrate intake is limited. Liver is the main site for gluconeogenesis, except during starvation, when the kidney becomes important in the process. Disorders of carbohydrate metabolism include diabetes mellitus, lactose intolerance, and galactosemia.