Metabolism and Temperature Regulation Help
Foods are first digested, then absorbed, and finally, metabolized. Metabolism refers to all the chemical reactions of the body. There are two aspects of metabolism: catabolism, the breaking-down process which provides energy (stored in ATP), and anabolism, the building-up process which requires energy. All metabolic reactions within the body, whether anabolic or catabolic, are catalyzed by enzymes. The substances in food that enter into metabolism are referred to as nutrients. They are classified as carbohydrates, lipids (fats), proteins, vitamins, minerals, and water.
The average human diet consists largely of polysaccharide and disaccharide carbohydrates. When digested, these molecules are broken down into the monosaccharides glucose, fructose, and galactose. The liver further converts fructose and galactose into glucose. Glucose is the molecule from which energy is formed. The equations for glucose metabolism are:
Glycolysis is more rapid than the aerobic pathway, but supplies much less energy and produces lactic acid, which causes early fatigue. Anaerobic activity can only be performed for a short period of time. The 10 steps of glycolysis take place in the cytoplasm of the cell. The Krebs cycle (9 steps, 8 enzymes) and the electron transport chain of oxidation-reduction reactions both take place in the mitochondria.
Not all glucose entering the cell is immediately catabolized to form energy. Extra glucose is linked together into a storage molecule, glycogen. When the body needs energy, glycogen, stored in the liver and muscle cells, is broken down and glucose is released into the blood. This inverse process, glycogenolysis, is spurred by the pancreatic hormone glucagon, and adrenal hormones epinephrine and norepinephrine.
Both protein and lipids can be converted to glucose by the process gluconeogenesis. Five hormones stimulate gluconeogenesis: cortisol, thyroxine, glucagon, growth hormone, and epinephrine.
Lipids are second to carbohydrates as a source of energy for ATP synthesis. Fats participate in the building of many cellular structures and hormones. Lipid metabolism is diagramed in Figure 20-2.
When total food intake exceeds the body's needs, the excess is converted into fat and stored. When stored fats are catabolized, the glycerol components may enter the glycolytic pathway to produce energy or glucose, the fatty acid components break down to form acetyl CoA. This is called beta oxidation. The anabolic process, leading from glucose or amino acids to lipids, is called lipogenesis.
Proteins play an essential role in cellular structure and function. Protein metabolism is diagrammed in Figure 20-3. Amino acids in proteins may be used as an energy source when other sources prove inadequate through the Krebs cycle.
Hormonal Regulation of Metabolism
Other factors that affect metabolic rate include increases in body size, body temperature, activity, and sympathetic stimulation.
Heat is continually being produced as a by-product of metabolism and is continually being lost to the surroundings. The body normally balances heat gain and loss.
Practice problems for these concepts can be found at:
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