Study Guide for Metabolism 8th Edition PDF

STUDY GUIDE FOR METABOLISM Metabolism is defined as the sum of all metabolic reactions that occur in the body. In order to maintain living tissue a constant expenditure of energy is required. This energy is obtained primarily through the metabolism (chemical conversion) of the energy in the...

STUDY GUIDE FOR METABOLISM Metabolism is defined as the sum of all metabolic reactions that occur in the body. In order to maintain living tissue a constant expenditure of energy is required. This energy is obtained primarily through the metabolism (chemical conversion) of the energy in the bonds of nutrient molecules (glucose, fatty acids, amino acids, and other nutrient molecules), into the bonds of ATP molecules. The principle metabolic pathway that carries out this conversion is known as aerobic metabolism and consists of two parts: glycolysis and cellular respiration. Both glycolysis and cellular respiration produce ATP. In glycolysis, nutrient molecules, such as glucose, fructose, and galactose, are converted into pyruvate. The pyruvate is subsequently converted into carbon dioxide and water through cellular respiration (citric acid cycle). The nutrient molecules (glucose, fatty acids, amino acids, etc.) that provide the energy needs by cells are obtained from two sources: 1) The digestion and absorption of ingested food, and 2) The breakdown of the glycogen, triglycerides, and proteins stores in the cells of the body. Which source the cells of the body rely on depends on the short- term metabolic state the body is in. Short-term metabolism can be divided into two states, the absorptive state (a.k.a. fed state) and the post-absorptive state (a.k.a. fasted state). The absorptive state occurs during the period following a meal, when digestion and absorption of nutrient molecules is occurring. Cells in this metabolic state rely on the metabolism of the absorbed nutrient molecules to meet their energy needs. The post- absorptive state occurs after the nutrients absorbed during a meal are no longer available to the cells (i.e. they have been used up or stored). Cells in the post-absorptive state rely on the breakdown of larger storage molecules (glycogen, triglycerides, and proteins) to provide the nutrient molecules (glucose, fatty acids, amino acids, etc.) that are needed for aerobic metabolism. Glycogen is used as a glucose storage molecule in liver cells and muscle fibers. Triglycerides are used as a fatty acid storage molecule in adipose cells. What happens to a molecule (such as glucose or a storage molecule) in the body depends on which metabolic state the body is in. The goal of the absorptive state metabolism is to use the absorbed nutrients, glucose, amino acids, and fatty acids as the source of the body's energy. You should know the ways that these nutrients are used during the absorptive state. The goal of post-absorptive state metabolism is to maintain blood glucose concentrations with in a set range, so that the neurons of the nervous system have an adequate supply of glucose for fuel. Neurons rely primarily on the metabolism of glucose to meet their energy needs. Neurons cannot use fatty acids or amino acids as a source of energy. However, if necessary, ketone bodies obtained from the liver can be used by neurons as an alternate energy source. During the post-absorptive state the liver is the primary source of glucose for all of the cells of the body. In the post- absorptive state, the glycogen stores in the liver are converted to glucose by a metabolic process called glycogenolysis (the conversion of glycogen to glucose). Muscle fibers are not able to convert their glycogen stores to glucose, so instead they convert it to pyruvate or lactate. The muscle fibers release this pyruvate or lactate into the blood where it circulates to the liver and is taken up and converted by the cells of the liver into glucose by a metabolic process called gluconeogenesis (the conversion of a non-carbohydrate precursor to glucose). Muscle fibers may also breakdown their proteins into amino acids and use these amino acids in cellular respiration, or convert them into pyruvate and release them into the blood for conversion into glucose by gluconeogenesis in the liver. During the post-absorptive state, adipose cells convert their stores of triglycerides into fatty acids, which are released into the blood. These fatty acids can be taken up by the cells of the body and used in cellular respiration. Some of these fatty acids are taken up by the liver and converted to ketone bodies by a process called -oxidation. Ketone bodies can be used by most cells (including neurons) in cellular respiration for energy. Regulation of the metabolic state of the cells is primarily under the control of the endocrine system. The minute-to-minute regulation of the metabolic state of the body is controlled by the ratios of two peptide hormones, insulin and glucagon. These two hormones are synthesized and secreted by the cells in islets of Langerhans of the pancreas. Insulin is synthesized and secreted by  cells in the islets of Langerhans, and glucagon is synthesized and secreted by  cells. The secretion of insulin and glucagon is largely controlled by the blood concentrations of glucose, and to a lesser degree by the levels of amino acids in the blood. As food is digested and nutrients are absorbed, blood glucose levels rise as a result of the absorption of glucose across the wall of the small intestine. The rise in blood glucose levels stimulate the  cells and inhibit the  cells. As a result, insulin levels rise in the blood and glucagon levels fall. Insulin acts on the cells of the body to shift them into the absorptive state metabolism. After the nutrients from a meal have been absorbed and used up, blood glucose levels fall. The fall in blood glucose levels reduces the stimulation of the  cells and removes the inhibition of the  cells. As a result, insulin levels fall and glucagon levels rise. It is important to understand that the shift to the post-absorptive metabolic state in most cells of the body is not due to the actions of glucagon. Glucagon’s primary target is the liver, where it stimulates glycogenolysis, gluconeogenesis, and ketogenesis. Instead the shift to the post-absorptive state metabolism is due to the lack of insulin acting on the cells of the body. That is, the breakdown of glycogen, proteins, and triglyceride stores in the liver, muscles, and adipose tissues during the post-absorptive state is the result of low insulin levels, rather than high glucagon levels. Suggested End of Chapter 22 questions: 8th edition: 1,3,5,6,7,9,10,11,12,13,14,15,16,17,19,20,22a,b,c,f,23,24,26,27,28,32