Medical brain notes 44

  Adenosine Triphosphate (ATP) Functions as an “Energy Currency” in Metabolism In the past, we have pointed out that car-bohydrates, fats, and proteins can all be used by cells to synthesize large quantities of adenosine triphosphate (ATP), which can be used as an energy source for almost all other cellular func-tions. For this reason, ATP has been called an energy “currency” in cell metabolism. Indeed, the transfer of energy from foodstuffs to most functional systems of the cells can be done only through this medium of ATP (or the similar nucleotide guanosine triphosphate, GTP).         An attribute of ATP that makes it highly valuable as an energy currency is the large quantity of free energy (about 7300 calories, or 7.3 Calories [kilocalories], per mole under standard conditions, but as much as 12,000 calories under physiologic conditions) vested in each of its two high-energy phosphate bonds. The amount of energy in each bond, when liberated b...

  Phosphocreatine Functions as an Accessory Storage Depot for Energy and as an “ATP Buffer” Despite the paramount importance of ATP as a coupling agent for energy transfer, this substance is not the most abundant store of high-energy phosphate bonds in the cells.  Phosphocreatine,  which also contains high-energy phosphate bonds, is three to eight times as abundant. Also, the high-energy bond (~) of phospho-creatine contains about 8500 calories per mole under standard conditions and as much as 13,000 calories per mole under conditions in the body (37°C and low con-centrations of the reactants). This is slightly greater than the 12,000 calories per mole in each of the two high-energy phosphate bonds of ATP. The formula for crea-tinine phosphate is the following: Unlike ATP, phosphocreatine cannot act as a direct coupling agent for energy transfer between the foods  and the functional cellular systems, but it can transfer energy interchangeably with ATP. When extra amo...

  Anaerobic Versus Aerobic Energy Anaerobic energy  means energy that can be derivedfrom foods without the simultaneous utilization of oxygen;  aerobic energy  means energy that can be derived from foods only by oxidative metabolism. it is noted that carbohydrates, fats, and proteins can all be oxidized to cause synthesis of ATP. However,  carbohydrates are theonly significant foods that can be used to provide energy without the utilization of oxygen;  this energy releaseoccurs during glycolytic breakdown of glucose or glyco-gen to pyruvic acid. For each mole of glucose that is split into pyruvic acid, 2 moles of ATP are formed. However, when stored glycogen in a cell is split to pyruvic acid, each mole of glucose in the glycogen gives rise to 3 moles of ATP. The reason for this difference is that free glucose entering the cell must be phosphorylated by using 1 mole of ATP before it can begin to be split; this is not true of glucose derived from glycogen be...

  Summary of Energy Utilization by the Cells With the background of the past few and of the preceding discussion, we can now synthesize a compos-ite picture of overall energy utilization by the cells, as shown in Figure 72–1. This figure demonstrates the anaerobic utilization of glycogen and glucose to form ATP and the aerobic utilization of compounds derived from carbohydrates, fats, proteins, and other substances to form additional ATP. In turn, ATP is in reversible equilibrium with phosphocreatine in the cells, and because larger quantities of phosphocreatine are present in the cells than ATP, much of the cells’ stored energy is in this energy storehouse. Energy from ATP can be used by the different func-tioning systems of the cells to provide for synthesis and growth, muscle contraction, glandular secretion, nerve impulse conduction, active absorption, and other cellular activities. If greater amounts of energy are demanded for cellular activities than can be provided by oxidat...

  Control of Energy Release in the Cell Rate Control of Enzyme-Catalyzed Reactions.  Before dis-cussing the control of energy release in the cell, it is nec-essary to consider the basic principles of  rate control  of enzymatically catalyzed chemical reactions, which are the types of reactions that occur almost universally throughout the body. The mechanism by which an enzyme catalyzes a chemical reaction is for the enzyme first to combine loosely with one of the substrates of the reaction. This alters the bonding forces on the substrate sufficiently so that it can react with other substances. Therefore, the rate of the overall chemical reaction is determined by both the concentration of the enzyme and the concen-tration of the substrate that binds with the enzyme. The basic equation expressing this concept is as follows: This is called the  Michaelis-Menten equation.  Figure 72–2 shows the application of this equation. Role of Enzyme Concentration in Regul...

  Metabolic Rate The  metabolism  of the body simply means all the chemical reactions in all the cells of the body, and the  meta-bolic rate  is normally expressed in terms of the rate ofheat liberation during chemical reactions. Heat Is the End Product of Almost All the Energy Released in the  Body.  In discussing many of the metabolic reactions in the preceding, we noted that not all the energy in foods is transferred to ATP; instead, a large portion of this energy becomes heat. On average, 35 per cent of the energy in foods becomes heat during ATP forma-tion. Then, still more energy becomes heat as it is trans-ferred from ATP to the functional systems of the cells, so that even under optimal conditions, no more than 27 per cent of all the energy from food is finally used by the functional systems. Even when 27 per cent of the energy reaches the func-tional systems of the cells, most of this eventually becomes heat. For example, when proteins are syn...

  Measurement of the Whole-Body Metabolic Rate Direct  Calorimetry  Measures  Heat  Liberated  from  the  Body. Because a person ordinarily is not performing any external work, the whole-body metabolic rate can be determined by simply measuring the total quantity of heat liberated from the body in a given time. In determining the metabolic rate by direct calorime-try, one measures the quantity of heat liberated from the body in a large, specially constructed  calorimeter.  The subject is placed in an air chamber that is so well insu-lated that no heat can leak through the walls of the chamber. Heat formed by the subject’s body warms the air of the chamber. However, the air temperature within the chamber is maintained at a constant level by forcing the air through pipes in a cool water bath. The rate of heat gain by the water bath, which can be measured with an accurate thermometer, is equal to the rate at which heat is liberated by the s...

  Energy Metabolism-Factors That Influence Energy Output Energy intake is balanced with energy output in healthy adults who maintain a stable body weight. About 45 per cent of daily energy intake is derived from carbohydrates, 40 per cent from fats, and 15 per cent from proteins in the average American diet. Energy output can also be partitioned into several measurable components, including energy used for (1) performing essential metabolic functions of the body (the “basal” metabolic rate); (2) performing various physical activities; (3) digesting, absorbing, and processing food; and (4) maintaining body temperature. Overall Energy Requirements for Daily Activities An average man who weighs 70 kilograms and lies in bed all day uses about 1650 Calories of energy. The process of eating and digesting food increases the amount of energy used each day by an additional 200 or more Calories, so that the same man lying in bed and eating a reasonable diet requires a dietary intake of about...

  Overall Energy Requirements for Daily Activities An average man who weighs 70 kilograms and lies in bed all day uses about 1650 Calories of energy. The process of eating and digesting food increases the amount of energy used each day by an additional 200 or more Calories, so that the same man lying in bed and eating a reasonable diet requires a dietary intake of about 1850 Calories per day. If he sits in a chair all day without exercising, his total energy requirement reaches 2000 to 2250 Calories. Therefore, the approximate daily energy requirement for a very sedentary man perform-ing only essential functions is 2000 Calories. The amount of energy used to perform daily physical activities is normally about 25 per cent of the total energy expenditure, but it can vary markedly in differ-ent individuals, depending on the type and amount of physical activity. For example, walking up stairs requires about 17 times as much energy as lying in bed asleep. In general, over a 24-hour peri...

  Basal Metabolic Rate (BMR)- The Minimum Energy Expenditure for the Body to Exist Even when a person is at complete rest, considerable energy is required to perform all the chemical reactions of the body. This minimum level of energy required to exist is called the  basal metabolic rate  (BMR) and accounts for about 50 to 70 per cent of the daily energy expenditure in most sedentary individuals (Figure 72–3). Because the level of physical activity is highly vari-able among different individuals, measurement of the BMR provides a useful means of comparing one person’s metabolic rate with that of another. The usual method for determining BMR is to measure the rate of oxygen utilization over a given period of time under the following conditions: 1.        The person must not have eaten food for at least 12 hours.   2.        The BMR is determined after a night of restful sleep.   3.    ...