CHAPTER 1 ENERGY SYSTEMS

Transcription

1 SECTION A CHAPTER 1 ENERGY SYSTEMS CHAPTER 1 ENERGY SYSTEMS Energy definitions Energy is the capacity to do work, and work has a mechanical definition, namely work = force x distance moved in the direction of the force. Energy and work are measured in joules (J). Chemical energy is energy that is produced by a complex series of chemical reactions, which can then be made available as kinetic energy (energy due to movement which results from muscular contractions), or potential energy which is stored energy due to gravity. Power is the rate at which energy is used, or the energy used per second which is measured in watts (W). Power can be calculated using the formula: power = energy (in joules) answer in watts time (in seconds) Energy sources and systems We derive our energy from food, namely carbohydrates (CHO),, and to a lesser extent proteins. Table 1.1 summarises the details of the major contributions from our diet to the provision of energy for exercise, how these contributions are absorbed, and where this energy is stored within the body. Table 1.1 summary of carbohydrate, fat and protein utilisation type of food / sources carbohydrate (CHO) sugars, rice, potatoes, pasta butter, oil, pastry, fried food proteins meat, eggs, milk, cheese, nuts function as a food fuel - how it is used Main energy supply. Absorbed as glucose in small intestine. Transported around body as blood glucose. Available for immediate energy. Excess stored as muscle and liver glycogen and as fat. Secondary energy supply. Absorbed as fatty acids and glycerol (called free fatty acids, FFAs) in the small intestine. Stored as triglycerides in adipose tissue. Triglycerides conveyed to the liver via the circulatory system. In the liver they are converted to glucose. Available as delayed ( minutes delay) energy source for long duration low intensity aerobic exercise. Absorbed as amino acids in the small intestine. Used for growth and repair by all tissues. Used as an energy source when the body is depleted of CHO and fat. Excess protein not needed for tissue repair is broken down and used as an energy supply. Protein can supply between 5 and 1% of energy needed to sustain prolonged exercise. energy content (kj g -1 ) percentage in a balanced diet 17 % % % The energy derived from carbohydrates, and proteins is stored in bodily tissues in the form of a high energy compound called adenosine triphosphate (ATP). figure 1.1 all muscle action uses ATP ATP - adenosine triphosphate ATP is the compound which stores energy and is therefore the energy currency linked to intensity and duration of physical activity. ATP exists in every living tissue and its breakdown gives energy for all life functions - this includes the action of the liver and the brain for example, as well as the contraction of muscle tissue. All muscular activity requires the availability and breakdown of ATP (figure 1.1). 122

2 APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE The energy released during tissue respiration is stored in the chemical bonds in ATP, and this energy is released during the reaction: ATP è ADP + P i + energy where ADP is adenosine diphosphate, and P i is a free phosphate radical. ATPase is an enzyme which facilitates this reaction, which is exothermic - it releases energy. The energy stored within ATP is only available as long as ATP is retained within the cells using it. In muscle cells during intense (flat-out) exercise, the stored ATP only lasts about 2 seconds. Therefore the ATP must be replaced as soon as possible so that exercise can continue. There are three processes by which this can happen: The ATP-PC system (also called the alactic anaerobic system). The lactic acid system (which is also anaerobic). The aerobic system. Resynthesis of ATP from ADP uses the reaction: energy + ADP + P i è ATP This is an endothermic reaction since energy is given to the molecule to enable the reaction to happen. This energy will be derived from food fuels. Anaerobic energy systems The ATP-PC system This system of replenishing of ATP from ADP is the predominant one for activity which lasts between 3 and 1 seconds, which means for high intensity maximum work, for example, flat out sprinting - the m sprint. No oxygen is needed - the process is anaerobic. The chemical reactions within this system are a coupled reaction in which ATP is resynthesised via phosphocreatine (PC) stored in muscle cell sarcoplasm. The following reactions take place: PC è P i + C + energy figure 1.2 changes in muscle ATP and PC energy + ADP + P i è ATP The two reactions together are called a coupled reaction and are facilitated by the enzyme creatine kinase (CK). The net effect of these two coupled reactions is: PC + ADP è ATP + C PC is re-created in muscle cells during the recovery process, which requires energy and is an endothermic reaction. During intense exercise, peak anaerobic power is attained within the first 5 seconds, and depletion of PC occurs between 7 and 9 seconds. STUDENT NOTE This process does not directly require glucose as an energy source - but the re-creation of PC during recovery will do so. % of resting value ATP level exhaustion muscle PC level time / seconds Look at the graph in figure 1.2 showing changes in muscle ATP and PC. After an initial small fall, the ATP level is maintained, then falls as the PC is used up because the energy from PC is being used to resynthesise ATP. This causes PC levels to fall rapidly to zero after about 1 seconds. The capacity to maintain ATP production at this point depends on the lactic acid system. The lactic acid system This system depends on a chemical process called glycolysis which is the incomplete breakdown of sugar. Figure 1.3 (see overleaf) shows the schematic layout of glycolysis. Anaerobic energy systems 13 3

3 SECTION A CHAPTER 1 ENERGY SYSTEMS The lactic acid system Carbohydrate from food you have eaten is stored as glycogen (in the muscles and liver). This glycogen is converted into glucose by the hormone glucagon released when blood glucose levels fall (when glucose is used during tissue respiration). The breakdown of glucose provides the energy to rebuild ATP from ADP. This is facilitated by enzymes such as glycogen phosphorylase (GPP) and phosphofructokinase (PFK). The whole process produces pyruvic acid. Glycolysis is anaerobic. It takes place in the sarcoplasm of the muscle cell. No oxygen is needed, and the end product of this reaction (in the absence of oxygen) is lactic acid. figure 1.3 the lactic acid system The enzyme facilitating the conversion from pyruvic acid to lactic acid is lactate dehydrogenase (LDH). As work intensity increases, lactic acid starts to accumulate above resting values, which produces muscle fatigue and pain. The resultant low ph inhibits enzyme action and cross-bridge formation, hence muscle action is inhibited and physical performance deteriorates. The lactic acid system is the predominant one used to resynthesise ATP in sport or activities in which the flat-out effort lasts up to 3- seconds. For example, a m run or a m swim. After exercise stops, extra oxygen is taken up to remove lactic acid by changing it back into pyruvic acid - this is the EPOC (Excess Post-exercise Oxygen Consumption, figure 1.4 the aerobic system sometimes called the oxygen debt), see page 18 below for the details of EPOC. 1 molecule of glucose 2ATP lactic acid glucose C 6 H 12 O 6 glycolytic enzymes (GPP, PFK) LDH pyruvic acid The aerobic system glycolysis 2ATP The aerobic energy system releases stored energy from muscle glycogen, and proteins. Figure 1.4 is a graphic of some of the details of the aerobic system showing how between 32 and 34 ATP molecules are resynthesised from one molecule of glucose - which is the food fuel created from the food we eat. This process will continue indefinitely until energy stores run out - or the exercise stops. Stage one - glycolysis This process takes place in the muscle cell sarcoplasm and is identical to the lactic acid system (anaerobic). ATP regenerated = 2ATP per molecule of glucose. Stage two - Kreb s cycle (citric acid cycle) This stage occurs in the presence of oxygen, and takes place in the muscle cell mitochondria within the inner fluid filled matrix. Here, 2 molecules of pyruvic acid combine with oxaloacetic acid (4 carbons) and acetyl coa (2 carbons) to form citric acid (6 carbons). The cycle produces H + and electron pairs, and CO 2 and 2 ATP. Also, free fatty acids (from body fat) facilitated by the enzyme lipoprotein lipase, or protein (keto acids - from muscle), act as the fuel for this stage. protein pyruvic acid acetyl coa citric acid O 2 Kreb's cycle H + ions and electrons oxaloacetic acid electron tr anspor t chain lactic acid 2ATP up to 32 ATP CO 2 The enzymatic catabolism (breakdown) of fat within the muscle cell mitochondria is termed b-oxidation. Energy derived from the breakdown of FFAs are the preferred food fuel as the duration of exercise increases. H O 2 144

4 rate of use of energy APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE Stage three - the electron transport chain The electron transport chain occurs in the presence of oxygen within the cristae (inner part) of the muscle cell mitochondria. Hydrogen ions and electrons have potential energy which is used to produce the ATP which is then released in a controlled step-by-step manner. Oxygen combines with the final H + ions to produce water and 32 ATP. Aerobic respiration In summary, the total effect of aerobic respiration is that it is an endothermic reaction: glucose + 36ADP + 36P i + 6O 2 è 6CO ATP + 6H 2 O figure 1.5 change in muscle glycogen Fat fuels produce 2 ATP less than glucose. during low intensity exercise Short-term response to aerobic activity The aerobic system requires carbohydrate in the form of glucose which is derived from glycogen stored in muscle cells (mostly slow twitch - SO type I) or in the liver. The graph in figure 1.5 shows how the rate of usage of muscle glycogen is high during the first 3 minutes of steady exercise - which has to be replaced if a sportsperson is to continue at the same rate. Hence consumption of energy drinks and bananas during a long tennis match. Endurance athletes can utilise FFAs during prolonged exercise sooner than untrained people. This training adaptation enables the trained athlete to not use glycogen up immediately, but save it for later on in an exercise effort, or when the intensity of exercise increases. This is called glycogen sparing and is illustrated in figure 1.6. The energy continuum % of initial muscle glycogen muscle glycogen time / hours This describes the process by which ATP is regenerated via the different figure 1.6 contribution of CHO and energy systems depending on the intensity and duration of exercise. for trained and untrained people Although all the systems contribute to ATP regeneration during any activity, one or other of the energy systems usually provides the major contribution for a given activity. Table 1.2 shows approximate proportions of ATP resynthesised via aerobic and anaerobic pathways for some sporting activities. Table 1.2 percentage contribution of the aerobic and anaerobic energy systems to different sports sport or event aerobic % anaerobic (all) % m sprint m sprint 1 9 m swim boxing 3 7 m run hockey 5 5 m rowing m cycle pursuit 7 3 3m run cross country run 9 1 marathon light to moderate exercise untrained person CHO 55% 45% low intensity exercise trained person CHO % % high intensity endurance exercise trained person CHO 35% 65% The graph in figure 1.7 overleaf shows how the different energy systems contribute resynthesis of ATP during flat-out exercise. Obviously, at reduced intensity of exercise, the contributions will be slightly different. But note that all systems are contributing from the start of exercise, only it takes some time for the lactic acid and aerobic systems to get going. The energy continuum 515

5 SECTION A CHAPTER 1 ENERGY SYSTEMS Short-term responses - thresholds The concept of a threshold applies to the time at which one particular system of ATP regeneration takes over from another as the major regenerator of ATP during flat out exercise - marked as T in figure 1.7. For example, ATP muscle stores are depleted within 2 seconds, and towards the end of this period the ATP-PC system has risen enough to be able to provide the ATP necessary for the exercise. Peak anaerobic power is attained within the first 5 seconds of flatout exercise, but depletion of PC occurs between 7 and 9 seconds. At this point, the lactic acid system has risen enough to be able to provide the ATP required for the next seconds or so. Hence the threshold between ATP-PC and lactic acid systems occurs between 7 and 9 seconds after the start of an exercise period. Long-term training effects - thresholds It is found that thresholds are delayed by training, so that the trained individual has a greater capacity for ATP-PC, has a greater lactic acid toleration, and more efficient ATP regeneration than the untrained person. figure 1.7 variation in contribution of energy systems % of maximum rate of energy production ATP store overall performance ATP-PC system ta d syst ac c l i 2s T 1s Ts T = threshold point e m aerobic system base rate 2hrs time 166 Other factors affecting the proportions of energy systems The level of fitness (whether adaptations to training have included enhancement of relevant enzymes - which would for example postpone levels of lactate accumulation). The availability of O 2 and food fuels. For example, a high CHO diet would assist replenishment of glycogen stores which would then be available for glycolysis. Practice questions 1) Define energy, and briefly describe how energy is released from food in the body. 5 marks 2) a) Identify the only form of usable energy in the body. 1 mark b) What is meant by an exothermic reaction? Illustrate this definition with an example. 2 marks c) What is meant by an endothermic reaction? Illustrate this definition with an example. 2 marks 3) Explain the specialist role of mitochondria in energy production. 4) a) An elite cyclist wants to discover her maximum output when sprinting. A load of 85 newtons is applied to the wheel of a cycle ergometer. The cyclist then sprints for 1 seconds and records a distance travelled of m. Assuming that no other force is acting on the cycle, calculate the power output of the cyclist. Show each stage of your calculation. b) Name the predominant energy system that would be utilised during this activity and describe how ATP is re-created within this system. 5) a) An elite swimmer performs a flat-out metre freestyle swim in 5 seconds. Describe how most of the ATP is regenerated during the swim. b) Sketch a graph, which shows the use of the appropriate energy systems against time during the swim. 6) a) Taking part in a triathlon involves swimming, cycling and running. Briefly describe how the aerobic energy system within the cell mitochondria supports this endurance event. 6 marks b) Construct a graph, which illustrates the food fuel usage against time during a triathlon race lasting 2 hours. 2 marks 7) Compare the relative efficiency of ATP production via the aerobic and anaerobic routes. Explain your answer. 3 marks

6 APPLIED PHYSIOLOGY TO OPTIMISE PERFORMANCE 8) Describe the predominant energy system being used in the following activities: shot put, metres breaststroke, a game of hockey, metres hurdles race, gymnastics vault, modern pentathlon. 6 marks Table 1.3 table of activities activity shot-put metres breaststroke a game of hockey metres hurdles race gymnastics vault modern pentathlon energy system figure 1.8 energy blocks for a variety of sports 9) Figure 1.8 represents the energy systems used during the following athletic events: metres, metres, 15 metres and marathon, but not necessarily in that order. a) Identify each event with an energy block and comment briefly on the reasons for your choice. 8 marks b) Using the same key, complete a block to show the approximate proportions of aerobic/anaerobic work during a basketball game. 2 marks A B C D aerobic ATP-PC lactic acid 1) The diagram in figure 1.9 is an energy continuum in relation to a variety of sports activities. a) Explain the concept the energy continuum. 2 marks b) At each end of the continuum examples of sporting activities have been omitted. Give one example of a sporting activity that is predominantly anaerobic and one example of a sporting activity that is predominantly aerobic. 2 marks c) Suggest two factors that need to be considered in evaluating sports activities on the basis of their relative position on the energy continuum. 2 marks d) Explain, using specific examples, why a game of hockey has aerobic and anaerobic components. figure 1.9 variation in contribution of energy system m swim boxing hockey rowing (m) 3m run crosscountry run aerobic anaerobic Practice questions 717