With the aid of diagrams provide a summary of how the following energy systems work.
Energy systems; introduction Energy systems are cellular levels processes used to produce Adenosine Triphosphate (ATP) figure 1. This is an adenosine molecule linked to three high-energy phosphates that acts as an energy store for the cell. The energy is released when ATPase, an enzyme, reacts with ATP to produce ADP and Pi, e.g.
ATP ADP + Pi There are three energy systems that do this; •The Creatine Phosphate System •The Glycolytic or Lactic Acid system •The Oxidative system (The Krebs cycle, Citric Acid Cycle or Tricarboxylic Acid Cycle) The first too are ANAEROBIC, the third is AEROBIC.
I.Creatine Phosphate (CrP) Summary: A cytoplasm based catabolic reaction in which Creatine Phosphate is degraded to Creatine to provide ATP; net profit of one ATP molecule; can proceed anaerobically.
Net reaction: CrP + ADP+H+—>ATP + Creatine Detail: During high-intensity exercise energy for ATP resynthesis is provided primarily by another high-energy phosphate compound called creatine phosphate (CrP), see figure 2. Cellular concentrations of CrP are 4-5 times greater than that of ATP and are generally concentrated in areas of contractile protein; skeletal muscle has 95%. CrP is like a match; when the muscle receives a nerve impulse from the brain instructing it to contract, it instantly releases its energy, as if the match had been struck.
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This gives a natural “reservoir” of energy to enable resynthesis of ATP to occur rapidly, 7Toler (1997), 8Vandenberghe (1996) and 9Feldman (1999), but it can only sustain work at maximal levels for about 5 – 15 seconds dependant on activity level and the individual’s personal physiological adaptations to exercise.
The system has two steps. Firstly, bond between creatine and phosphate splits energy is liberated, as CrP has a higher potential energy than ATP, sufficient energy is released to resynthesise ADP to ATP. This reaction is catalysed by the enzyme creatine kinase.
CrP Cr + Pi + energy The energy created in the split’s useless to the cell directly; so in step two it’s used to convert ADP and Phosphate to ATP and thus a source of useable energy to the cell. The process of ATP-CrP regeneration is the most rapid pathway for providing muscular energy.
Energy ADP + Pi ——————> ATP This is a 1:1 ratio in that one Creatine phosphate delivers one ATP molecule. This is not a very efficient system but it’s very fast; the chemical name for Creatine is N-(aminoiminomethyl)-N-methyl glycine or methylglycocyamine, figure 3. It’s synthesized in the liver, pancreas, and kidney from the three amino acids – L-Arginine, Glycine and L-Methionine. Following its biosynthesis, creatine is transported to the skeletal muscles, heart, brain, and other tissues where it’s phosphorylated (the red area in figure 3) and stored. This allows the muscle cells to have about a 15 second energy source on tap that does not require oxygen to be used, during maximal exercise.
Very fast sports, e.g. the 100m sprint, powerlifting, are based almost totally on this energy system, (see table 2 page 13).
II.Glycolysis – the Lactic Acid System.
Summary: A series of cytoplasm based catabolic reactions in which glucose is degraded to pyruvate (pyruvic acid) to provide ATP and NADH (nicotinamide adenine dinucleotide) as well as molecules for the anabolic pathways; net profit of two ATP molecules; hydrogen is released; can proceed anaerobically.
Net reaction: Glucose + 2 ADP + 2 Pi + 2 NAD+—>2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O Detail: Glycolysis, also called the Embden-Myerhoff pathway, is the sequence of reactions, which converts a glucose molecule into two pyruvate molecules with the production of NADH and ATP. Specific enzymes control each of the different reactions, as shown in figure 5. Glucose comes either directly from digestion, or from short-term storage in the muscles or long-term storage in the liver. There is a net gain of two (2) ATP at the end of glycolysis (reaction shown above), using glucose as the source and three (3) from glycogen. This is because glucose needs to be converted to glucose 6-phosphate to enter the glycolytic pathway that requires the use of one ATP, (see figure 4).
... O2 6 CO2 6 H2O Energy Glycolysis Oxidative phosphorylation Pyruvate Acetyl CoA and Krebs cycle Oxidative phosphorylation ATP/glucose Glycolysis (2), Krebs (2 GTP), oxidative ... respiration can produce a maximum of 38 ATP per glucose molecule. Anaerobic respiration can produce 2 ATP per glucose molecule. As a result, aerobic respiration is ...
All the reactions of glycolysis take place entirely in the cytosol due to the abundance of free-floating ingredients such as ADP, NAD+, and inorganic phosphates. Glycolysis itself does not require oxygen and can proceed under both aerobic and anaerobic conditions.
Glycolysis can generally be divided into two main phases. Phase one encompasses the first four steps of glycolysis, see figure 5. During this first phase, phosphate is added to the glucose molecule. The glucose molecule is now split into two three-carbon glyceraldehyde-3-phosphate (PGAL) molecules. This phase of glycolysis cannot occur without the input of energy and phosphate from two molecules of ATP.
In the second phase of glycolysis (steps 6-10 in figure 5), energy harvesting begins, firstly with the reduction of NAD+ to NADH by the oxidation of PGAL, storing some of the energy from glucose in NADH’s energy rich electrons and secondly enough energy is released to add a second phosphate group to PGAL, forming 1,3-bisphosphoglyceric acid. At this point, glycolysis is finally ready to make ATP. Substrate-level phosphorylation occurs when one of the phosphates of 1,3-bisphosphoglyceric acid is transferred to ADP. The three-carbon molecule that remains is then rearranged to form phosphoenolpyruvic acid, becoming pyruvate when it gives up its phosphate to a second ADP. In this way, each PGAL from the first half of glycolysis is used to make two molecules of ATP and one molecule of pyruvic acid.
Through glycolysis, a small amount of the chemical energy that started out in glucose ends up in ATP and NADH, about 5% of it; most of the energy remains in pyruvate, which under aerobic conditions is used to either make more ATP in the mitochondria via the Krebs cycle or under anaerobic conditions builds up as lactate.
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The system supports moderate and high intensity exercise that lasts for up to 45 seconds or so.
Other sugars can be catabolised by glycolysis via their conversion to molecules that can be fed into the pathway in its first phase: •Fructose —-> 2 glyceraldehyde 3-phosphate •Lactose –> glucose + galactose, •Galactose –> glucose 1-phosphate –> glucose 6-phosphate •Mannose —> mannose 6-phosphate –> fructose 6-phosphate During periods when glycolytic metabolism exceeds oxidative phosphorylation (Krebs cycle and the electron transport chain) glycolysis’s end product; lactate, is passed to the blood and thence to the Liver where it’s converted back to glucose; under a process called gluconeogenesis and thence back to the blood and cells as fuel. This is known as the Cori cycle, or lactic acid cycle.
III. Aerobic System (Krebs cycle) using 1. Carbohydrate and 2. Fat.
1.Carbohydrate Summary: The degradation (oxidation) of the 2-carbon acetyl group of acetyl coenzyme-A (acetyl-CoA) through a cyclic sequence called the Krebs cycle (KC).
Carbohydrate enters the cycle through the conversion of one glucose molecule to two pyruvate molecules that are then in turn converted to two molecules of acetyl-CoA. Fatty acids are readily used in the system and this is covered in section 2 below. Electrons in the oxidations are transferred to NAD and to FAD, and a pyrophosphate bond is generated in the form of guanosine triphosphate (GTP).
This high-energy phosphate is readily transferred to ADP to form ATP. As Figure 6 illustrates, this is a truly continuous cycle.
Net reaction: Acetyl-CoA + 2H2O + 3NAD+ + Pi + GDP + FAD —>2CO2 + 3NADH + GTP + CoA-SH + FADH2 + 2H+ Details: Krebs cycle (KC) or the Citric Acid Cycle (CAC) or Tricarboxylic Acid Cycle (TCA), occurs in mitochondria and is the final common catabolic pathway to completely oxidise fuel molecules (monosaccharides and free fatty acids (FFAs)) under aerobic conditions. Sir Hans Krebs worked out the details of the cycle in the 1930’s. Two carbons enter the cycle as acetyl-CoA and two carbons leave as CO2. In the course of the cycle, four oxidation-reduction reactions take place to yield reduction potential in the form of three molecules of NADH and one molecule of FADH2. A high-energy phosphate bond (GTP) is also formed.
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Pyruvate, the product of anaerobic glycolysis, is produced in the cytosol. It’s moved into the mitochondrial matrix by active transport where it’s used to form acetyl-CoA by oxidative decarboxylation; this forms the link between aerobic and anaerobic carbohydrate catabolism and thus is also termed ‘the common degradation product’.
The first reaction of the cycle occurs when acetyl-CoA transfers its two-carbon acetyl group to the four-carbon compound oxaloacetate, forming citrate, a six-carbon compound. The citrate then goes through a series of chemical transformations, losing first one and then a second carboxyl group as carbon dioxide. Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the KC, three molecules of NAD+ are reduced to NADH. In Step 6, electrons are transferred to the electron acceptor FAD rather than to NAD+. Because two acetyl-CoA molecules result from each glucose, the cycle must turn twice to process each molecule. At the end of each turn of the cycle, the four-carbon oxaloacetate is left, and the cycle is ready for another turn. Only four moles of ATP are produced directly by a substrate-level phosphorylation with each turn of the KC, this is not much more than the two moles from glycolysis. The rest of the ATP that is formed during aerobic respiration is produced by the electron transport system and chemiosmosis see figure 7.
2.Fat Summary: The process of fatty acid (FA) oxidation is termed b-oxidation (beta oxidation) since it occurs through the sequential removal of 2-carbon units by oxidation at the b-carbon position of the fatty acyl-CoA molecule. Each round of b-oxidation produces one mole of NADH, one mole of FADH2 and one mole of acetyl-CoA, which then enters the KC, and under goes the reactions as described above. The oxidation of FAs yields significantly more energy per carbon than carbohydrates; b-oxidation of one mole of oleic acid (18-carbon FA) yields 146 moles of ATP; ~441 molecules, compared with 114 moles from 18 glucose carbon atoms.
Net reaction: Each reaction is specific to the length of the carbon backbone of the fat involved; the overall reaction for the b-oxidation of palmitic acid (palmitoyl-CoA) may be written as follows: C16-SCoA + 7 FAD + 7 NAD+ + 7 H2O + 7 CoASH 8 Acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+ Detail: Fatty acids are a major source of energy for most tissues. The first step in their utilization is by beta-oxidation of fatty acyl-CoAs. In this process the size of the fatty acy-lCoA is reduced in a sequence of steps. A sequence of four enzymatic reactions splits the molecule at the single CC bond between the (alpha) and (beta) carbons, hence beta-oxidation. Fat is a highly concentrated energy source. Muscle is the main tissue that burns (oxidises) fat. High rates of fat oxidation (fat oxidation phase/ FO phase) occur during the later stages of aerobic exercise; the exact point at which it begins cannot be specified because as transition from the Lactate Phase to the FO Phase is gradual. It’s important to note that in order to obtain energy efficiently from fat, glucose must be burned simultaneously, hence the expression: “Fat burns in the fires of glucose.” The primary sources of fatty acids (FAs) for b-oxidation are dietary and mobilisation from cellular stores. FAs from the diet can are delivered from the gut to cells via transport in the blood. FAs are primarily stored as triacylglycerols within adipocytes of adipose tissue. In response to energy demands, they can be mobilised for use by peripheral tissues. This release is controlled by a complex series of interrelated cascades that result in the activation of hormone-sensitive lipase.
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The stimulus to activate this cascade, in adipocytes, can be glucagon, epinephrine or b-corticotropin. These hormones bind cell-surface receptors that are coupled to the activation of adenylate cyclase upon ligand binding. The resultant increase in cAMP leads to activation of protein kinase A (PKA), which in turn phosphorylates and activates hormone-sensitive lipase. This hydrolyses FAs at carbons 1 or 3. The resulting diacylglycerols are substrates for either hormone-sensitive lipase or for the non-inducible enzyme diacylglycerol lipase. Finally the monoacylglycerols are substrates for monoacylglycerol lipase. The net result of the action of these enzymes is three moles of free fatty acid (FFA) and one mole of glycerol. The FFAs diffuse from adipose cells, combine with albumin in the blood, and are thereby transported to other tissues, where they passively diffuse into cells.
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b.Explain when the different systems might be used in the body The cells of the body utilise the breakdown of ATP to liberate energy to then undertake the functions necessary for life. There is only a limited supply of ATP in the body at any time, roughly 85g, as such only a few seconds (<10 seconds) of maximal energy release, and thus muscular work, can occur ‘on tap’, i.e. instantaneously at any one time. It’s not really known why the body has such low ATP stores, however, it’s quite heavy and the average sedentary person uses an amount of ATP equivalent to 75% of their body weight each day, so small amounts with rapid resynthesis is probably good, or we’d all be 75% heavier than our current weight, 10Houston (2001).
To maintain energy release ADP must be resynthesised to ATP on a regular basis within each cell as it’s used, and the resynthesis occurs within a specific metabolic pathway, the selection of which is dependent upon the volume and mode of exercise performed, i.e. the intensity. In general the higher the volume, and more isometric the mode then the more reliance on aerobic based energy pathways as the both the Creatine and the glycolytic systems become exhausted relatively quickly as energy reservoirs.
Carbohydrate is a crucial fuel for exercise and its main supplier into the blood is the liver – either directly from its own glucose store (glycogen) or by turning fat and protein into glucose (‘gluconeogenesis’).
However, the snag is that only limited amounts of glycogen can be stored in the liver and muscles (table 1).
The liver weighs only about 2kg, compared to the body muscle mass; weighing 10-20 times this, as such hard working muscles could totally deplete blood glucose in a few minutes. Simply, the liver can nowhere near supply glucose at a rate that the muscle can utilise.
Table 1: The body’s Fuel stores gkcal Carbohydrate Liver glycogen110451 Muscle glycogen2501025 Glucose in body fluids1562 Total3751538 Fat Subcutaneous780070980 Intramuscularly1611465 Total796172445 11Adapted from Wilmore & Costill (1999).
Thus, to protect the blood levels of glucose, and to protect the glucose supply to the brain (the brain is highly susceptible to low blood glucose as it’s virtually the only fuel it uses and it doesn’t have it’s own stores) and other peripheral tissues, muscle has somehow to be prevented from exhausting the blood glucose. And the method whereby this is done is threefold: •first, muscle is given a good store of glucose as glycogen; •second, muscle is prevented from using much glucose directly from the blood during muscular work; and •third, muscle is switched over to the massive fat energy store as soon as the glycogen / blood glucose runs low.
Thus as exercise intensity or duration increases, the contribution of the aerobic system to energy production initially decreases as the glycolytic system and the CrP system are used, this creates a characteristic continuum curve shown in figure 9, but then increases with their exhaustion and within the aerobic system, fat becomes an increasingly important energy source. Table 2 gives varying dependencies on the three systems and in Figure 10 an illustrated example is given.
As the rate of energy extraction from fat is much lower than carbohydrate, 5Journal of Applied Physiology, more oxygen is required to gain the same amount of energy, and hence more blood needs to be supplied, which in turn requires a higher heart rate. Since circulation is improved as a result of training, better fat utilisation is possible because of that increased supply,15Fox & Bowers.
Table2; Dependency of various sports activates on the different energy systems.
ActivityDependence on:Time DurationHrs:mins:secs Creatine PhosphateGlycolyticMitochondrial Kicking a ballHighLowLow0:0:05 Power liftingHighModerateLow0:0:05 Throwing EventsHighLowLow0:0:10 Running up stairsHighLowLow0:0:10 Pole VaultingHighModerateLow0:0:10 Jumping EventsHighLowLow0:0:10 100-200m SprintHighModerateLow0:0:10-0:0:30 50-100m SwimHighModerateLow0:0:10-0:0:30 Internal LiftingHighModerateLow0:0:30-0:2:00 400-800m SprintHighHighModerate0:0:60-0:3:00 200-400 m SwimHighHighModerate0:2:00-0:5:00 1500m run ModerateModerateHigh0:3:30-0:6:00 5-10k runLowLowHigh0:12:00-0.30:00 Marathon Low LowHigh2:0:00-4:0:00 Source: 17Cavangh & Kram (1985) There are a number of artificial means by which the fuel for exercise can be altered which for completeness of answer are summarised in table 3 below.
Table 3: Artificial means of altering the fuel balance for exercise.
1The ingestion of caffeine in sufficient quantities (about 5 mg/kg of body weight) can cause free fatty acid levels to peak after about 60 minutes and remain elevated for about three hours at about three to four times that of normal levels. The effect is delayed by about two hours if sugar is also taken at the same time.
2The drug Heparin has similar properties to that of caffeine. Although it has been used in an attempt to extend endurance performances, research has not been consistent in replicating the effects and benefits that it’s suggested to produce.
3A high carbohydrate meal causes blood insulin to rise and stay elevated for 60 to 90 minutes. Since insulin inhibits performance because it slows free fatty acid mobilization and the breakdown of glycogen in the liver, the body has to rely primarily on muscle glycogen and a small amount of glucose in the blood for energy. Those sources are used rapidly, hypoglycaemia could result (evidenced by dizziness, a feeling of weakness, or nausea), and endurance is reduced. 2Foster and Costill (1978) found reductions of 19 percent in endurance capacity in subjects who ingested 75 grams of glucose prior to performing a maximum exercise at 80 percent of VO2max. This would suggest it’s not wise to ingest any form of carbohydrate within two hours before a performance. This no longer is generally recommended although it’s necessary for individuals who are susceptible to reactive hypoglycaemia. Thus, testing for reactivity in athletes is important so that the best precompetition regimen can be established.
4The ingestion of glucose or carbohydrates during exercise can marginally prolong performance. It has no effect on muscle glycogen but it does spare the use of liver glycogen if it can be assimilated into the circulatory system in time. The rate of emptying from the stomach and absorption into the blood stream determine the value of this supplement. Emptying is facilitated by the glucose being diluted as a cool drink taken in resting or calm circumstances.
5The rate of muscle glycogen use appears to be increased in hot conditions.
6However the body’s athletic conditioning also plays a role in what fuel is chosen. Firstly specific training can be done to increase the creatine system and also to condition the muscles to resist the effects of lactic acid build-up from the glycolytic system. In addition as specific athletic fitness alters the call on fat versus carbohydrate oxidation and glycogen depletion is stalled, 2Foster & Costill (1978).
In addition regular training increases the total number of mitochondria in the muscles, thus making the ‘battery’ they provide larger, this not only improves endurance, 12Holloszy (1975) but delays the need to switch to fat, 13Holloszy (1967) and 14Dudley (1975), making muscle more efficient at a given intensity.