The VCE Phys Ed Network

ATP - Energy for muscular contraction

Adenosine Triphosphate (ATP), is the immediate usable form of chemical energy for muscular activity. This is one of the most important of the energy rich compounds which is stored in all cells, particularly muscle cells. All forms of chemical energy available from the food we eat must eventually be transferred into ATP before they can be utilized by the muscle cell. Once depleted, ATP must be replaced for the body to maintain muscular contractions. There are three systems that operate to maintain the body's energy.

Energy can be defined as the capacity to perform work. Work is defined as the application of a force through a distance.

Energy cannot be created or destroyed, it is transformed from one form to another. It is the transformation of chemical energy in the human body to mechanical energy that enables human movement. The energy to allow this movement to occur comes from food. However, the energy released during the breakdown of food - carbohydrates, fats, protein - is not directly used to perform work, it is used to resynthesize ATP. ATP is stored in the muscle cells. The energy released during the breakdown of ATP is the only source of energy that can be used directly by the muscle cell to perform work (physical activity). Because the muscle cells only store a very small amount of ATP (enough for 1-2 seconds of maximal activity) and it is only through the breakdown of ATP that energy is released for work; the body must have ways of putting the ATP molecule back together again.

The energy systems

There are three systems where chemical energy is made available for the rebuilding of ATP:

1. ATP-PC or phosphagen system,
   
2. Lactic acid or anaerobic glycolysis system 
3. Aerobic or oxygen system.

All three suppliers of energy for ATP rebuilding operate from the same principle - energy liberated from the breakdown of food and phosphagens is used to put the ATP molecule back together again - to resynthesize it. Two of the three systems, the ATP-PC system and Lactic Acid system are termed anaerobic. Anaerobic means 'without oxygen'. Thus anaerobic metabolism refers to the rebuilding of ATP through chemical reactions that do not require oxygen. The other system is aerobic and needs oxygen to metabolise fuels. All three energy systems are initiated at the start of exercise and none function independently of one another. This is called energy system interplay - the main supplier of energy changes but all energy systems have some contribution to energy output.

The ATP-PC System

The first energy system in terms of order of predominance and power (the rate at which energy is liberated) is the ATP-PC system. This works on the interaction between ATP and a fuel called phosphocreatine which is stored in the muscles. This fuel is also called creatine phosphate, it is not a food fuel although it is present in many food sources that come from animals. 

ATP (adenosine triphosphate) is stored in the muscles – we have 50g worth, enough for 1-2 seconds of intense physical activity. When ATP is used for muscular contraction it is split. The breakage of the outermost bond causes a phosphate to be released so that ADP (Adenosine Diphosphate) and Pi (Inorganic phosphate remains. The bond breakage releases energy. Notice how one phosphate is removed from ATP is split, because it is no longer associated with the rest of the molecule it is said to be an inorganic phosphate. This can actually contribute to fatigue as can ADP, but luckily these to molecules can be re-used to make ATP. However, for them to be reused, a bond of a fuel needs to be broken so that the ADP can be rejoined to the phosphate.

Creatine phosphate is useful fuel for this, however we only have limited supplies of it in our muscles – enough for about 10 seconds of high-intensity physical activity. When CP/PC is split, energy is released that goes towards ATP resynthesis. This a very fast reaction and this is why it can supply energy initially in explosive activities. The ATP-PC system is the predominant provider of energy between 1-10 seconds. As PC deplete the anaerobic glycolysis (lactic acid system increases its output). When PC is almost fully depleted the ATP-PC only contributes about one percent to overall power output. PC stores are replenished through the aerobic recovery and a passive recovery most (where the athlete rests completely) is most efficient. During exercise PC stores cannot be properly depleted. This is why you see long jumpers for example lie down after they have finished jumping. After 30 seconds of rest PC stores are about 70% replenished. After 180 seconds they are approximately 98% replenished.

ATP-PC SYSTEM SUMMARY:

FOOD FUEL:  None, PC is not a food fuel.

FUEL INPUT: Creatine Phosphate (PC/CP)

Energy Output (Yield): 1 ATP molecule from each PC molecule

Energy system dominance: 1-10 seconds

Peak power output: 3-5 seconds

Fatigue factors: Increased body temperaSince the ATP-PC system energy has the highest rate of energy liberation, depletion of PC stores is a significant contributor to fatigue because the athlete cannot maintain the same power output. Also contributing to fatigue are the accumulation of inorganic phosphates and ADP caused particularly by the use of this system.

Lactic Acid System

Glucose is a versatile fuel. It can be broken down both aerobically and anaerobically. When we eat carbohydrates (CHO) we are able to extract glucose from them in digestion. This glucose travels through the bloodstream, some is stored in the liver and some in the muscles. Glucose is stored as glycogen. When we need it for energy, we must first convert glycogen back to glucose – this process does not release ATP yet. Note that the Nelson textbook is incorrect, its diagrams show ATP being released during glycogen breakdown (catabolism) when it in fact occurs during glucose break down. This process is called glycolysis and it occurs in the cytosol. This is the fluid that is within all muscles cells. The process does not need oxygen and results in a net output of 2 ATP molecules per glucose molecule – a greater yield than the ATP-PC system. However this energy is released more slowly then the ATP-PC system. Glucose is converted into pyruvic acid and because no oxygen is present is is converted into lactic acid. This lactic acid then dissociates into two parts: lactate and hydrogen ions. The lactic acid system as with both the anaerobic systems has a finite capacity - it is limited because it produces fatiguing by-products, namely hydrogen ions.

Place of reaction: Anaerobic glycolysis occurs in the cytosol.

FOOD FUEL:  Carbohydrates

FUEL INPUT: Glucose/Glycogen

Energy Output (Yield): 2 ATP molecule from each glucose molecule

Energy system dominance: 10-30 seconds

Peak power output: 10-15 seconds

Fatigue factors: Hydrogen ion (H+) accumulation decreases muscle pH and increases muscle acidity and this interferes with glycolytic enzymes which assist with the breakdown of glucose. Lactate concentration correlates with fatigue but does not directly cause fatigue. This area always developing and research continues...

Aerobic System

The aerobic system is the most efficient but therefore slowest of all the energy systems because it completely breaks down all food fuels. It also has the ability to catabolise fats and protein but it does not break down PC. The aerobic system requires oxygen for its last two stages. The first stage is glycolysis, it is exactly the same process that occurs in the lactic acid system - that is, it does not require oxygen and it occurs in the cytosol of the cell. However, when pyruvic acid forms, it is transported into the mitochondria to be further metabolised because oxygen is available.

What are mitochondria? They are organelles (cellular components) that house the enzymes and membranes necessary for oxidation of fuels to occur and for ATP to be extracted from these fuels. Only aerobic processes occur in a mitochondrion. The second stage of aerobic glycolysis occurs when pyruvic acid is broken down in the krebs or citric acid cycle. This process does not require oxygen but it relies on oxygen being present. Another 2 ATP are released at this stage and carbon dioxide is given off as a by-product. The next stage is electron transport. The hydrogen ions (released during glycolysis which cause fatigue if they are not removed) are used to generate 32 ATP. They then combine with oxygen to form water - a by-product of electron transport. Heat is also released and this can contribute to fatigue especially if the athlete is not adequately hydrate.

Place of reaction: Glycolysis occurs in the cytosol, krebs cycle and electron transport in the mitochondria.

Fatigue factors: Increased body temperature adversely effects enzymes and can lead to heat stroke and increased perceived effort. If the athlete is dehydrated this means that they need to conserve water and this further contributes to the increased body temperature because sweating decreases. The blood becomes more concentrated and therefore cardiac output decreases and less oxygen goes to the muscles. The blood instead travels to the skin where vasodilation (expansion of blood vessels) occurs in an effort to release heat through radiation to the atmosphere. This is different to sweating which involves the body's heat being used to evaporate water (sweat). Depletion of glycogen stores is another factor which contributes to fatigue. When hypoglycaemia (low blood glucose) sets in, the body is forced to utilise more fat and this requires more oxygen. Fortunately the by-products released during aerobic glycolysis are not toxic and this explains why this system is able to last indefinitely.