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E1–E4 The Role of ATP, & the ATP-PC, (Alactic) lactate & Aerobic Energy Systems GapFill
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Adenosine phosphate (ATP) is the immediately accessible form of energy for living cells. Hydrolysis (breakdown) of ATP is catalysed by the enzyme ATPase, releasing the energy required to drive cell processes, and in the case of muscles, for contraction. As well as the energy produced, breakdown of ATP releases adenosine diphosphate (ADP) and phosphate (P), which are used to resynthesise ATP in the reversible reaction: ADP + P + energy. The energy used in this reaction comes from the three main energy systems, placed on an energy continuum which details the differing rates of ATP resynthesis between them. The predominant system used depends on the intensity and of exercise (i.e. the demand for ATP at the muscle and the capacity of each system to provide this).
The ATP-PC ( ) system provides the most rapid resynthesis of ATP for exercise, so is used at the onset of exercise regardless of intensity. Continual use of this system depends on the intensity of exercise. During maximal-intensity anaerobic activity, this system will be the predominant supplier of energy for up to ~ seconds. Once this limited store of (PC) in the muscle is depleted, the ATP demand at the muscle exceeds the rate at which it can be supplied, and so exercise intensity must be reduced as a consequence.
The system takes over as the predominant supplier of energy. This is also used for anaerobic energy production, with reactions taking place starting with glucose as substrate, which is converted into energy, and as a by-product. This is associated with the reduction in the pH of the cell and direct inhibition of the processes involved in muscle contraction. Fitter individuals are able to resist fatigue to this harmful by-product longer than their untrained counterparts. This system can last for up to ~ minutes, depending on the intensity of exercise, before the resynthesis of ATP can no longer meet its demands at the muscle. As a consequence, exercise intensity must be reduced even further.
The aerobic energy system uses both and carbohydrates as fuel for exercise, generating energy in the of the cell. Because of the large amounts of these fuel sources stored in the muscle, liver, and adipose tissue, this energy system can be used almost indefinitely. Therefore, it is the predominant energy system used in long-duration, low-intensity exercise. At the onset of exercise, there is an oxygen lag of ~2–5 minutes, which means that even when exercise is completed at a low intensity from the outset, it would still take a delay before aerobic means of resynthesising ATP begin to take place.
The duration at which energy supply can last with each system is correlated with the length of time each takes to recover. It is also dependent on the intensity and duration of each exercise bout, but it is often quoted that around 50% of initial PC stores are resynthesised after ~30 seconds of recovery, with this system making an almost full recovery in ~3–4 minutes. The system takes a while longer to recover (roughly anywhere between 20 minutes and 2 hours), due to the time taken to remove the harmful waste products. Full recovery of the system depends on the amount of fuel used, but, in the worst case, would take only 24–48 hours. However, during intermittent exercise there is enough fuel available within this energy system to maintain supplies of ATP.