The three energy systems (ATP-PC, glycolytic and aerobic), their fuels, sites, yields and by-products, the energy continuum and intensity thresholds, EPOC, recovery and the factors affecting which system predominates.

What this dot point is asking

AQA wants you to describe the three energy systems used to resynthesise ATP, including their fuel, site, controlling enzyme, yield and by-products, explain the energy continuum and the intensity and duration at which each predominates, and explain EPOC and the recovery process.

Why we need energy systems

All muscular contraction is powered by adenosine triphosphate (ATP). ATP is broken by the enzyme ATPase into adenosine diphosphate (ADP) and an inorganic phosphate, and it is the energy released in this exothermic reaction that drives the cross-bridge cycle in muscle. The body stores only enough ATP for about 2 to 3 seconds of maximal work, so it must be resynthesised continuously. Rebuilding ATP from ADP and phosphate is endothermic (it requires energy), and the three energy systems differ in how they supply that energy: how quickly, from which fuel, and at what cost in by-products. Understanding which system dominates is the key applied idea AQA tests, because it explains why a sprinter and a marathon runner train and fatigue so differently.

The ATP-PC system

The reaction is a single coupled step: $\text{PC} \rightarrow \text{P} + \text{C}$, with the energy released used to drive $\text{ADP} + \text{P} \rightarrow \text{ATP}$. Because it involves no oxygen and only one chemical reaction, it provides energy almost instantly and at the highest rate of any system. Its limitation is capacity: PC stores are exhausted in roughly 8 to 10 seconds, after which the muscle must rely on glycolysis. PC stores are fully replenished within about 3 minutes of rest using oxygen, which is why recovery between maximal efforts matters so much for sprinters and games players.

The glycolytic (lactic acid) system

The anaerobic glycolytic system breaks down glucose (from blood glucose or muscle and liver glycogen) by glycolysis in the sarcoplasm, controlled by enzymes such as phosphofructokinase (PFK), to produce 2 ATP and pyruvate. Without sufficient oxygen the pyruvate is converted by lactate dehydrogenase (LDH) into lactic acid (lactate). It is the predominant system for high-intensity work lasting from about 10 seconds up to 3 minutes, such as a 400 m run or a sustained attacking phase in a game. The accumulation of lactate and the associated fall in pH (acidosis) inhibits enzyme activity and impairs contraction, which is the fatigue mechanism candidates must name. The point at which lactate begins to accumulate faster than it can be removed is the lactate threshold (OBLA), a key concept linked to endurance training.

The aerobic system

The high yield comes at the cost of speed: the multi-stage process and dependence on oxygen delivery make it the slowest to supply ATP. During very long, lower-intensity exercise (over about 20 minutes) the body increasingly uses beta oxidation of fats, which yields even more ATP per molecule but requires more oxygen, sparing limited glycogen stores.

The energy continuum, EPOC and recovery

The three systems do not work in isolation; the energy continuum describes how the predominant system changes with the intensity and duration of activity, and the relative contributions blend rather than switching cleanly. A games player, for example, moves continuously along the continuum as intensity rises and falls.

After exercise the body consumes more oxygen than at rest to recover, known as EPOC (excess post-exercise oxygen consumption), formerly called oxygen debt. It has two components: the fast (alactacid) component, which restores ATP and phosphocreatine stores and resaturates myoglobin with oxygen, and the slow (lactacid) component, which removes lactic acid, maintains a raised heart and breathing rate, and supports tissue repair and an elevated temperature and metabolism.