Where does the energy for muscle contraction come from across different intensities and durations?
The three energy systems (ATP-PC, anaerobic glycolytic and aerobic), their fuels, by-products and yields, the energy continuum, and the recovery process including EPOC.
A focused WJEC A-Level PE answer on the three energy systems, covering ATP-PC, anaerobic glycolysis and the aerobic system, their fuels and by-products, the energy continuum, and recovery including EPOC and the oxygen debt.
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What this dot point is asking
WJEC wants you to explain how ATP is resynthesised by the three energy systems, name the fuel, site, by-products and yield of each, place them on the energy continuum, and describe the recovery process including EPOC (the oxygen debt) and its two components.
ATP, the energy currency
The three energy systems
- ATP-PC (alactic) system
- Phosphocreatine (PC) stored in muscle is broken down by creatine kinase, releasing energy to rejoin ADP and a phosphate into ATP. It is very fast and anaerobic, with no lactate produced, but the PC store lasts only about 8 to 10 seconds. It dominates in explosive actions such as a jump or short sprint.
- Anaerobic glycolytic (lactic) system
- Glycogen is broken down to glucose and then, in glycolysis, to pyruvate. Without sufficient oxygen, pyruvate is converted to lactate. This releases energy anaerobically for a net yield of 2 ATP per glucose. It predominates in efforts of roughly 10 seconds to 3 minutes, such as a 400 m run, but the build-up of lactate (and the associated hydrogen ions) causes fatigue.
- Aerobic system
- With enough oxygen, glucose and fats are fully broken down through glycolysis, the Krebs cycle and the electron transport chain, producing carbon dioxide and water and a large yield of up to 38 ATP per glucose molecule. It has no fatiguing by-product, so it powers prolonged, lower-intensity exercise such as a marathon, but it is comparatively slow to produce ATP.
The energy continuum
All three systems operate at the same time. The energy continuum describes how the predominant system changes with the intensity and duration of the activity. A games player moves continually along it: sprinting (ATP-PC), sustained high-intensity running (glycolytic), and jogging or recovering (aerobic). The crossover between systems is gradual, not a sharp switch.
Recovery and EPOC
In the fast component, oxygen resynthesises the muscle's stored ATP and phosphocreatine and reloads oxygen onto myoglobin and haemoglobin; it is repaid within a few minutes. In the slow component, lactate is converted back to pyruvate and either oxidised for energy or used to resynthesise glycogen (gluconeogenesis); this also supports the raised heart rate, breathing rate and body temperature, and can take much longer.
Examples in context
Example 1. The oxygen debt after a sprint. A 200 m runner gasps for breath for minutes after finishing because EPOC must restore ATP and PC quickly and then clear the lactate built up during the race. The continued deep breathing supplies the oxygen needed for both components.
Example 2. Fat as a fuel in endurance. During a long, easy run, the aerobic system uses a higher proportion of fats (beta-oxidation) to spare glycogen. Trained endurance athletes adapt to use fat more readily, conserving glycogen for later in the event, a typical WJEC application of aerobic metabolism.
Try this
Q1. Name the fuel and the by-product of the anaerobic glycolytic system. [2 marks]
- Cue. Fuel: glycogen (glucose). By-product: lactate.
Q2. Explain why the ATP-PC system can only supply energy for a short time. [2 marks]
- Cue. The phosphocreatine store in muscle is small and is used up within about 8 to 10 seconds, so it cannot resynthesise ATP beyond that.
Q3. Describe the two components of EPOC and what each restores. [4 marks]
- Cue. Fast (alactacid) component restores ATP and PC and resaturates myoglobin/haemoglobin; slow (lactacid) component removes lactate and replenishes glycogen while supporting the raised heart rate, breathing and temperature.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20196 marksA games player sprints, jogs and walks repeatedly during a match. Explain how the three energy systems contribute to their performance, referring to the predominant system at different points.Show worked answer →
All three systems work at once, but one predominates depending on intensity and duration.
During short maximal sprints (up to about 10 seconds) the ATP-PC system predominates, using phosphocreatine to rephosphorylate ADP very rapidly with no fatiguing by-product.
During sustained high-intensity efforts (about 10 seconds to 3 minutes) the anaerobic glycolytic system predominates, breaking down glycogen to pyruvate and then lactate, which causes fatigue.
During low-intensity jogging and recovery between efforts, the aerobic system predominates, fully oxidising glycogen and fats to carbon dioxide and water with a large ATP yield and no fatiguing by-product.
Markers reward matching each system to an intensity, naming the fuel and by-product, and the idea that the body moves along the energy continuum.
WJEC 20214 marksExplain what is meant by EPOC and describe two processes that take place during the recovery period.Show worked answer →
EPOC (excess post-exercise oxygen consumption) is the extra oxygen consumed after exercise, above resting levels, to restore the body to its pre-exercise state. It is also called the oxygen debt.
In the fast (alactacid) component, oxygen resaturates myoglobin and haemoglobin and resynthesises stored ATP and phosphocreatine.
In the slow (lactacid) component, lactate is converted back to pyruvate and oxidised or used to resynthesise glycogen (gluconeogenesis); oxygen also supports the raised heart rate, breathing and temperature.
Markers reward the definition of EPOC, and at least two specific recovery processes such as PC resynthesis and lactate removal.
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