Energy Systems: ATP-PC, Lactic Acid and Aerobic
Muscle contraction is fuelled directly by ATP (adenosine triphosphate) breaking down into ADP, phosphate and energy. Three energy systems resynthesise (replenish) ATP.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn that all muscle contraction is fuelled directly by ATP breaking down to ADP + Pi, releasing energy, and that the three energy systems exist to resynthesise (replenish) ATP.
- Learn the fuel, the presence or absence of oxygen, the by-product(s) and the approximate duration for each of the three energy systems: ATP-PC, lactic acid (anaerobic glycolysis) and aerobic.
- Learn the word equation for aerobic respiration and for the lactic acid system, including which by-products each one produces.
- Learn to match a sporting example to each system based on the intensity and duration of the effort: short and maximal for ATP-PC, high intensity for around 10 seconds to 2-3 minutes for lactic acid, and low-to-moderate intensity over a long duration for aerobic.
- Learn that the three systems work together on a continuum rather than in strict isolation, and that the dominant system shifts as an activity's intensity and duration change, e.g. from ATP-PC into the lactic acid system as a sprint is extended.
- For explain questions, always state the specific reason a system suits an activity - its duration/intensity match, its by-products, its rate of ATP production - rather than just naming the system.
- For extended questions, take a named sport or a phase within it, identify which system(s) dominate at each stage, and justify the choice using intensity, duration and by-products.
Worked example
A 400m sprinter takes around 50 seconds to complete the race, working at a very high intensity throughout. Identify the main energy system used for the first few seconds of the race and the main energy system used for the remainder of the race, and explain why the athlete is likely to experience muscle fatigue in the final stages.
- Identify the system used for the very start of the race: the ATP-PC system, since it provides immediate energy for the maximal effort of the initial acceleration but only lasts up to about 10 seconds.
- Identify the system used for the remainder of the roughly 50-second race: the lactic acid (anaerobic glycolysis) system, since the effort is too intense to be sustained aerobically but lasts longer than the ATP-PC system can supply alone.
- Explain the cause of fatigue: the lactic acid system produces lactic acid as a by-product.
- Continue the explanation: lactic acid accumulates faster than it can be removed at this intensity, causing the muscles to become acidic.
- State the effect: this disrupts muscle contraction, causing the fatigue felt in the final stages of the race.
Practice questions
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Q1Name the molecule that is broken down to directly release energy for muscle contraction.Show answer
Answer: ATP (adenosine triphosphate).
Q2State the fuel used by the ATP-PC system.Show answer
Answer: Phosphocreatine (PC).
Q3Name the by-product of the lactic acid (anaerobic glycolysis) energy system.Show answer
Answer: Lactic acid.
Q4Write the word equation for aerobic respiration.Show answer
Answer: Glucose + oxygen -> carbon dioxide + water (+ energy).
Q5State roughly how long the ATP-PC system can supply energy for before it is depleted.Show answer
Answer: Up to about 10 seconds.
Q6Give one sporting example best suited to the aerobic energy system.Show answer
Answer: Marathon running, or any low-to-moderate intensity, long-duration activity.
Q7Which energy system produces lactic acid as its by-product?Show answer
Answer: The lactic acid (anaerobic glycolysis) system.
Q8State one reason the aerobic system is not suitable for a 100m sprint.Show answer
Answer: It produces energy too slowly to meet the very high, immediate intensity demand of a maximal sprint - the ATP-PC system supplies energy far faster over that short duration.
Exam-style questions
Written in the style of a GCSE Physical Education exam paper, with a full mark scheme.
A tennis player plays a long rally lasting around 40 seconds at high intensity, followed by a 20-second rest before the next point. Explain which energy system is dominant during the rally and why lactic acid builds up.
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Using named examples from a game of your choice, analyse how the three energy systems work together to supply energy for a team sport performer, and evaluate which system contributes most to the performer's overall success.
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See real GCSE Physical Education past-paper questions, with official mark schemes →
Free printable worksheet
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