Aerobic vs anaerobic respiration: GCSE PE explained
Aerobic respiration is the release of energy from glucose using oxygen, producing carbon dioxide and water as waste. Anaerobic respiration is the release of energy from glucose without oxygen, producing lactic acid as waste. In sport, your body switches between the two depending on how hard you are working: A steady jog runs on aerobic respiration, while a 100 m sprint runs almost entirely on anaerobic respiration.
This guide covers the two word equations, when each system is used in sport, the role of lactic acid and oxygen debt, and the exam-ready definitions AQA examiners look for in GCSE PE Paper 1.
Aerobic uses oxygen
Slow, long-lasting energy release. Used during steady cardio like long-distance running, cycling, and swimming.
Anaerobic skips oxygen
Fast, short-burst energy release. Used during sprints, weightlifting, and explosive movements lasting under a minute.
Duration is the key clue
If the activity lasts more than around 60 seconds at a steady effort, it is aerobic. Short, all-out bursts are anaerobic.
The two word equations
You need to learn both equations word-for-word for AQA GCSE PE. They appear in nearly every Paper 1 exam, usually as a one or two-mark recall question. Get the wording exactly right and the marks are easy.
The aerobic equation involves four substances: Glucose and oxygen as the inputs, and carbon dioxide and water as the waste outputs. The anaerobic equation is shorter because there is no oxygen and no carbon dioxide or water produced.
| System | Word equation | Energy released |
|---|---|---|
| Aerobic respiration | Glucose + oxygen → carbon dioxide + water (+ energy) | Large amount, released slowly |
| Anaerobic respiration | Glucose → lactic acid (+ energy) | Smaller amount, released quickly |
Common equation mistakes Forgetting to include glucose as an input. Writing oxygen on the right-hand side of the anaerobic equation (there is no oxygen in anaerobic respiration). Spelling lactic acid as lactose or lactate at GCSE. Missing the word energy in brackets at the end.
When each system is used in sport
Your body never sticks to one system. It uses both at the same time and shifts the balance based on how hard the working muscles are demanding energy. The shift depends on intensity and duration, not on the sport itself.
A marathon is almost entirely aerobic because the runner sets a pace they can maintain for hours. A 100 m sprint is almost entirely anaerobic because the body cannot deliver oxygen to the muscles fast enough. A football match is a mix of both: Aerobic during the steady jog between plays, anaerobic during the sprints and tackles.
| Activity | Main system | Why |
|---|---|---|
| Marathon | Aerobic | Steady pace held for 2 hours or more |
| 100 m sprint | Anaerobic | Maximum effort under 12 seconds |
| Long jump approach | Anaerobic | Explosive sprint under 6 seconds |
| 1500 m run | Mostly aerobic with anaerobic finish | Steady pace then sprint finish |
| Football match | Both | Steady jog with repeated sprints |
| Weightlifting (1 rep max) | Anaerobic | Single explosive effort under 5 seconds |
Lactic acid and oxygen debt
Lactic acid is the waste product of anaerobic respiration. As you keep working anaerobically, lactic acid builds up in the working muscles. This is what causes the burning feeling in your legs near the end of a 400 m race and is one reason your performance drops if you keep pushing.
When you finish exercising, your body still needs oxygen to break down the lactic acid that has built up. The extra oxygen your body takes in after exercise is called the oxygen debt or excess post-exercise oxygen consumption (EPOC). This is why you keep breathing heavily for several minutes after stopping a hard sprint.
Exam answer for oxygen debt Oxygen debt is the amount of extra oxygen the body needs after exercise to break down the lactic acid built up during anaerobic respiration. To repay the debt, you continue breathing deeply and quickly until the lactic acid has been converted to carbon dioxide and water.
Comparing the two systems side by side
A six or eight-mark question on AQA Paper 1 often asks you to compare aerobic and anaerobic respiration. The mark scheme is structured: One mark for each clear point of comparison, with extra marks for linking the comparison to a sporting example.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen used | Yes | No |
| Speed of energy release | Slow | Fast |
| Amount of energy released | Large | Smaller |
| Waste products | Carbon dioxide and water | Lactic acid |
| Duration it can be sustained | Hours | Up to around 60 seconds |
| Typical sporting example | Marathon, long-distance cycling | Sprint, weightlifting |
Worked example: A six-mark exam question
Question: Compare aerobic and anaerobic respiration, using examples from sport. (6 marks)
Model answer: Aerobic respiration uses oxygen and releases energy slowly over a long period, while anaerobic respiration does not use oxygen and releases energy quickly in short bursts. Aerobic respiration produces carbon dioxide and water as waste, while anaerobic respiration produces lactic acid. A marathon runner uses aerobic respiration because they need to maintain a steady pace for over two hours. A 100 m sprinter uses anaerobic respiration because the activity is too short and too intense for the body to supply enough oxygen. Anaerobic respiration causes lactic acid to build up, which leads to muscle fatigue, while aerobic respiration can be sustained for much longer without fatigue.
This answer scores six marks because it gives three clear comparisons, two sporting examples, and links lactic acid to fatigue.
Examiner tip For any compare-style question, write your answer in pairs. State the aerobic point, then immediately state the anaerobic equivalent. Examiners can tick off comparison marks quickly when your structure makes the points obvious.
Key facts to memorise for the exam
- Aerobic equation: Glucose + oxygen → carbon dioxide + water (+ energy)
- Anaerobic equation: Glucose → lactic acid (+ energy)
- Aerobic is slow, long-lasting, and uses oxygen
- Anaerobic is fast, short-burst, and produces lactic acid
- Lactic acid causes the burning feeling and muscle fatigue
- Oxygen debt is the extra oxygen taken in after exercise to break down lactic acid
- Marathon = aerobic, 100 m sprint = anaerobic, football = both
- Six-mark answers need comparisons in pairs plus a sporting example
