Respiration (B1.3)
Respiration is the process that releases energy from glucose for all cellular processes. It is tested on virtually every Gateway A paper — expect both equation recall and a 6-mark application question on exercise physiology.
What is respiration?
Respiration is an exothermic chemical reaction that transfers energy from glucose to ATP. The energy is used for:
- Muscle contraction
- Active transport
- Protein synthesis
- Maintaining body temperature
⚠ Critical distinction: respiration is NOT breathing. Breathing is the mechanical ventilation of the lungs; respiration is the chemical process inside every cell.
Aerobic respiration
Uses oxygen; takes place in the mitochondria; releases large amounts of energy.
Word equation:
glucose + oxygen → carbon dioxide + water
Symbol equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Approximately 36–38 ATP molecules are made per glucose molecule.
Anaerobic respiration
Occurs when oxygen supply is insufficient (e.g. during intense exercise). Takes place in the cytoplasm. Releases much less energy (only ~2 ATP per glucose).
In animals (and humans):
glucose → lactic acid
Lactic acid builds up in muscles → muscle fatigue → burning sensation. The lactic acid must eventually be oxidised to CO₂ and water using oxygen — this creates the oxygen debt.
In plants and yeast (anaerobic fermentation):
glucose → ethanol + carbon dioxide
This is exploited in brewing (ethanol production) and bread-making (CO₂ makes dough rise).
Response to exercise
During exercise:
- Heart rate increases → more blood pumped per minute → more oxygen and glucose delivered to muscles.
- Breathing rate (ventilation) increases and breathing becomes deeper → more gas exchange.
- Blood vessels to muscles dilate (vasodilation) → more blood flow to muscles.
If exercise is intense enough that aerobic respiration alone cannot supply enough ATP, anaerobic respiration begins.
Oxygen debt
After strenuous exercise, the body needs extra oxygen to:
- Oxidise the lactic acid produced anaerobically into CO₂ and water.
- This extra oxygen demand above resting level = the oxygen debt (sometimes called excess post-exercise oxygen consumption, EPOC).
This is why breathing and heart rate remain elevated for some time after exercise stops — the body is repaying the oxygen debt.
Comparing aerobic and anaerobic respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Location | Mitochondria | Cytoplasm |
| ATP yield | ~36–38 per glucose | ~2 per glucose |
| Products | CO₂ + water | Lactic acid (animals) OR ethanol + CO₂ (yeast/plants) |
| End product removed? | Exhaled / excreted | Lactic acid removed via blood → liver |
Common Gateway-paper mistakes
- Writing that anaerobic produces CO₂ in animal cells — only ethanol fermentation in yeast/plants produces CO₂.
- Forgetting that respiration happens in ALL cells all the time, not just during exercise.
- Saying "respiration provides oxygen" — it USES oxygen (aerobic) or doesn't use oxygen (anaerobic).
- Confusing oxygen debt with being "out of breath" — debt is the extra oxygen needed AFTER exercise to metabolise lactic acid.
- Placing aerobic respiration in the cytoplasm — it's in the mitochondria.
➜Try this— Quick check
A runner sprints 400 m. At the end of the sprint her heart rate and breathing rate remain elevated for 10 minutes. Explain this using the concept of oxygen debt.
- During the sprint, muscles used anaerobic respiration, producing lactic acid.
- After the sprint, elevated breathing and heart rate supply extra oxygen (above resting level).
- This extra oxygen oxidises the lactic acid to CO₂ and water.
- Once the debt is repaid, rates return to normal.
AI-generated · claude-opus-4-7 · v3-ocr-combined-science