Respiration
What is Respiration?
Respiration is a series of chemical reactions in cells that releases energy from organic molecules (usually glucose). It is NOT breathing (ventilation) — it occurs in every living cell.
Energy released by respiration is used for:
- Active transport (moving substances against a concentration gradient)
- Muscle contraction
- Protein synthesis
- Maintaining body temperature (in mammals and birds)
- Cell division
Aerobic Respiration
Aerobic respiration uses oxygen to break down glucose fully, releasing maximum energy.
Word equation: Glucose + Oxygen → Carbon dioxide + Water
Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Where it occurs: Mainly in the mitochondria (the enzymes for the final stages are located here).
Energy yield: ~2800 kJ per mole of glucose — far more efficient than anaerobic respiration.
Aerobic respiration is used by cells under normal conditions when oxygen is available.
Anaerobic Respiration
Anaerobic respiration occurs when oxygen is not available or cannot be supplied fast enough — e.g., during intense exercise.
In animals (and humans):
Word equation: Glucose → Lactic acid
Occurs in: cytoplasm
Energy yield: Much less than aerobic (~200 kJ per mole) — glucose is only partly broken down.
Lactic acid builds up in muscles → muscle fatigue, cramp and soreness.
Oxygen debt (EPOC):
After intense exercise, you continue to breathe heavily — this extra oxygen is used to:
- Oxidise lactic acid to CO₂ and water
- Restore ATP, creatine phosphate and muscle glycogen levels
In yeast (and some microorganisms):
Word equation: Glucose → Ethanol + Carbon dioxide
This is called fermentation. Used in:
- Making bread (CO₂ makes dough rise)
- Brewing beer and wine (ethanol produced)
Note: Yeast can switch between aerobic (when O₂ is available) and anaerobic (when O₂ is absent) respiration.
Comparing Aerobic and Anaerobic Respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Where? | Cytoplasm → mitochondria | Cytoplasm only |
| Glucose fully broken down? | Yes | No |
| Products | CO₂ + H₂O | Lactic acid (animals) or ethanol + CO₂ (yeast) |
| Energy released | Much (efficient) | Little (less efficient) |
| Used when? | Normal conditions | Intense exercise / no O₂ |
Effects of Exercise on the Body
During exercise:
- Muscle cells require more energy → respiration rate increases
- Heart rate and breathing rate increase to deliver more oxygen and glucose to muscles, and remove CO₂
- If exercise intensity exceeds oxygen supply → anaerobic respiration begins
- Lactic acid builds up → muscles fatigue; you feel "the burn"
- After exercise: heart and breathing rates remain elevated until the oxygen debt is repaid
AI-generated · claude-opus-4-7 · v3-wjec-combined-science