The process by which glucose is broken down in the presence of oxygen to release energy (ATP), producing carbon dioxide and water as by-products. Occurs in the mitochondria.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP energyGlucose + Oxygen → Carbon dioxide + Water + Energy
Energy released is used for: muscle contraction, active transport, protein synthesis, cell division, maintaining body temperature.
2. Anaerobic Respiration
Anaerobic respiration
Incomplete breakdown of glucose without oxygen. Releases much less energy than aerobic respiration.
In humans (muscles)
In yeast
Equation
Glucose → lactic acid + small amount of ATP
Glucose → ethanol + CO₂ + small amount of ATP
Products
Lactic acid
Ethanol + carbon dioxide
Energy yield
Much less than aerobic
Much less than aerobic
When it occurs
During intense exercise when O₂ supply is insufficient
In absence of oxygen (brewing, bread making)
Oxygen debt
Lactic acid accumulated during anaerobic respiration must be broken down after exercise — this requires oxygen. The extra oxygen needed is the oxygen debt. It is repaid by continued heavy breathing after exercise.
Lactic acid — not lactate for O-Level
At O-Level, the product of anaerobic respiration in humans is stated as lactic acid. "Lactate" is the correct biochemical term, but exam mark schemes at this level accept lactic acid.
3. Lung Structure
Structure
Function
Trachea
Airway from throat to bronchi; supported by cartilage rings
Bronchi
Two branches, one to each lung
Bronchioles
Smaller branches within each lung
Alveoli
Tiny air sacs where gas exchange occurs
Diaphragm
Muscular sheet below lungs; contracts to aid inhalation
Intercostal muscles
Between ribs; external muscles contract to raise ribs during inhalation
Ventilation (breathing)
Inhalation
Exhalation
Diaphragm
Contracts (flattens)
Relaxes (domes)
Intercostal muscles
External: contract (ribs up and out)
External: relax (ribs down and in)
Thorax volume
Increases
Decreases
Air pressure in lungs
Decreases below atmospheric → air in
Increases above atmospheric → air out
4. Gas Exchange in Alveoli
Alveoli are adapted for efficient gas exchange:
Enormous number (~300 million) → very large total surface area (~70 m²).
Thin walls (one cell thick) → short diffusion distance.
Moist surfaces → gases dissolve before diffusing across.
Good ventilation → continuously replenishes O₂ and removes CO₂.
O₂ diffuses from alveolar air into blood (high → low O₂ concentration). CO₂ diffuses from blood into alveolar air (high → low CO₂ concentration). Both move by diffusion — no energy required.
Aerobic Respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP energy
Occurs in mitochondria. Releases maximum energy (~36 ATP per glucose).
Anaerobic Respiration (animals)
Glucose → Lactic acid + small amount of ATP
No O2 required. Only 2 ATP produced. Lactic acid must be removed afterwards.
Must-Know for Exam
Aerobic: in mitochondria, produces ~36 ATP, products are CO2 + H2O
Anaerobic (animals): produces lactic acid, only ~2 ATP per glucose
Anaerobic (yeast): produces ethanol + CO2 (fermentation)
Alveolus features for gas exchange: one cell thick, large surface area, moist lining, rich capillary network
Oxygen debt = lactic acid produced during anaerobic respiration that must be repaid aerobically
5. Common Exam Traps
Trap 1 — Breathing is not respiration
Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is the chemical process in cells that releases energy from glucose. They are linked but not the same process.
Trap 2 — Inhaled air still contains oxygen; exhaled air still contains CO₂
Inhaled air: ~21% O₂, ~0.04% CO₂. Exhaled air: ~16% O₂, ~4% CO₂. Exhaled air is not 100% CO₂ — it still contains a significant amount of oxygen. Never say "we breathe out pure CO₂".
Trap 3 — Steep concentration gradient maintenance
When explaining alveolar adaptations, always link each feature to maintaining a steep concentration gradient for diffusion — this is the key mechanism being maximised. "Good blood supply brings CO₂ to the alveolus and removes O₂ quickly, maintaining the gradient."
Key Terms — Flashcard Review
Tap each card to reveal the definition.
Aerobic respiration
C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP. In mitochondria. Maximum ATP yield (~36 ATP per glucose).
Anaerobic (humans)
Glucose -> lactic acid + small amount of ATP. Occurs in muscles when O2 is insufficient during intense exercise.
Anaerobic (yeast)
Glucose -> ethanol + CO2 + small amount of ATP. Used in fermentation for brewing and bread-making.
Alveolus
Air sac in lungs. Thin walls (one cell thick), moist lining, huge surface area, good blood supply. Site of gas exchange.
Oxygen debt
Lactic acid accumulated during anaerobic respiration. Must be oxidised aerobically after exercise to remove it.
Explanation: O₂ diffuses passively from alveolar air (high [O₂]) into blood (low [O₂]). No energy required.
Question 6 of 8
Alveoli have a large surface area primarily to:
Explanation: A large surface area increases the rate of diffusion of O2 from air into blood and CO2 from blood into air (Fick's law). Alveoli also have thin walls, moist lining, and a rich blood supply to further maximise exchange.
Question 7 of 8
Lactic acid accumulates in muscles during sprinting because:
Explanation: During intense exercise, muscles cannot receive enough O2 quickly enough. They switch to anaerobic respiration (glucose to lactic acid + small ATP). Lactic acid causes the burning sensation and eventual muscle fatigue.
Question 8 of 8
Continued rapid breathing after vigorous exercise serves to:
Explanation: Oxygen debt: the extra O2 needed after exercise to break down accumulated lactic acid. The liver converts lactic acid to glucose, or it is oxidised to CO2 + H2O. Breathing stays fast until all lactic acid has been removed.