Respiration and Gas Exchange
What this domain tests
Questions check the stages and sites of aerobic and anaerobic respiration, ATP accounting, the human respiratory tract, the mechanism of breathing, and how O2 and CO2 are carried in blood.
Core rules
- Breathing is ventilation; respiration is the controlled release of energy from glucose as ATP.
- Aerobic: glycolysis in cytoplasm (glucose → 2 pyruvate, net 2 ATP, 2 NADH); link reaction and Krebs cycle in the mitochondrial matrix; electron transport chain on the inner membrane - about 36–38 ATP per glucose.
- Anaerobic in muscle: pyruvate → lactic acid, 2 ATP; in yeast: pyruvate → ethanol + CO2, 2 ATP.
- Lactic acid build-up causes fatigue and an oxygen debt repaid by continued heavy breathing.
- Human tract: nostrils, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. Alveoli give a huge, thin, moist, vascular surface for diffusion.
- Breathing in: diaphragm contracts and flattens, external intercostals lift ribs, thoracic volume rises, pressure falls, air enters.
- Tidal volume about 500 mL; vital capacity is the maximum voluntary exchange.
- Oxygen travels mostly as oxyhaemoglobin; carbon dioxide mostly as bicarbonate (about 70%), some as carbaminohaemoglobin and dissolved in plasma.
Worked example
After a 100 m sprint a runner keeps panting. Reasoning: muscle oxygen supply fell short, so pyruvate was reduced to lactic acid for quick ATP. The lactic acid must be oxidised back once oxygen is available, so heart rate and breathing stay high to deliver extra O2 - this is the oxygen debt being repaid in the liver.
Traps that cost marks
Treating breathing and respiration as synonyms; placing glycolysis in the mitochondrion; saying CO2 is carried mainly as carbaminohaemoglobin; claiming ATP is produced in the lungs; forgetting yeast makes ethanol, not lactic acid.