Respiration and Gas Exchange
Aerobic and anaerobic respiration
Respiration releases energy from glucose inside cells. Aerobic respiration uses oxygen and gives carbon dioxide and water, releasing much energy. Anaerobic respiration in muscles gives lactic acid and less energy; in yeast it gives ethanol and carbon dioxide (fermentation). A frequent trap is confusing respiration with breathing: breathing is ventilation, respiration is a chemical process in cells. Another trap is writing 'respiration makes energy'; energy is released, not made.
Gas exchange surfaces
Effective exchange surfaces share features: large surface area, thin (short diffusion distance), moist, and good ventilation or blood supply to maintain a steep concentration gradient. In humans the alveoli provide this; in plants the stomata and air spaces in spongy mesophyll do the same job.
Ventilation and smoking
Know the mechanics: inhaling contracts the diaphragm and intercostal muscles, raising the ribcage and increasing volume, lowering pressure so air enters. Smoking damages cilia and alveoli, causing bronchitis, emphysema and cancer. Exam questions often ask you to explain why emphysema reduces gas exchange: fewer, larger alveoli mean less surface area.
Worked example
Explain why an athlete breathes faster after exercise. Muscles respire anaerobically, producing lactic acid; extra oxygen is needed to break it down, so breathing rate and depth increase. Rule of thumb: exercise raises carbon dioxide, which stimulates faster, deeper breathing.
Practical note
Limewater turns milky with carbon dioxide, proving that exhaled air contains more carbon dioxide than inhaled air.