Respiration
What you must understand
Respiration is the release of energy from food in living cells. It is not breathing. Aerobic respiration uses oxygen and releases more energy: glucose + oxygen → carbon dioxide + water + energy. Anaerobic respiration releases less energy. In muscles it produces lactic acid; in yeast it produces ethanol and carbon dioxide (fermentation).
Know the structure of the respiratory system: nose, trachea, bronchi, bronchioles, alveoli. The alveoli are the gas exchange surface: many of them, thin walls, moist, and surrounded by capillaries, giving a large surface area and a short diffusion distance. Breathing movements: intercostal muscles and diaphragm contract to inhale, increasing volume and lowering pressure; they relax to exhale.
Learn how to test for the products: limewater turns milky with carbon dioxide; cobalt chloride paper turns from blue to pink with water. Explain the effect of exercise: more respiration, more carbon dioxide, faster breathing and heart rate.
Traps examiners set
- Writing "respiration is breathing" or describing respiration as only gas exchange.
- Forgetting that anaerobic respiration in humans still occurs when oxygen is short, and that lactic acid must later be broken down using oxygen (oxygen debt).
- Saying energy is "made". Energy is released; ATP is not usually required at this level.
- Mixing up which gas is breathed in and out; exhaled air has more carbon dioxide and water vapour but is not pure carbon dioxide.
Worked example
Explain why a runner breathes faster after a race. During the race, muscles respire anaerobically, building up lactic acid. Extra oxygen is needed to break down the lactic acid, so breathing and heart rate stay high: this is the oxygen debt.
Rule of thumb: link every respiration question back to energy release, and every gas exchange question back to surface area and diffusion gradient.