Respiration and Gas Exchange
What you must understand
Distinguish aerobic respiration (glucose + oxygen → carbon dioxide + water + energy) from anaerobic respiration in muscles (glucose → lactic acid + some energy) and in yeast (glucose → ethanol + carbon dioxide + some energy). Energy is used for muscle contraction, growth, active transport, maintaining body temperature and nerve impulses.
Know the gas exchange surfaces: alveoli (large surface area, thin walls, moist, rich blood supply, short diffusion distance), gills in fish (counter-current flow maintains a concentration gradient), stomata and leaves in plants. Learn the breathing mechanism - intercostal muscles and diaphragm, pressure changes, inhalation and exhalation - and the effect of exercise on breathing and heart rate.
Learn the tests for carbon dioxide (limewater turns milky) and oxygen (glowing splint relights), and how to interpret simple respirometer or germinating-seed experiments.
Traps examiners set
- Saying respiration "makes" energy - it releases energy from glucose.
- Confusing breathing (ventilation) with respiration (chemical reaction in cells).
- Claiming plants only photosynthesise - they respire all the time.
- Forgetting that anaerobic respiration in muscles creates an oxygen debt repaid by extra breathing.
Worked rule of thumb
When asked why a surface is efficient, use the diffusion equation idea: rate increases with surface area and concentration difference, and decreases with distance. Name the feature, then state how it affects each term.