Biology - digestion, respiration, circulation, and excretion
What the exam tests
Human digestion, gas exchange, circulation and excretion, including enzyme action and the route of substances around the body.
Core rules
- Enzymes are specific biological catalysts; amylase digests starch to sugars, protease digests protein to amino acids, lipase digests fats to fatty acids and glycerol.
- Villi increase surface area for absorption; bile emulsifies fats but does not digest them.
- Breathing ventilates the lungs; gas exchange occurs across alveoli by diffusion.
- Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy. Anaerobic respiration in muscles produces lactic acid.
- The heart has four chambers; the left ventricle pumps oxygenated blood to the body. Arteries carry blood away from the heart, veins return it.
- Kidneys filter blood, reabsorb useful substances and excrete urea in urine.
Worked example
Trace oxygen from an alveolus to a leg muscle: alveolus → blood in pulmonary capillary → pulmonary vein → left atrium → left ventricle → aorta → artery → muscle capillary → muscle cell. At the muscle, oxygen diffuses from high concentration in the capillary to lower concentration in the cell, where it is used in aerobic respiration.
Traps
- Arteries are not always oxygenated: the pulmonary artery carries deoxygenated blood.
- The left side of the heart handles oxygenated blood; the right side handles deoxygenated blood.
- Enzymes are not used up, and they work at an optimum temperature and pH.