Cells, nutrition, digestion, respiration, circulation and excretion
This domain links cell structure to how the body obtains and uses energy. Expect labelling of cells, food tests, enzyme action, the digestive system, the heart and blood vessels, respiration equations and the kidney.
Cells
Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells add a cellulose cell wall, chloroplasts and a permanent vacuole. Magnification = image size ÷ actual size. Diffusion is net particle movement from high to low concentration; osmosis is water moving across a semi-permeable membrane from a dilute to a more concentrated solution; active transport requires energy.
Nutrition and digestion
Food provides carbohydrates, proteins, lipids, vitamins, minerals and water. Enzymes are biological catalysts: amylase breaks starch to maltose, protease breaks protein to amino acids, lipase breaks lipid to fatty acids and glycerol. High temperature or the wrong pH denatures them. Mechanical digestion (teeth, stomach churning) increases surface area; chemical digestion uses enzymes. Bile, made in the liver and stored in the gall bladder, emulsifies fats and neutralises stomach acid. Villi in the small intestine give a large surface area, thin walls and a rich blood supply for absorption; the large intestine reabsorbs water.
Respiration, circulation and excretion
Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy, in mitochondria. Anaerobic in muscle gives lactic acid plus some energy; in yeast it gives ethanol and carbon dioxide. The heart has four chambers; arteries carry blood away, veins return it, capillaries exchange materials. Excretion removes metabolic waste - the kidneys remove urea and excess water as urine.
Worked example
A cell is drawn 40 mm long; its actual length is 0.05 mm. Magnification = 40 ÷ 0.05 = ×800.
Traps
- Confusing respiration with breathing.
- Saying enzymes are 'killed' rather than denatured.
- Mixing up diffusion and osmosis.