IGCSE Biology — Core: Life Processes, Systems and the Language of Explanation

Biology at Core is examined as three verbs — state, describe, explain — and almost every mark is available to a candidate who keeps the levels apart: a cell is a unit, a tissue is a group of similar cells, an organ is a group of tissues doing one job, and a system is organs working together. The seven life processes (movement, respiration, sensitivity, growth, reproduction, excretion, nutrition) are the definition of 'living' the papers use, and a question asking whether a virus or a seed is alive is answered against that list item by item, not by opinion. Cell structure then supplies the comparison that recurs all year: plant cells have a cellulose wall, a permanent vacuole with cell sap and chloroplasts; animal cells have none of the three, and both have nucleus, cytoplasm, mitochondria and membrane. Specialisation is examinable as 'one structure, one function': a root-hair cell's long projection and thin wall for surface area and short diffusion distance; the palisade layer's position and chloroplast count for light absorption; a ciliated, goblet-lined trachea for moving mucus; a sperm's mitochondria and acrosome; xylem as hollow dead tubes reinforced with lignin and phloem as living sieve tubes with sieve plates. Diffusion, osmosis and active transport must be distinguished by what moves and in which direction: diffusion is net movement down a concentration gradient; osmosis is water from dilute to concentrated solution across a partially permeable membrane; active transport moves ions against the gradient through a protein carrier using energy from respiration — the only one of the three that needs a supply of energy, which is why root-hair cells are respiring. Transport answers are always the same three-part sentence — the vessel, the substance, and the mechanism: the heart pumps, arteries carry under pressure with thick elastic walls, veins return with a low pressure and semilunar valves, capillaries are one cell thick for exchange. Blood: red cells carry oxygen with haemoglobin and no nucleus, white cells defend by phagocytosis and by antibody, platelets clot, plasma carries the dissolved substances. Coronary heart disease is described as the narrowing of arteries feeding the heart muscle by fatty deposits, with risk factors named and a stent described as holding them open; defence against disease covers vaccination (a weakened pathogen stimulating antibody production and memory, giving immunity without illness) and the limits of antibiotics for viruses.