IB Science Command Terms and Marking Scheme Logic
IB Science examination responses are marked against a specific mark scheme that allocates marks to command-term-specific answer components. Misunderstanding command terms is the single most common source of lost marks in IB Science HL examinations. 'Outline' (1–2 marks): provide a brief, concise account of the main features — no explanation of mechanism required. 'Describe' (2–4 marks): provide a detailed account of what happens — facts, observations, sequences of events, but not necessarily 'why.' 'Explain' (4–8 marks): give a mechanistic account of why something happens — the process, pathway, or causal chain that produces the outcome. This is the highest-demand command term in Science and requires showing understanding of mechanism. 'Evaluate' (4–6 marks): make a judgment based on evidence, identifying strengths, limitations, and overall significance. Marking scheme logic for 'explain' questions: each mark in the mark scheme is allocated to a specific mechanistic step or causal link. A response that correctly describes the outcome without explaining the mechanism receives partial marks. For Biology HL: 'Explain how the sodium-potassium pump maintains the resting membrane potential' requires: (1) pump actively transports 3 Na⁺ out and 2 K⁺ in; (2) using ATP hydrolysis; (3) creating a net outward positive charge movement; (4) establishing electrochemical gradient with inside negative relative to outside (resting potential −70mV). Each mechanistic step earns a mark — omitting the ATP detail or the charge asymmetry loses marks.