IB Biology: The Cell Membrane & Transport — FAQs

Most marks vanish because students describe rather than explain — writing 'water moves through osmosis' instead of naming the concentration gradient, the direction, and the membrane structure involved. Examiners want the mechanism (channel proteins, ATP, gradients), not just the vocabulary term attached to a vague sentence.

Diffusion is passive movement of any particle down its concentration gradient; osmosis is specifically water moving through a partially permeable membrane down its water potential gradient; active transport moves solutes against their gradient and requires ATP and a carrier protein. All three matter for exam precision.

The fluid mosaic model explains why membranes are selectively permeable and dynamic rather than fixed — phospholipids move laterally, proteins float and shift, and cholesterol regulates fluidity. Exam questions often ask you to link this model directly to transport mechanisms, so knowing structure feeds directly into function marks.

Both SL and HL need the fluid mosaic model, phospholipid structure, membrane proteins (channel, carrier, receptor, glycoprotein), and cholesterol's role in fluidity. According to the IB Biology guide, first exams for the current syllabus were 2025, and membrane structure sits under Structure 2.4.

Yes — HL students go further into active transport mechanisms, including the sodium-potassium pump and its role in nerve impulses, plus endocytosis/exocytosis in more mechanistic detail. SL focuses on the core concepts: diffusion, osmosis, facilitated diffusion, and a general treatment of active transport.

The classic IB practical uses beetroot discs at different temperatures to measure pigment leakage as a proxy for membrane damage — higher temperatures denature membrane proteins and disrupt the phospholipid bilayer, increasing permeability. You should be able to describe the method, identify variables, and explain the trend.

Ask one question every time: does it need energy? Passive processes (diffusion, facilitated diffusion, osmosis) move substances down their gradient with no ATP. Active transport and bulk transport (endo/exocytosis) always need ATP because they move things against a gradient or move large molecules the membrane can't cross directly.

Structure your answer around water potential: state which solution has the higher water potential, say water moves from high to low water potential across the partially permeable membrane, and link this to the biological consequence (cell swelling, plasmolysis, turgor). Naming the direction explicitly is what separates a 2-mark answer from a 4-mark one.

Water potential (Ψ) combines solute potential and pressure potential: Ψ = Ψs + Ψp. Pure water has Ψ = 0; adding solute always lowers it (more negative). Water always moves from a less negative (higher) to a more negative (lower) water potential — that's the rule examiners expect you to apply, not just state.

Both cover the fluid mosaic model, osmosis and active transport, but IB Biology asks students to write full explanatory answers using command terms like 'explain' and 'describe', rather than mostly multiple-choice or short-answer recall. Your child needs to practise writing structured, mechanism-based responses, not just memorise definitions.

The most effective revision combines concise notes on membrane structure with topical practice questions specifically on transport mechanisms, since this is where mark loss concentrates. On revisionprep.com, the Biology Revision Notes and Topical Worksheets for this unit are built around the exact command terms IB examiners use.

Yes — Paper 1 typically tests membrane structure and transport type recognition through data-based multiple choice, while Paper 2 asks longer 'explain' or 'describe' questions on osmosis, active transport and the sodium-potassium pump. Practising both formats matters, since the skills examiners reward differ between them.