MYP Biology: Diffusion & Osmosis FAQs

Two similar-sounding processes get taught back-to-back, and the vocabulary blurs together. Diffusion is the net movement of any particles down a concentration gradient; osmosis is the special case where only water crosses a selectively permeable membrane. Marking MYP 4-5 scripts for years, the repeated slip is students writing 'diffusion of water' instead of naming osmosis specifically.

Diffusion is the net movement of particles — gases, solutes, even smells — from an area of high concentration to low concentration, and it needs no membrane at all. Osmosis is one specific type of diffusion: only water molecules move, and only across a selectively permeable membrane, from a dilute to a more concentrated solution.

Picture a membrane with tiny holes that let water through but not sugar molecules. Osmosis is water moving through that membrane from the side with more water (a dilute solution) to the side with less (a concentrated solution), evening things out. In MYP Biology, you'll model this with cells placed in solutions of different concentrations.

Water always moves toward the solution with a higher solute concentration, trying to balance things out either side of the membrane. Put a plant cell in salty water and it loses water and plasmolyses; put an animal cell in pure water and it gains water and can burst (lysis) because it has no cell wall to resist the pressure.

Common mistake: writing 'water moves from low to high concentration' without saying which concentration — solute or water. Examiners want you to specify direction using the solute (water moves from a dilute, low-solute solution to a concentrated, high-solute solution) and to name the structure correctly: a selectively permeable membrane, not just 'membrane'.

Weigh your potato (or dialysis tubing) sample before soaking, then again after — those are your two raw numbers. Percentage change in mass equals (final mass minus initial mass) divided by initial mass, times 100. A negative answer means the tissue lost water through net osmosis outward; a positive answer means it gained water.

For full marks against Criterion A, use: concentration gradient, selectively permeable (not just 'permeable') membrane, equilibrium, solute, and solvent, plus the tonicity terms hypertonic, hypotonic and isotonic. MYP doesn't require the DP term 'water potential' — that's introduced later — but naming the membrane type correctly is what separates level 5-6 answers from level 7-8.

Level 7-8 answers under Criterion A don't just describe what happens — they explain why, using precise terminology, and link it to a real biological context, like why root hair cells are adapted for water uptake by osmosis. I tell every student I teach: name the gradient, name the membrane, then explain the biological consequence.

The classic is potato or apple cylinders soaked in a range of sucrose concentrations, measuring mass change to find the concentration at which mass doesn't change — that tells you the cell sap's approximate concentration. Some schools substitute dialysis tubing 'artificial cells' or egg-membrane experiments, but the underlying variable and calculation stay the same.

Explaining the mechanism sits under Criterion A: Knowing and understanding. If you run the potato-and-sucrose practical yourself, you're also assessed against Criterion B: Inquiring and designing, for planning the method, and Criterion C: Processing and evaluating, for graphing results, calculating percentage change, and evaluating reliability.

All three move substances across membranes, but only active transport needs energy from ATP. Diffusion and osmosis both move substances down a concentration gradient — high to low — for free; active transport moves substances against the gradient, from low to high concentration, which is why active transport happens near mitochondria.

The core mechanism doesn't change, but the depth does. According to the IB, first assessment for the current Biology guide was 2025, and DP Biology builds in quantitative work — calculating water potential, interpreting osmolarity data, and linking surface-area-to-volume ratio to diffusion rate — that MYP simply doesn't require.

Yes — diffusion and osmosis sit within the Cell Biology content that's fair game for the on-screen MYP eAssessment in Sciences, typically tested through short-response and diagram-labelling item types rather than long written explanations. Practising with digital-style questions, not just paper past papers, is worth doing before that exam.

Kitchen experiments make this click faster than any diagram. Soak a raisin in water overnight and it plumps up (osmosis in); put celery in food-dye water and watch the colour travel up the stem (diffusion). Ask your child to explain out loud which direction the water or dye moved, and why.

Diffusion and osmosis are genuinely abstract — nothing about a concentration gradient is visible, so students who do fine with structural topics like organ systems can stumble here. It's usually not a comprehension gap but a vocabulary one, mixing up which way water moves. Targeted practice on labelling and directionality usually fixes it within a couple of worksheets.