Diffusion, osmosis, active transport
Three processes move substances across membranes. Diffusion: particles move from higher to lower concentration (down the gradient); passive. Osmosis: water molecules move from a dilute (high) to a more concentrated (low) solution across a partially permeable membrane; passive. Active transport: movement against the gradient (low→high) using respiration energy — the only one moving up-gradient, so the only one costing energy.
Four factors set the rate of movement
Rate of diffusion or osmosis rises with: (1) a steeper concentration gradient; (2) higher temperature (more kinetic energy); (3) larger surface area; (4) shorter distance (thinner membrane). Exchange surfaces — alveoli, villi, root hair cells — have a large surface area and short distance, and a good blood/water supply keeps the gradient steep. A breeze past a leaf carries water vapour away, maintaining the gradient.
SA:V: small cells have a large ratio
Surface-area-to-volume ratio (SA:V) = surface area ÷ volume. For a cube of side : surface area , volume , so . As an object grows, volume rises faster than surface area, so SA:V falls. A small cell has a large SA:V, so diffusion supplies the whole volume. A large organism has a small SA:V, so it needs specialised exchange surfaces (lungs, gills, villi) plus a transport system.
Drawn from real examiner reports.
Osmosis direction and definition
The osmosis definition needs three parts: movement of water molecules, from a dilute (high water) to a more concentrated (low water) solution, across a partially permeable membrane. Two errors: reversing the direction, and calling the water itself 'more/less concentrated'. Say water moves from a dilute to a more concentrated solution.
June 2023 Paper 1B Q03(c) — the commonest reason marks were lost on red blood cells in distilled water / salt solution was confusion over the direction of water movement; the report advises stating water moves from a dilute to a more concentrated solution and warns that calling water more/less concentrated is confusing.
Only plant cells plasmolyse
Match the vocabulary to the cell type. An animal cell has no wall: in a dilute solution it bursts (lysis), in a concentrated one it shrinks (crenates). A plant cell has a wall: dilute → turgid, concentrated → flaccid, and may plasmolyse (membrane pulls off the wall). Calling a red blood cell 'plasmolysed' earns no credit.
June 2023 Paper 1B Q03(c)(ii) — some candidates called a red blood cell in concentrated salt solution 'plasmolysed'; this gained no credit, as plasmolysis is a plant-cell term (animal cells shrink / crenate).
Dissolved solutes do not give energy
When cells sit in a glucose or salt solution, some candidates claim the dissolved solute 'provides the cell with energy'. It does not — these questions are about the osmotic movement of water, driven by the concentration difference. Energy from respiration is used only in active transport, never in osmosis. Describe what the water does, not what the solute supplies.
June 2024 Paper 2B — strong answers recognised pure water enters cells by osmosis and bursts them; a common error was suggesting the dissolved glucose or salt would provide the cells with energy.
Up the gradient means active transport
Diffusion and osmosis are passive — they move substances down a gradient, no respiratory energy. Active transport moves particles against the gradient (low→high) using respiration energy. Discriminator: if a substance moves up its gradient — e.g. mineral-ion uptake by root hairs — it is active transport, stopping without respiration.
The wall, not the membrane, stops bursting
Bacteria and plant cells in water do not burst because the cell wall resists expansion; a red blood cell, with no wall, bursts. Two flagged errors: crediting the membrane with preventing bursting (it is not strong enough), and saying the wall 'stops water crossing it' — water still enters by osmosis; the wall only resists the swelling.
June 2023 Paper 1BR Q1(a)(iii) — explaining why bacteria do not burst in water but red blood cells do needed the cell-wall role plus osmosis with a direction; errors were crediting the membrane and saying the wall stops water crossing it.
Use the term concentration gradient
Vague wording like 'the concentration was high so water moved' loses marks. Examiners want the key term concentration gradient (or, for osmosis, water moving from dilute to concentrated). In water-loss and diffusion questions, state that a steeper or maintained gradient drives faster movement, rather than quoting bare 'high' and 'low' values.
June 2023 Paper 2B — on a water-loss item, stronger answers explained an increased or maintained concentration gradient; the report stresses the key term concentration gradient over vague wording.
Percentage-change: divide by initial
Percentage change in mass = (final − initial) ÷ initial × 100. Keep the sign (gain +, loss −), divide by the initial mass not the final, and round as asked. The concentration giving zero change matches the cell contents.
Osmosis question? Define it, give direction
Whenever osmosis appears, define it and give the direction of water movement — dilute to more concentrated, across a partially permeable membrane. Refer to a labelled concentration, not a vague 'concentration'. This fixes most lost marks on cells-in-solution questions.
SA:V: show area and volume separately
In a surface-area-to-volume calculation, write the surface area and the volume as separate steps before dividing — partial credit is given for a correct area or volume even if the ratio is wrong. For a cube, SA:V = 6 ÷ L, but still show the steps.
Substances move into and out of cells in three ways. Learn each definition in its precise, mark-scheme form (a vague answer scores zero).
| Process | Mark-scheme definition | Energy? | Direction |
|---|---|---|---|
| Diffusion | Net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient) | Passive (no energy) | Down the gradient |
| Osmosis | Net movement of water molecules from a dilute solution (high water concentration) to a more concentrated solution (low water concentration) across a partially permeable membrane | Passive (no energy) | Down the water gradient |
| Active transport | Movement of particles against a concentration gradient (from low to high concentration) using energy released by respiration | Uses energy | Up the gradient |
Factors that increase the rate of movement (statement 2.16):
Define osmosis.
Define osmosis. (3 marks)
