Diffusion — all four clauses
Learn the definition word for word: diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), as a result of the random motion of particles. Every clause is separately markable, so all four must appear. Diffusion is also passive — the organism supplies no energy (no ATP is used), because the process runs on the particles' own kinetic energy.
Crossing the cell membrane — what moves and why
Particles cross the cell membrane in either direction, always down their own gradient. Into a respiring cell: oxygen and glucose (used up inside). Out of it: carbon dioxide (made inside, so higher inside). This drives gas exchange at alveoli, gills and stomata, and moves digested food and mineral ions between cells. Diffusion is only fast enough over short distances — it cannot supply a large organ alone, which is why exchange surfaces are thin.
(Extended) Rate of diffusion — the four factors
(Extended) Four factors set the rate: surface area (larger -> faster: more room to cross at once), temperature (higher -> faster: more kinetic energy, faster random motion), concentration gradient (steeper -> faster: a bigger imbalance, so more net movement per second) and distance (shorter -> faster: less path to cross). In the agar-cube practical, smaller cubes have a larger surface-area-to-volume ratio (SA:V), so dye reaches the centre faster.
Drawn from real examiner reports.
Incomplete or reversed definition
Three things get dropped. "Net" — without it, particles seem to move one way only. The direction — "lower to higher" describes active transport, not diffusion. The cause — "random motion" is separately credited and often omitted. Some claim particles are attracted to the low-concentration side; no such attraction exists. Write the full sentence, not a paraphrase.
Flagged Nov 2022 P32 Q11a; Nov 2023 P32 Q10c
Reactants diffuse in, products diffuse out
Asked which substances diffuse into a respiring cell, candidates frequently answer carbon dioxide and water — the products, which diffuse out. A respiring cell uses up oxygen and glucose, keeping them lower inside, and makes carbon dioxide, which builds up inside. Check whether the question wants a reactant (in) or a product (out) before naming anything.
Flagged Nov 2022 P33 Q4cii; Jun 2023 P32 Q10bi
Naming a factor is not explaining it
Extended questions credit the mechanism, not the label. "Large surface area" alone scores nothing; "more room for particles to cross at once" scores. "Thin" alone scores nothing; "thin gives a short diffusion distance" scores. Same for all four gas-exchange surface features: large surface area, thin, good blood supply, good ventilation.
Flagged Nov 2022 P42 Q10cii (B11 gas-exchange surface features)
Diffusion ≠ osmosis (water only)
Diffusion is the net movement of any particles — gases or solutes — down their own concentration gradient. Osmosis is the special case of water molecules moving across a partially permeable membrane, from a dilute solution to a concentrated one. If the question involves water and a partially permeable membrane, the term wanted is osmosis, not diffusion.
Diffusion ≠ active transport
Diffusion is passive: particles move down a gradient and the cell does no work. Active transport moves particles against the gradient, from low to high concentration, and needs energy from respiration. So if particles are travelling from a low to a high concentration, the process cannot be diffusion — check the gradient direction before you name the process.
Net movement ≠ total movement
Particles never stop moving randomly in every direction. "Net movement" describes only the overall drift, which is down the concentration gradient. At equilibrium the net movement is zero but the particles are still moving. Diagrams that freeze the particles once the concentrations are equal, or that show arrows crossing the membrane in one direction only, lose the mark.
Rate is not the same as distance
Rate of diffusion is the distance diffused per unit time (e.g. mm/min), not the distance reached. Answers that quote a bare distance where a rate is asked for, or that give the right number with no unit, drop marks. Divide distance by time — that way round — and state the unit the data supports.
The four-step rate answer
Name the factor -> state the direction of change ("increasing the surface area...") -> explain the mechanism at particle level (more room to cross / more kinetic energy / bigger imbalance / shorter path) -> state the effect on rate. Four steps, four marking points.
Match the command word
Describe — name the pattern only (the rate increases or decreases). Explain — the pattern plus the particle-level mechanism. State the importance of diffusion — link it to a real biological process such as gas exchange or nutrient uptake, not just "molecules move".
Never restate the question
"Diffusion is faster because there is more diffusion" earns nothing. Every explanation must name a cause: particles, kinetic energy, concentration gradient or diffusion distance. If your sentence would still read the same with the factor deleted, it is not an explanation.
Cambridge 0654 spec reference: Section B3 "Movement into and out of cells", sub-topic B3.1 (Core + Extended). This leaf covers the definition of diffusion, the fact that some substances cross the cell membrane by diffusion, the importance of diffusion of gases and solutes in organisms, and (Extended) the factors that influence the rate of diffusion.
Out of scope for B3.1 (covered elsewhere in B3): osmosis and active transport — these are named only briefly below to sharpen the boundary of the diffusion definition.
| Term | Mark-scheme-precise definition |
|---|---|
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient), as a result of the random motion of particles |
| Concentration gradient | The difference in the concentration of a substance between two regions |
| Cell membrane | The structure (not the cell wall) that controls which substances enter and leave a cell, including by diffusion |
Substances that diffuse into a respiring cell: oxygen and glucose (the reactants of respiration; higher concentration outside).
Substances that diffuse out of a respiring cell: carbon dioxide, and water () (products of respiration; higher concentration inside).
Diffusion is passive — it requires no energy input from the organism, because it results only from the particles' own random motion. (Contrast: active transport, below.)
Importance of diffusion in organisms:
(Extended) State the four factors that influence the rate of diffusion, and the direction of their effect.
(a) Define diffusion. (3 marks)
(b) Name one substance that diffuses INTO a cell during respiration, and one substance that diffuses OUT of a cell during respiration. (2 marks)
