Osmosis — the word-perfect definition
Osmosis is the diffusion of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a partially permeable membrane — a membrane that lets water through but not larger solutes like sucrose. All three clauses are separately markable, so write all three. Osmosis is passive: no energy (no ATP) is used, because water moves down its own gradient by random motion.
Cell responses — burst, turgid, plasmolysed
Water enters or leaves a cell by osmosis whenever inside and outside concentrations differ. Animal cells (red blood cells) have no wall: in a dilute solution they swell and may burst (lysis); in a concentrated one they shrink/crenate. Plant cells have a cellulose wall resisting expansion, so they turn turgid not burst, and plasmolysed when enough water leaves. Turgor pressure keeps non-woody stems and leaves firm; without it the plant wilts.
(Extended) Water potential — higher to lower
(Extended) Water potential measures the tendency of water molecules to move out of a solution. Water moves from higher (less negative) to lower (more negative) water potential through a partially permeable membrane — the same direction as dilute to concentrated. Pure water has the highest water potential, defined as zero; any dissolved solute lowers it. Root-hair cell sap is more negative than the dilute soil water, so water enters the root by osmosis.
Drawn from real examiner reports.
Water moves dilute → concentrated
A "concentrated" solution sounds like it should push water away, so candidates write that water moves from concentrated to dilute. Water moves TOWARD the concentrated side, down the water's own concentration gradient; the solute stays put because it cannot cross. On Extended papers it appears as "lower to higher water potential". State both sides explicitly, every time.
Flagged Nov 2023 P22 Q2 · Jun 2023 P41 Q10a
Osmosis ≠ "absorption"
Writing that the tissue "absorbs" the water earns nothing. Examiners want the named process — osmosis — plus its mechanism: diffusion of water molecules down a water-concentration (or water-potential) gradient through a partially permeable membrane. "Absorption", "soaks up" and "sucks in" describe the outcome, not the process, and are not credited.
Flagged Nov 2022 P32 Q1bii
Turgid ≠ flaccid ≠ plasmolysed
These are three ordered states, not synonyms for "lost water". Turgid — maximum water, firm, contents pressed against the wall. Flaccid — some water lost, soft, but the membrane is still against the wall. Plasmolysed — so much water lost that the membrane has visibly pulled away from the wall. Only that visible gap justifies "plasmolysed"; "looking limp" is flaccid.
Flagged Nov 2023 P21 Q2
Cell wall ≠ cell membrane
Osmosis crosses the cell membrane, which is partially permeable. The cell wall (plant cells only) is fully permeable to water and solutes and controls nothing entering or leaving — it only stops a turgid cell bursting. Naming the wall as the partially permeable structure is the single most repeated cell error across 0654 papers, and it costs the definition mark.
Flagged Nov 2022 P32 Q1ai, P43 Q4ci · Jun 2023 P32 Q10aii
Osmosis is not just diffusion
Diffusion is the net movement of any particle — solute or gas — down its own concentration gradient, and needs no membrane. Osmosis is the special case for water molecules only, and the partially permeable membrane is part of the definition. Defining osmosis without naming water, or without naming the membrane, loses marks even though the general idea is right.
No NET movement ≠ no movement
At the sucrose concentration where a potato cylinder shows zero change in mass, candidates write "no osmosis is happening". Water molecules are still crossing the membrane in both directions — there is simply no net movement, because the two rates are equal. The mark scheme wants the word "net"; "osmosis has stopped" is wrong.
The cell wall does not shrink
In a plasmolysis diagram the cell wall keeps its shape — only the membrane and cytoplasm shrink inward, leaving a gap that fills with the external solution. Candidates draw the wall shrinking with the contents, leave the vacuole the same size in all three states, or omit the gap entirely. That gap between wall and membrane is the whole point of the drawing.
Explain = direction + osmosis + comparison
Three steps, every time: state the direction of net water movement; name the process osmosis (usually its own mark); then give the comparison that caused it — "the solution outside was more dilute / had a higher water potential than the cell contents".
Describe vs explain
Describe wants only what happened — the mass rose, the cell went limp. Explain wants that plus the cause: the water-concentration or water-potential comparison, and the word osmosis. Answering an "explain" with a description is a routine way to lose half the marks.
Percentage change: divide by the original
Percentage change in mass divides by the original mass, never the final one: . Keep the sign — a negative value means the tissue LOST mass, so water left the cells.
Say water potential on Extended
Core answers may use "dilute/concentrated". (Extended) answers should switch to water potential, remembering that a higher water potential is the less negative one. Both describe the same direction; mixing them mid-sentence is where reversals creep in.
Cambridge 0654 spec reference: Section B3 "Movement into and out of cells", sub-topic B3.2 (Core + Extended). This leaf covers the definition of osmosis, the effect of immersing plant tissue in solutions of different concentration, the effects of osmosis on animal and plant cells, and (Extended) water potential and the importance of osmosis in water uptake/loss.
Out of scope for B3.2 (covered elsewhere in B3): the general definition and rate factors of solute diffusion (B3.1); active transport against a gradient (B3.3). The xylem/transpiration-pull mechanism of long-distance water transport in plants belongs to B8, not this leaf.
| Term | Mark-scheme-precise definition |
|---|---|
| Osmosis | The diffusion of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane |
| Partially permeable membrane | A membrane that allows small molecules (e.g. water) to pass through, but not larger solute molecules |
| Turgid | A plant cell that has taken in the maximum water it can by osmosis; firm, with contents pressing on the cell wall (turgor pressure) |
| Flaccid | A plant cell that has lost some water but the cell membrane has not yet pulled away from the cell wall; soft, not firm |
| Plasmolysed | A plant cell that has lost so much water by osmosis that the cell membrane has visibly pulled away from the cell wall |
| (Extended) Water potential | A measure of the tendency of water molecules to move out of a solution; pure water has the highest (least negative) water potential |
Osmosis is passive — like diffusion, it requires no energy input from the organism, resulting only from the random motion of water molecules down their own concentration (or water-potential) gradient.
Define osmosis.
(a) Define osmosis. (3 marks)
(b) Name the structure water molecules cross when they enter a plant cell by osmosis, and state whether it is fully or partially permeable. (2 marks)
