Osmosis — the four-element definition
The net movement of water molecules, from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Each phrase is separately creditworthy: net (water crosses both ways, but more one way), water molecules (solutes stay put), the direction, and the membrane clause that separates osmosis from diffusion. In a living cell that membrane is the cell membrane.
Plant cells — turgid, flaccid, plasmolysed
In a dilute solution water enters, the vacuole swells and the cytoplasm presses outwards on the cell wall: the cell is turgid and firm (turgor pressure). At equal concentration there is no net movement. In a concentrated solution water leaves and the cell goes flaccid and limp; if enough leaves, the cytoplasm and cell membrane pull away from the wall — plasmolysis. The wall stops a turgid cell bursting, and plasmolysis reverses in a dilute solution (deplasmolysis).
Animal cells and the mass investigation
An animal cell has no cell wall, so in a dilute solution it swells and may burst (lyse); in an isotonic solution it is unchanged; in a concentrated solution it crenates. Osmosis is investigated with visking (dialysis) tubing — sucrose sealed inside gains mass in water because only water crosses — or with potato cylinders in a range of sucrose concentrations. Both are measured as percentage change in mass, and the concentration giving zero change is isotonic with the cell sap.
Drawn from real examiner reports.
Osmosis without the membrane clause
The most heavily penalised osmosis error. "Water moves from a dilute to a concentrated solution" drops two creditworthy elements: net movement, and through a partially permeable membrane — the phrase that distinguishes osmosis from ordinary diffusion. "Osmosis is the diffusion of water" is only a start; add the membrane and the water-potential direction before it scores.
Flagged s23 P11 Q7; P21 Q5 · osmosis/diffusion confused s23 P11 Q6
Only water crosses, not sucrose
Candidates describe sucrose moving out of visking tubing, or out of a potato cell. Solute molecules are too large to pass a partially permeable membrane — only water molecules move by osmosis. A sucrose-filled bag gains mass because water enters, not because sugar leaves. Writing "particles move" rather than "water molecules" also loses the mark.
Flagged s23 P11 Q7; P21 Q5 (sucrose described as leaving the dialysis tubing)
Percentage change: divide by initial mass
The denominator is always the initial mass: (final − initial) ÷ initial × 100. Using the final mass gives a systematically different, wrong answer — a cylinder going 5.0 g to 4.3 g is −14%, but dividing by 4.3 gives −16.3%. The initial mass is what the change is being measured against, so it belongs on the bottom every single time.
Flagged s23 P41 Q2a i; w22 P32 Q3b ii; s22 P31 Q1c i
A mass loss is a negative answer
If water leaves the tissue, (final − initial) is negative and so is the percentage change — write −12%, not 12%. Dropping the sign reverses the biology: it claims water entered. Sanity-check against the solution used — more concentrated than the cell sap means water leaves, mass falls, and the answer must come out negative.
Flagged s23 P41 Q2a i; w22 P32 Q3b ii; s22 P31 Q1c i
Turgid ≠ plasmolysed
A plant cell in pure water becomes turgid, not plasmolysed — pure water has the higher water potential, so water enters. Plasmolysis is the opposite case: a very concentrated solution, water leaving, cytoplasm pulled off the wall. Equally, a cell in a concentrated solution goes flaccid, never turgid. Reversing these two is a recurring MCQ trap.
Flagged s23 P23 Q5 (cell in pure water wrongly said to plasmolyse)
Cell wall ≠ cell membrane
Osmosis occurs through the cell membrane (cell-surface membrane), which is partially permeable. The cell wall is fully permeable — it restricts nothing, so it cannot be the osmotic barrier, and "the cell wall acts as the partially permeable membrane" scores zero. The wall's role is mechanical: it resists the swelling of a turgid cell and stops it bursting.
Flagged s22 P23 Q6 (partially permeable membrane is the cell membrane, not the wall)
Concentration gradient ≠ water potential
Where the question is about water potential, the mark scheme wants those exact words — not "concentration gradient", and not "more dilute". Pure water has the highest water potential (taken as zero); adding solute makes it lower, i.e. more negative. Write "from higher water potential to lower water potential" and the terminology mark is secured.
Flagged s23 P41 Q2a ii; w22 P41 Q3c; s22 P31 Q1
Say the direction in the marked words
Make every direction statement read "from higher water potential (dilute solution) to lower water potential (concentrated solution)". That one sentence satisfies both the Core and the Supplement phrasing at once, so it is worth using by default rather than deciding under pressure.
Name the membrane and name the state
If structure is mentioned, say "through the partially permeable cell membrane". If the outcome is asked for, name the state outright: plant cell → turgid, or flaccid/plasmolysed; animal cell → swells and may burst, or crenates. Marks attach to the named term.
Show the substitution before the answer
Write (final − initial) ÷ initial × 100 with the numbers substituted in, then the answer. The substitution secures the method mark even if the arithmetic slips, and seeing the initial mass on the bottom makes the denominator error obvious before you commit to it.
Read the result back as biology
Do not stop at the number. A mass gain means water entered, so the solution was more dilute (higher water potential) than the cell sap; a loss means the reverse; and the concentration where a percentage-change graph crosses zero is isotonic with the cell sap.
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Every element of this definition is mark-scheme essential:
| Required element | Common wrong version (no credit) |
|---|---|
| net movement | "movement" alone |
| water molecules | "water" alone scores partially; "particles" does not |
| higher water potential to lower | "from dilute to concentrated" is the loosest form accepted; mark schemes have demanded "water potential" on Core papers as well as Extended |
| partially permeable membrane | omitting the membrane entirely |
Water potential (Extended): a measure of the tendency of water molecules to move from one region to another. Pure water has the highest water potential (set at zero). Adding solute lowers the water potential (makes it more negative). Water moves by osmosis from higher (less negative) to lower (more negative) water potential.
Partially permeable membrane: allows water molecules to pass freely but restricts the passage of larger solute molecules (e.g. sucrose, starch). In a living cell, this is the cell membrane (cell-surface membrane) — not the cell wall.
Define osmosis.
(a) State the definition of osmosis.
(b) Name the structure in a plant cell that acts as the partially permeable membrane for osmosis.
(c) Explain why the cell wall does not act as this membrane.
