1.3.2: Osmosis
Not started yet — this one needs some love.
Osmosis: movement of water molecules only from a solution of lower solute concentration (more water) to a solution of higher solute concentration (less water) through a partially permeable membrane.
Water moves down its water concentration gradient (from dilute to concentrated).
Turgid: plant cell that has gained water by osmosis — vacuole is full, cell is firm (important for support).
Plasmolysed: plant cell that has lost water — cell membrane pulls away from cell wall (in very concentrated external solution).
Isotonic solution: same concentration as cell contents → no net movement of water.
% change in mass = (final mass − initial mass) ÷ initial mass × 100.
Common exam mistakes
'Water moves from high concentration to low concentration' is incorrect unless you specify water concentration — without the word 'water', concentration refers to solute, making the statement wrong. Correct phrasing: 'water moves from a dilute solution (high water concentration) to a concentrated solution (low water concentration)'.
Even if you correctly say 'water concentration' at first, do not then write 'down a concentration gradient' without specifying it is the water concentration gradient — mixing the two terms in the same answer contradicts itself and loses marks.
Only water molecules move through the partially permeable membrane — not the solute, not the whole solution, not the salt.
Must mention the partially permeable membrane for full marks — it is what distinguishes osmosis from general diffusion.
'The salt dries out the potato' or 'salt draws water out' describes the effect but not the mechanism — osmosis involves water moving down its water concentration gradient through a partially permeable membrane.
The cell membrane is the partially permeable membrane across which osmosis occurs; the cell wall provides rigid structural support and prevents the cell from bursting once turgid. The cell wall does NOT stop water from entering — do not confuse the two.
Osmosis is a passive process — it does not require energy.