D2.3 Water potential. Practice questions with markscheme.

• A: water enters in hypotonic, leaves in hypertonic, no net movement in isotonic solutions;

• A: the soil solution is more concentrated than the cell sap of the root cells, so there is net movement of water out of the roots by osmosis instead of in; the cells lose water and turgor pressure and become flaccid, so the non-woody plant wilts; the cell walls stop plant cells from bursting, and extra solutes inside a cell would draw water in, not out;

• D: 250 cm³ × 3.6 % ÷ 1000 cm³ = 0.9 %, so the fluid is isotonic with plasma and there is no net movement of water into or out of the blood cells; B (1.8 %) and C (2.7 %) are hypertonic, so red blood cells would lose water and shrink (crenate); A (0.32 %) is hypotonic, so they would take in water, swell and could burst;

• (a) [3 max]
  • glucose and fructose dissolve: they form hydrogen bonds with water molecules / are each surrounded by a shell of water molecules, whereas starch stays undissolved;
  • one starch molecule gives thousands of dissolved sugar molecules, so the solute concentration of the cell sap rises (greatly);
  • the cell sap becomes hypertonic to the solution outside the cells / in the cell walls;
  • water moves into the cells by osmosis, from the less concentrated to the more concentrated solution;

  Do not accept 'water moves from high to low water concentration' as the direction point.

• (b) [3 max]
  • rain water is hypotonic to (much more dilute than) the cell sap of the ripe flesh;
  • water that passes through the skin enters the flesh cells by osmosis;
  • the protoplasts swell and press against their cell walls, so turgor pressure rises;
  • the cell walls resist expansion, so the cells do not burst;
  • the many swelling cells enlarge the flesh, which pushes outwards on the skin until the skin is stretched further than it can stand and splits, OWTTE;

  Do not accept 'the cells burst' as the cause of splitting.

• (c) [1 max]
  • their carbohydrate is still (mostly) undissolved starch, so their cell sap has fewer dissolved solute molecules / a lower solute concentration;
  • so the sap is less hypertonic to rain water, less water enters and turgor pressure rises less, OWTTE;
  • the skin of a green fruit may be stronger / more elastic than the skin of a ripe fruit, so it stretches without splitting, OWTTE;

  Accept a suggestion about the skin as an alternative to the solute points.

• in both, water enters by osmosis (down the water-potential gradient);
• the plant cell's (cellulose) wall resists expansion, so (hydrostatic) pressure builds, turgor;
• (turgor) opposes further net water entry, reaching equilibrium;
• animal cells lack a wall, so no pressure develops, they keep swelling and may burst/lyse, OWTTE;

• (a) [1]
  • ≈ 0.4 mol dm⁻³ (accept 0.40–0.45), where the change in mass is (closest to) zero;
• (b) [3 max]
  • the solution is hypotonic / has a lower solute concentration than the potato cells;
  • water enters the cells by osmosis (across the partially permeable membranes);
  • (so) the tissue gains mass;
  • cells become (more) turgid, OWTTE;

  Accept ψ notation.

• (c) [1]
  • to remove surface liquid, so only the water inside the tissue is weighed / a fair comparison, OWTTE;
• (d) [2]
  • the cylinders' initial masses differed (slightly);
  • percentages allow fair comparison / control for initial size, OWTTE;

  Content pivot toward experimental skills.

• A: in fresh water the cytoplasm is strongly hypertonic to the surroundings, so water enters continuously and must be expelled; adding salt reduces the difference in solute concentration, so less water enters and the vacuole needs to empty less often; B fails because the contractile vacuole expels water, not salt; C reverses the direction of change; D fails because such organisms have no cell wall, which is why they need a contractile vacuole;

• ψX = −1000 + 300 = −700 kPa;
• ψY = −800 + 250 = −550 kPa;
• ψZ = −600 + 100 = −500 kPa;
• water moves from higher (less negative) to lower (more negative) ψ, so from Z → Y → X (net movement toward cell X), OWTTE;

Award ECF for a direction consistent with the candidate's calculated values.

• (a) [1]
  • statements 3 and 5;

  Both are needed for the mark.

• (b) [4 max]
  • (statement 3) water leaves but the solutes stay inside the cell / the same amount of solute is now in less water;
  • so the cell sap becomes more concentrated and ψs falls / becomes more negative, not towards zero;
  • (statement 5) once the plasma membrane has pulled away from the wall, the protoplast no longer pushes on the wall and does not pull on it either, so ψp stays at zero;
  • the wall keeps its shape while the protoplast shrinks away from it / the space between the wall and the membrane fills with the external solution, OWTTE;
  • negative pressure potentials occur where sap is transported under tension, in xylem vessels, not in living cells with walls, OWTTE;
  • ψp cannot fall below zero in a plasmolysed cell, so the cell's water potential equals its solute potential, OWTTE;

  Award [2 max] if only one statement is corrected. Accept ψ notation or the words solute and pressure potential.

• (c) [2]
  • ψp = 0 kPa, because the cells are plasmolysed / the protoplasts no longer press on the walls;
  • ψs = −1200 kPa, because the water potential of the cells equals that of the solution and ψs = ψw − ψp = −1200 − 0;

  Allow ECF for ψs from a candidate's stated ψp. Do not accept −1000 kPa, the starting value.

• (a) [2]
  • percentage plasmolysed increases with sucrose concentration;
  • slowly at first then steeply / steepest between 0.3 and 0.5 mol dm⁻³ (S-shaped), OWTTE;
• (b) [3]
  • concentrated sucrose is hypertonic to the cells;
  • water leaves the (protoplast of the) cell by osmosis (down its gradient);
  • the protoplast/cytoplasm shrinks and (the membrane) pulls away from the wall;
• (c) [1]
  • ~0.39 to 0.40 mol dm⁻³; read between 0.3 (24 %) and 0.4 (52 %);
• (d) [2]
  • individual cells vary (in solute concentration/response);
  • a larger sample gives a more reliable estimate of the true proportion / reduces the effect of chance/anomalies, OWTTE;