Biology  by Bradford
IB Biology 2025 · Theme D · Continuity and change

D2.3 Water potential. Practice questions with markscheme.

47 original IB-style questions on D2.3, written from the 2025 guide: 23 multiple-choice, 16 short-answer, 4 data-based, 2 extended-response part, 2 drawing. Below is a 22-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

7 statements at SL and HL, 4 additional higher level.

  1. D2.3.1SL / HL Solvation with water as the solvent
  2. D2.3.2SL / HL Water movement from less concentrated to more concentrated solutions
  3. D2.3.3SL / HL Water movement by osmosis into or out of cells
  4. D2.3.4SL / HL Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions
  5. D2.3.5SL / HL Effects of water movement on cells that lack a cell wall
  6. D2.3.6SL / HL Effects of water movement on cells with a cell wall
  7. D2.3.7SL / HL Medical applications of isotonic solutions
  8. D2.3.8HL Water potential as the potential energy of water per unit volume
  9. D2.3.9HL Movement of water from higher to lower water potential
  10. D2.3.10HL Contributions of solute potential and pressure potential to the water potential of cells with walls
  11. D2.3.11HL Water potential and water movements in plant tissue

In the bank for D2.3

  • 23 multiple-choice
  • 16 short-answer
  • 4 data-based
  • 2 extended-response part
  • 2 drawing
  • 18 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

Make your own

The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAAAAAAAhQAO8gB
Biology · topic quiz
Standard level · topic practice, not an exam format
35 minutes22 marks

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Covers D2.3 Water potential
Name:
1.

Solution P contains 0.1 mol dm⁻³ sucrose and solution Q contains 0.3 mol dm⁻³ sucrose. The two solutions are separated by a membrane that is permeable to water but not to sucrose. Which statement is correct?

[1]
  1. P is hypertonic to Q, so there is net movement of water from Q to P
  2. P is hypotonic to Q, so there is net movement of water from Q to P
  3. P is hypotonic to Q, so there is net movement of water from P to Q
  4. P is hypertonic to Q, so there is net movement of water from P to Q
2.

Why must fluids used to replace blood volume in a patient be isotonic with blood plasma?

[1]
  1. So that the fluid contains the same concentration of nutrients and gases as the blood plasma it replaces
  2. So that water does not move into or out of blood cells by osmosis, which would damage them
  3. So that the fluid clots in the same way as blood if the patient starts to bleed again
  4. So that the fluid is at the same temperature as the patient’s blood and does not cool the tissues
3.

A red blood cell is placed in pure water. What happens, and why?

[1]
  1. It swells and bursts, because water enters by osmosis and there is no cell wall to resist the pressure
  2. It shrinks and becomes crenated, because water leaves by osmosis into the more concentrated surroundings
  3. It is unchanged, because pure water is isotonic to the cytoplasm and there is no net movement of water
  4. It swells but does not burst, because the plasma membrane is rigid enough to resist the pressure
4.

Explain why turgor pressure develops in plant cells but not in animal cells placed in a hypotonic solution.

[3]
5.

Explain how sodium chloride dissolves in water.

[3]
6.

A red blood cell and a leaf epidermis cell are both placed in distilled water. Explain the different outcomes.

[3]
7.

Explain what happens to a plant cell placed in a strongly hypertonic solution.

[3]
8.

Draw and label a diagram of a plant cell that has been bathed in a hypertonic solution for 30 minutes.

[3]
9.

Explain the different effects of hypotonic and hypertonic solutions on animal and plant cells.

[4]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Show the markscheme

Markscheme BbB-EAAAAAAAAhQAO8gB

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. D2.3.2 [1]
  • C: P has the lower solute concentration, so it is hypotonic to Q, and water moves by osmosis from the less concentrated to the more concentrated solution; A and D wrongly call the more dilute solution hypertonic; B has the terminology right but the direction of water movement reversed;
2. D2.3.7 [1]
  • B: hypotonic fluid would cause lysis and hypertonic fluid crenation of red blood cells;
3. D2.3.5 [1]
  • A: animal cells lack a wall; a hypotonic solution causes lysis;
4. D2.3.4 [3 max]
  • 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;
5. D2.3.1 [3 max]
  • water molecules are polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms;
  • the positive sodium ions are attracted to the (partially negative) oxygen atoms of water molecules;
  • the negative chloride ions are attracted to the (partially positive) hydrogen atoms of water molecules;
  • these attractions pull the ions away from the crystal / overcome the (ionic) attractions between the ions;
  • each ion becomes surrounded by a shell of water molecules (hydration shell / solvation), which keeps the ions apart in solution, OWTTE;

Accept "solvation" or "dissolving". Do not accept water forming covalent bonds with the ions.

6. D2.3.3 [3 max]
  • distilled water is hypotonic to both cells, so water enters (both) by osmosis;
  • the red blood cell has no wall, so it swells and bursts / lyses;
  • the plant cell's wall resists expansion, so it becomes turgid (pressure builds);
  • (turgor) prevents bursting / net entry stops at equilibrium, OWTTE;
7. D2.3.4 [3 max]
  • the solution has a higher solute concentration than the cytoplasm / is hypertonic to it;
  • water leaves the cell (and vacuole) by osmosis;
  • the protoplast shrinks and the membrane pulls away from the wall / plasmolysis;
  • the cell becomes flaccid (and the tissue wilts), OWTTE;
8. D2.3.6 [3 max]
  • cell wall drawn as a rigid outline that has kept its shape, labelled;
  • plasma membrane (enclosing the cytoplasm and vacuole) drawn shrunken and pulled away from the cell wall, labelled, showing the cell as plasmolysed;
  • the space between the cell wall and the plasma membrane labelled as containing the external (hypertonic) solution;
  • vacuole drawn smaller than in a turgid cell, with cytoplasm and nucleus shown inside the shrunken protoplast;
  • annotation stating that water has left the cell by osmosis / the contents no longer press on the wall, OWTTE;

Do not accept a cell wall drawn shrunken with the membrane, or the membrane drawn outside the wall. Award [3 max].

9. D2.3.3-D2.3.4 [4 max]
  • in hypotonic solutions water enters cells by osmosis (down the water-potential gradient);
  • animal cells swell and may burst/lyse (no wall);
  • plant cells become turgid, the wall resists expansion and prevents bursting (turgor supports the plant);
  • in hypertonic solutions water leaves cells by osmosis;
  • animal cells shrink/crenate;
  • plant cells become flaccid and the membrane pulls away from the wall / plasmolysis;
  • (hence isotonic fluids are used for animal tissues in medicine), OWTTE;

Award converse statements.

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