IB DP Biology: Water potential and osmosis (DP Biology) – Revision Notes

These revision notes cover section D2.3 (Water potential) of the IB Biology guide, first assessment 2025, for DP Biology. Statements D2.3.1 to D2.3.7 are SL and HL. Statements D2.3.8 to D2.3.11 are additional higher level, so they are HL only. For full explanations and worked examples, read the study guide.

Test yourself with the practice set. Other links: IB DP Biology hub, printable checklist, free 10-minute diagnostics and Membranes and membrane transport.

Key terms

Term	Meaning
Solvation	Solvent molecules surrounding and attracted to solute particles
Osmosis	Net movement of water across a partially permeable membrane towards the higher solute concentration
Hypertonic	Higher solute concentration than the solution it is compared with
Hypotonic	Lower solute concentration than the solution it is compared with
Isotonic	Equal solute concentration
Dynamic equilibrium	Water crosses in both directions at equal rates, so there is no net change
Turgor pressure	Outward pressure of the protoplast on the cell wall
Plasmolysis	Plasma membrane pulling away from the cell wall as the protoplast loses water
Crenation	Shrinkage of an animal cell with a puckered membrane in a hypertonic medium
Water potential (HL)	Potential energy of water per unit volume, compared with the reference of pure water at 20 °C and normal air pressure

D2.3.1: Solvation in five lines

• Water is polar: oxygen slightly negative, hydrogens slightly positive.
• Positive ions attract the oxygen ends; negative ions attract the hydrogen ends.
• Polar molecules form hydrogen bonds with water molecules.
• A shell of water molecules then surrounds each solute particle and keeps it dispersed.
• Non-polar substances cannot do this, so they stay undissolved.

D2.3.2 and D2.3.3: Direction of water movement

Method in steps

1. Find the solute concentration on each side of the membrane.
2. Check the membrane is impermeable to the solute (or that the solute crosses far more slowly than water).
3. Label the sides hypertonic (higher solute) and hypotonic (lower solute), or isotonic if equal.
4. State that net movement of water is from the hypotonic side to the hypertonic side.
5. If isotonic, write “no net movement; water still crosses in both directions (dynamic equilibrium)”.

Cell placed in	Net water movement	Animal cell	Plant cell
Hypotonic medium	In	Swells, may burst	Turgid
Isotonic medium	None	Normal shape	Flaccid
Hypertonic medium	Out	Crenated	Plasmolysed

Wording to use: “The solution outside has a lower solute concentration than the cytoplasm, so there is net movement of water into the cell by osmosis.” Never write “water moves to where there is less water”.

D2.3.4: Plant tissue experiment

Variables

• Independent: sucrose concentration.
• Dependent: percentage change in mass (or length).
• Controlled: size and shape of pieces, volume of solution, temperature, time, blotting, tissue source.

Calculations

• Percentage change = (final − initial) ÷ initial × 100.
• Isotonic concentration: where the graph of percentage change crosses zero. If you only have two points either side, use straight-line interpolation.
• Mean ± SE gives the precision of each mean. SE = SD ÷ √n. You do not have to memorise the formulae.
• Error bars drawn as SE: bars that do not overlap suggest a real difference between means.

Worked reminder. A piece of tissue goes from 1.50 g to 1.38 g. Change = −0.12 g. Percentage change = −0.12 ÷ 1.50 × 100 = −8.0%. Mass fell, so the solution was hypertonic to the cells and water left.

Limitations to quote: surface liquid left on pieces after blotting; pieces cut from different regions of the tissue; too few repeats; time too short to reach equilibrium.

D2.3.5: Cells without a wall

• Hypotonic medium: swell and burst.
• Hypertonic medium: shrink and crenate.
• Freshwater unicellular organisms: water enters constantly. A contractile vacuole collects it and expels it, using ATP.
• Multicellular animals: keep tissue fluid isotonic to the cells so cells do not swell or shrink harmfully.

D2.3.6: Cells with a wall

• Hypotonic medium: water enters, protoplast presses on the wall, turgor pressure develops. The cell is turgid and does not burst because the wall resists.
• Hypertonic medium: water leaves, protoplast shrinks, membrane pulls away from the wall. The cell is plasmolysed.
• Gap in a plasmolysed cell: filled with the external solution, since the wall is permeable.

D2.3.7: Isotonic solutions in medicine

Use	Why isotonic
Intravenous saline, glucose-saline and similar fluids	No net water movement into or out of blood cells, so they keep their shape
Bathing a donor organ before transplantation	Organ cells neither swell nor shrink while outside the body

Remember: distilled water given into a vein would be hypotonic to the blood, so red blood cells would swell and burst.

D2.3.8 and D2.3.9: Water potential (HL only)

• ψw = potential energy of water per unit volume. Units: kPa.
• Reference: pure water at 20 °C and normal air pressure = 0 kPa.
• Absolute potential energy cannot be measured, so everything is relative to the reference.
• Dissolved solutes lower ψw, so cells and solutions have negative values.
• Water moves from higher (less negative) ψw to lower (more negative) ψw, down the potential energy gradient.
• Order from highest to lowest: −100, −350, −800 kPa. Water moves from −100 towards −800.

D2.3.10: ψw = ψs + ψp (HL only)

Quantity	Sign	Notes
ψs, solute potential	Zero or negative	0 for pure water; more solute, more negative
ψp, pressure potential	Usually positive in cells	0 in open solutions; negative in xylem sap under tension
ψw, water potential	Zero or negative	Sum of the two

Worked reminder. Cell with ψs = −650 kPa and ψp = +400 kPa. ψw = −650 + 400 = −250 kPa. In a solution with ψw = −250 kPa there is no net movement of water.

D2.3.11: Explaining tissue changes (HL only)

Hypotonic solution. Outside ψw is higher than inside ψw. Water enters. ψp rises as the wall resists. Net movement stops when ψw inside equals ψw outside.

Hypertonic solution. Outside ψw is lower than inside ψw. Water leaves. ψp falls. At ψp = 0, ψw = ψs (incipient plasmolysis). If outside ψw is lower still, the cell plasmolyses.

Answer frame: (1) compare ψw inside and outside; (2) state direction of water movement; (3) say what happens to ψp and ψs; (4) state the end result for the cell.

Quick self-test

1. State the meaning of hypertonic.
2. A cell contains 0.25 mol dm⁻³ of solute and sits in 0.40 mol dm⁻³ of a solute that cannot cross the membrane. State the direction of net water movement.
3. Explain why a cell in an isotonic solution is not “static”.
4. A piece of tissue rises from 1.80 g to 2.07 g. Calculate the percentage change in mass.
5. A set of nine repeats has an SD of 1.5. Calculate the SE.
6. Name the structure that removes excess water from a freshwater single-celled organism.
7. Distinguish plasmolysis from crenation.
8. Give one reason for bathing a donor organ in an isotonic solution.
9. (HL) A cell has ψs = −500 kPa and ψp = +200 kPa. Calculate ψw.
10. (HL) Cell A has ψw = −300 kPa and cell B has ψw = −450 kPa. State the direction of net movement and the reason.
11. (HL) A cell has ψs = −900 kPa. State ψw at incipient plasmolysis.
12. (HL) State two situations where ψp is not positive.

Answers

1. Having the higher solute concentration of two solutions compared.
2. Out of the cell, because the solution is hypertonic to the cell contents.
3. Water molecules still cross the membrane in both directions at equal rates (dynamic equilibrium); only the net change is zero.
4. (2.07 − 1.80) ÷ 1.80 × 100 = +15.0%.
5. SE = 1.5 ÷ √9 = 0.5.
6. Contractile vacuole.
7. Plasmolysis is a plant (walled) cell’s membrane pulling away from the wall; crenation is a wall-less animal cell shrinking with a puckered surface. Both happen in a hypertonic medium.
8. Any one: cells do not swell or shrink by osmosis; the organ’s tissue is not damaged by water movement.
9. −500 + 200 = −300 kPa.
10. From A to B, because A has the higher (less negative) water potential.
11. ψp = 0, so ψw = ψs = −900 kPa.
12. In an open container of solution (ψp = 0) and in xylem sap under tension (ψp negative).

Where marks are usually lost

• Describing direction in terms of “water concentration” instead of solute concentration.
• Writing “no movement” for an isotonic solution.
• Using plasmolysis for animal cells and crenation for plant cells.
• Forgetting that the percentage change divides by the initial value, not the final one.
• Reading the isotonic concentration from the wrong axis or from the point of greatest mass.
• Calling large SE bars “more reliable” instead of “less precise”.
• Giving positive water potential values for a solution (HL).
• Treating ψp as always positive (HL).
• Saying the cell wall stops water entering, rather than resisting expansion.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section D2.3 (Water potential), under Theme D: Continuity and change.