IB DP Biology: Water potential and osmosis (DP Biology)

This study guide covers 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 for both SL and HL. Statements D2.3.8 to D2.3.11 are additional higher level, so they are HL only and are marked that way below.

Use it with the revision notes and the practice set. See also the IB DP Biology hub, the printable checklist and a free 10-minute diagnostic. The membrane mechanisms behind osmosis, including aquaporins, are in Membranes and membrane transport.

What this unit covers

Statement	What you must be able to do	Level
D2.3.1	Explain solvation, with water as the solvent	SL and HL
D2.3.2	Describe water movement from less to more concentrated solutions, using hypertonic, hypotonic and isotonic	SL and HL
D2.3.3	Predict net water movement into or out of cells; explain dynamic equilibrium	SL and HL
D2.3.4	Measure and analyse changes in plant tissue length and mass; use standard deviation and standard error	SL and HL
D2.3.5	Explain effects of water movement on cells without a wall	SL and HL
D2.3.6	Explain turgor pressure and plasmolysis in walled cells	SL and HL
D2.3.7	Describe medical uses of isotonic fluids	SL and HL
D2.3.8	Define water potential as the energy of water per unit volume	HL only
D2.3.9	Explain movement from higher to lower water potential	HL only
D2.3.10	Use ψw = ψs + ψp for cells with walls	HL only
D2.3.11	Explain tissue changes in terms of ψs and ψp	HL only

D2.3.1: Solvation

A solvent surrounds dissolved particles and keeps them dispersed. Solvation is the process by which solvent molecules cluster around a solute particle. Water is a good solvent for many substances because its molecules are polar: the oxygen carries a small negative charge and each hydrogen a small positive charge.

• Ionic solutes. In sodium chloride, each Na⁺ ion attracts the oxygen ends of several water molecules. Each Cl⁻ ion attracts the hydrogen ends. These attractions pull the ions out of the crystal lattice and a shell of water molecules surrounds each ion, which stops it rejoining the lattice.
• Polar molecules. Glucose has several –OH groups. Hydrogen bonds form between these groups and water molecules, so glucose dissolves.
• Non-polar substances. Hydrocarbon chains cannot form these attractions, so they are poorly soluble.

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

Osmosis is the passive net movement of water across a partially permeable membrane, from a solution with lower solute concentration to one with higher solute concentration. Always give the direction using solute concentration; do not describe it as movement towards a lower “water concentration”.

Compare two solutions with these terms:

• Hypertonic: the solution with the higher solute concentration.
• Hypotonic: the solution with the lower solute concentration.
• Isotonic: two solutions with equal solute concentration.

The terms are relative. A solution is hypertonic to one and hypotonic to another.

Worked example 1. A cell has contents equivalent to 0.30 mol dm⁻³ of solute. It is placed in a solution of 0.12 mol dm⁻³ of a solute that cannot cross the membrane.

1. The surrounding solution has the lower solute concentration, so it is hypotonic to the cell contents. The cell contents are hypertonic to the solution.
2. Net water movement is towards the higher solute concentration, so water enters the cell.

In an isotonic environment water molecules still cross the membrane in both directions, at equal rates. There is no net change, but movement continues. This is dynamic equilibrium, not an absence of movement.

D2.3.4: Investigating plant tissue

Plant tissue such as potato, parsnip or beetroot is cut into pieces of the same size and placed in solutions of different concentration, usually sucrose. Cells with a wall gain or lose water, so the tissue changes in mass and length.

Method in steps

1. Cut cylinders with a cork borer and trim them to equal length. Blot and record starting mass and length.
2. Place each in a different sucrose concentration, with repeats at each concentration. Keep volume, temperature and time constant.
3. Remove, blot with the same care each time, and record final mass and length.
4. Calculate percentage change: (final − initial) ÷ initial × 100.
5. Plot percentage change against concentration. The concentration where the line crosses zero is where there is no net water movement. At that point the solution is isotonic with the tissue cells.

Percentage change allows for pieces that start at slightly different sizes.

Worked example 2. A parsnip cylinder has a starting mass of 2.00 g. After an hour in 0.0 mol dm⁻³ sucrose it is 2.31 g. Change = +0.31 g, so the percentage change is 0.31 ÷ 2.00 × 100 = +15.5%. Another cylinder falls from 2.10 g to 1.86 g, a change of −0.24 g, which is −11.4% (to 3 s.f.).

Worked example 3: finding the isotonic concentration. Mean percentage mass change of parsnip cylinders (five per concentration):

Sucrose / mol dm⁻³	0.0	0.2	0.4	0.6	0.8
Mean change / %	+14.2	+7.6	−3.1	−11.8	−17.9

The sign changes between 0.2 and 0.4. Assume a straight line between these points. The line falls by 10.7 percentage points over 0.2 mol dm⁻³. It falls from +7.6 to zero over a fraction 7.6 ÷ 10.7 of that interval. So the zero is at 0.2 + 0.2 × (7.6 ÷ 10.7) = 0.34 mol dm⁻³. The cell sap is isotonic with about 0.34 mol dm⁻³ sucrose.

Standard deviation and standard error

Repeats at each concentration allow you to judge reliability. You do not need to memorise the formulae for these statistics.

• Standard deviation (SD) describes how spread out the repeats are around their mean. A small SD means the repeats agree closely.
• Standard error (SE) describes how precisely the mean has been estimated. It is the SD divided by the square root of the number of repeats, n. More repeats give a smaller SE.
• SE is often drawn as error bars on a graph. Bars that do not overlap suggest the means differ in a real way, although they do not prove it.

Worked example 4. The five percentage changes for 0.2 mol dm⁻³ were +8.1, +6.9, +7.5, +9.0 and +6.4.

1. Mean = 37.9 ÷ 5 = 7.58% (quoted as 7.6% in the table).
2. A calculator gives SD = 1.02.
3. SE = 1.02 ÷ √5 = 0.46.

The mean can be quoted as 7.6 ± 0.5% (mean ± SE). Comparing SDs as percentages of the means shows whether length or mass gave the more reliable results.

D2.3.5: Cells without a cell wall

Animal cells and many unicellular organisms have only a plasma membrane to contain the cytoplasm.

• Hypotonic medium: water enters, the cell swells and may burst (lysis), because nothing resists expansion.
• Hypertonic medium: water leaves, the cell shrinks, and the membrane shows a puckered outline. This is crenation in cells such as red blood cells.
• Freshwater unicellular organisms live in a medium hypotonic to their cytoplasm. Water enters continually. A contractile vacuole collects the excess water and expels it, which uses ATP. In a saltier medium the vacuole slows because less water enters.
• Multicellular animals keep the tissue fluid bathing the cells isotonic to the cells. Without this, cells would swell or shrink in ways that damage tissues. Homeostasis of blood and tissue fluid concentration prevents this.

D2.3.6: Cells with a cell wall

A cell wall resists expansion.

• Hypotonic medium: water enters the vacuole and cytoplasm. The protoplast pushes against the wall, which pushes back. This outward pressure is turgor pressure, and the cell becomes turgid. The wall prevents bursting.
• Hypertonic medium: water leaves, the protoplast shrinks and the plasma membrane pulls away from the wall. This is plasmolysis. The gap between wall and membrane fills with the external solution, because the wall is freely permeable to water and solutes. A plasmolysed cell has lost turgor, so non-woody tissue wilts.
• Isotonic medium: there is no net movement and the cell is flaccid, with little turgor.

D2.3.7: Isotonic fluids in medicine

• Intravenous fluids. Patients who are dehydrated or cannot drink may be given sterile saline that is isotonic to blood plasma. About 0.9 g of sodium chloride per 100 cm³ is a standard value, equal to roughly 154 mmol dm⁻³. Isotonic fluid does not cause net water movement into or out of blood cells, so they keep their shape. Pure water would be hypotonic and red blood cells would swell and burst.
• Bathing organs for transplant. A donor organ is kept in a cold isotonic solution so its cells neither swell nor shrink while it is out of the body.

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

Water potential (ψw) measures how much potential energy a volume of water holds. The absolute amount cannot be measured, so each value is a comparison with a reference: pure water, at 20 °C and normal atmospheric pressure, is defined as 0. The usual unit is the kilopascal (kPa).

Because pure water is zero, adding solute lowers the water potential, so the values for solutions and cells are negative. Water moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential. This is movement down a potential energy gradient: free water molecules have more potential energy than those held by solute attractions. A change from −300 kPa to −600 kPa is a move to a lower water potential.

D2.3.10: Solute and pressure potential (HL only)

For a cell with a wall:

ψw = ψs + ψp

• Solute potential (ψs): the effect of dissolved solutes. It is zero for pure water and negative for any solution; more solute gives a more negative value.
• Pressure potential (ψp): the effect of physical pressure. In a turgid cell the wall pushes back, so ψp is positive. It is zero in an open container of solution. In xylem vessels where sap is pulled upwards under tension, ψp can be negative.

Worked example 5. A plant cell has ψs = −800 kPa and ψp = +500 kPa. It is placed in an open solution with ψs = −600 kPa.

1. Cell: ψw = −800 + 500 = −300 kPa.
2. Solution: ψp = 0, so ψw = −600 kPa.
3. Water moves from −300 to −600 kPa, so water leaves the cell.

D2.3.11: Explaining tissue changes (HL only)

• Hypotonic solution (higher ψw outside): water enters the cells. This dilutes the vacuole slightly, so ψs rises a little. The main change is that ψp rises as the wall resists. Net entry stops when ψw inside equals ψw outside; in pure water that is zero, so ψp = −ψs.
• Hypertonic solution (lower ψw outside): water leaves. ψp falls. When ψp reaches zero, ψw equals ψs: this is incipient plasmolysis. If the outside ψw is still lower, the protoplast shrinks away from the wall and the cell plasmolyses.

In example 5, ψp would reach zero when the cell’s ψw is −800 kPa. The solution at −600 kPa is not low enough for that, so the cell stays turgid at a lower ψp until ψw inside matches −600 kPa.

Common errors

• Writing “water moves to where there is less water”. Use solute concentration, or water potential at HL.
• Saying nothing moves in an isotonic solution. Water still crosses in both directions.
• Calling a plant cell in strong sugar solution “crenated”. That is plasmolysed.
• Forgetting that ψs is negative, so adding solute decreases ψw.

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.