IB Biology HL D2.3 Water potential Questions

Calculate water potential, interpret plant-tissue osmosis data and explain plant and animal cell responses in IB Biology HL D2.3.

43 questions in this syllabus scope, with question parts counted together.

Exam analysis

HL · 2010–2025

Papers with this topic22 / 117in the analysed sample

Latest appearanceNovember 2025

Tagged question parts28

Paper types5

Most tested objectives

D2.3.5—Effects on cells without wall8 papers
D2.3.2—Water movement3 papers
D2.3.3—Osmosis into/out of cells3 papers
D2.3.4—Changes in plant tissue3 papers
D2.3.6—Effects on cells with wall3 papers

Common question formats

Definition or recall
Process explanation
Calculation
Data analysis
Diagram interpretation

Recent exam appearances

November 2025Paper 2 HL · TZ3Q4(a)[ 3 ]

November 2025Paper 2 HL · TZ1Q9(c)[ 4 ]

November 2025Paper 1A HL · TZ1Q20[ 1 ]

117 papers analysed · Tagging incomplete

Exam points

Define osmolarity; apply osmosis to root uptake and reduced water movement in saline soil.
Plan fair plant-tissue osmosis tests using controlled variables and percentage mass change.
Plot percentage mass change against solution concentration and read the zero-change intercept.
Interpret plant-tissue graphs relative to the isotonic point to identify or count solutions causing mass loss.
Interpret hypotonic and hypertonic effects from red-cell swelling, haemolysis or crenation data.
Explain how contractile vacuoles remove excess water from freshwater protists.
Explain how water entry builds turgor and cellulose walls support plant cells.
Explain how isotonic plasma prevents net osmosis and mismatched fluids damage cells.
Use Ψw = Ψs + Ψp to calculate unknown potentials with correct signs and kPa units.
Infer plant-cell water and pressure-potential changes from movement between solutions.
Predict how hypotonic solutions alter root-cell pressure and solute potentials.

Covered syllabus points

D2.3.1—Solvation with water D2.3.2—Water movement D2.3.3—Osmosis into/out of cells D2.3.4—Changes in plant tissue D2.3.5—Effects on cells without wall D2.3.6—Effects on cells with wall D2.3.7—Medical applications D2.3.8 (HL)—Water potential D2.3.9 (HL)—Movement from higher to lower potential D2.3.10 (HL)—Solute and pressure potential D2.3.11 (HL)—Water potential in plant tissue

Exam points

Explain water’s solvent properties for an ionic or polar solute by linking water’s polarity to ion attraction or hydrogen bonding.

[Maximum number: 7]

Hydrogen bonds can exist both within and between molecules in living organisms and have an impact on their structure and function. Explain the importance of hydrogen bonding for living organisms.

Exam points

Define osmolarity as the total concentration of solute particles per litre of solution.
Identify osmosis as the process that moves water into plant root cells.
Identify that increased soil salinity decreases water movement from soil into plant roots.

[Maximum number: 1]

Vibrio fischeri is a bioluminescent (light-producing) bacterium in the Pacific Ocean. V. fischeri grown in the laboratory often has less luminescence. An investigation was carried out to look at the effect of the external osmolarity (in mOsmol) on the bacterium's luminescence, at different densities of bacteria. V. fischeri cultures were grown in three different concentrations of artificial seawater. Relative luminescence is the total luminescence divided by the density of bacteria.

Key: osmolarity
- 1300 mOsmol
- 810 mOsmol
- 570 mOsmol

Define osmolarity.

Exam points

Use a blood-cell osmolarity table to identify the isotonic NaCl concentration, using no net water movement or minimal haemolysis as evidence.

[Maximum number: 1]

The table shows the percentage of red blood cells that have ruptured (hemolysis) when placed in sodium chloride solutions of different molar concentrations.

Deduce with a reason which sodium chloride concentration shown in the table is closest to the osmolarity of the blood cells.

Exam points

Describe controls for a plant-tissue osmolarity experiment, such as equal sample size, immersion time, temperature and blotting method.
Explain why percentage mass change lets plant-tissue samples with different starting masses be compared fairly.
Plot percentage mass change against solution concentration and use the zero-change intercept to estimate plant-tissue osmolarity.
Use an isotonic sucrose concentration to determine how many potato cylinders lost mass across the tested solutions.

[Maximum number: 3]

An experiment was carried out to study osmosis in plant cells. Small cylinders of zucchini (Cucurbita pepo) were cut and placed in different sucrose solutions at $25^{\circ} \mathrm{C}$ . The figure shows the percentage changes in mass after 24 hours.

(a)

Estimate the solute concentration of the zucchini cells.

[ 1 ]

$0.36 \mathrm{~mol} \mathrm{dm}^{-3} / \mathrm{M}$

Units required Allow a range of $0.35-0.37 \mathrm{~mol} \mathrm{dm}^{-3} / M$ .

(b)

Explain one reason for calculating the percentage changes in mass.

[ 2 ]

Exam points

Predict water entry, swelling or lysis when red blood cells are placed in distilled water or another hypotonic solution.
Interpret red-cell morphology or haemolysis data to distinguish hypotonic water gain from hypertonic crenation and water loss.
Explain how a freshwater protist uses increased contractile-vacuole activity to expel excess water in a dilute medium.

[Maximum number: 2]

The micrograph shows part of a cardiac muscle cell.

Explain the changes that would occur in a cardiac muscle cell placed in a hypotonic solution.

Exam points

Explain how water entry by osmosis and restriction by the cell wall build pressure above atmospheric pressure in plant cells.
Outline water loss, reduced pressure and plasmolysis when walled plant tissue is placed in a hypertonic solution.
Relate cellulose in the cell wall to structural support and prevention of bursting when internal pressure is high.

[Maximum number: 1]

Onion (Allium cepa) epidermis was placed in pure water and observed with a light microscope using high magnification.

What would happen to these cells if they were transferred to a hypertonic solution?

Cells would gain mass.

Cells would take in water by osmosis and swell.

Cells would burst open, releasing their content.

Cell membranes would detach from walls at some points.

Exam points

Explain that isotonic plasma and tissue fluid match cell osmolarity and therefore prevent net osmosis.
Explain how hypotonic or hypertonic plasma or tissue fluid causes cell swelling and bursting or water loss and crenation.

[Maximum number: 4]

Living organisms have mechanisms for preventing or reducing change.

Explain the need for isotonic conditions in human blood plasma and tissue fluid.

No public sample questions are available for this syllabus point yet.

Exam points

Use Ψw = Ψs + Ψp to calculate plant-cell solute potential from given water and pressure potentials, keeping the negative sign and kPa units.

[Maximum number: 1]

Two plant cells are shown as they appear in different solutions X and Y .

What happens to water and pressure potential inside the cell if it is moved from solution X to solution Y ?

Water potential

Pressure potential

does not change

decreases

does not change

increases

Exam points

Explain how water entering a root cell raises pressure potential and makes solute potential less negative.

[Maximum number: 3]

(a) Explain how the pressure potential and solute potential change inside a plant root cell when it is placed into a solution that is more hypotonic than the cell's cytoplasm.

Explain how the pressure potential and solute potential change inside a plant root cell when it is placed into a solution that is more hypotonic than the cell's cytoplasm.

a. (when cell is placed in a hypotonic solution) surrounding/outside (of the root hair cell) is more dilute / has higher water potential

OR

water will enter the root hair cell

OR

water moves from high water potential to low water potential;

b. (positive) pressure potential will increase / become more positive;

c. (negative) solute/osmotic potential will increase / become less negative;

d. water potential inside the cell will increase / become less negative;

e. $\Psi_{w}=\Psi_{s}+\Psi_{p}$ ;

f. when cell's water potential equals that of the solution, (net) water movement stops;

For e. allow word equation:

$\Psi_{w}=$ water potential

$\Psi_{s}=$ solute potential

$\Psi_{p}=$ pressure potential

[Maximum number: 3]

Key: osmolarity
- 1300 mOsmol
- 810 mOsmol
- 570 mOsmol

(a)

Define osmolarity.

[ 1 ]

(b)

Student osmosis experiments often involve putting plant tissue such as potato cylinders in several salt solutions of different concentrations and measuring the mass before and after immersion. Outline how data collected from such an experiment could be used to estimate the osmolarity of the plant tissues.

[ 2 ]

[Maximum number: 4]

The table shows the percentage of red blood cells that have ruptured (hemolysis) when placed in sodium chloride solutions of different molar concentrations.

(a)

Explain the effect of placing red blood cells in distilled water (0.000 M NaCl).

[ 3 ]

(b)

Deduce with a reason which sodium chloride concentration shown in the table is closest to the osmolarity of the blood cells.

[ 1 ]

All question bank results loaded

a	red blood cells are hypertonic/more concentrated/have lower water potential/higher solute concentration (than distilled water)/vice versa;
b	water moves into cells by osmosis;
c	water moves from an area of higher water potential/concentration/ solution (in water) to lower water potential/concentration (in cell) OR water moves from a more dilute solution to a more concentrated solution OR water moves from hypotonic to hypertonic solution;
d	through a (selectively) permeable membrane;
e	cells swell and (eventually) burst/ complete/100\% hemolysis;

a	the change in mass indicates whether the tissue has gained/lost water
b	the pieces of tissue will not all be the same mass «at the beginning of the experiment»
c	to compare the relative changes in mass

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

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