IB Biology SL D2.3 Water potential Questions

Explain osmosis, interpret plant-tissue mass data and predict plant, animal and protist cell responses in IB Biology SL D2.3.

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

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam analysis

SL · 2010–2025
Papers with this topic21 / 141in the analysed sample
Latest appearanceNovember 2025
Tagged question parts30
Paper types5

Most tested objectives

  1. D2.3.5—Effects on cells without wall9 papers
  2. D2.3.4—Changes in plant tissue5 papers
  3. D2.3.2—Water movement4 papers
  4. D2.3.6—Effects on cells with wall3 papers
  5. D2.3.1—Solvation with water1 paper

Common question formats

  • Definition or recall
  • Process explanation
  • Calculation
  • Data analysis
  • Diagram interpretation

Recent exam appearances

November 2025Paper 1A SL · TZ1Q12[ 1 ]
May 2025Paper 2 SL · TZ1Q6(c)[ 7 ]
May 2025Paper 1A SL · TZ1Q12[ 1 ]

141 papers analysed · Tagging incomplete

Exam points

  • State osmosis requirements and infer water movement toward higher solute concentration.
  • Use isotonicity to explain no net osmosis and predict lysis or crenation in wall-less cells.
  • Explain how contractile vacuoles remove excess water from freshwater protists.
  • Predict plant-cell turgor or plasmolysis from hypotonic or hypertonic exposure.
  • Calculate percentage mass change and use the zero-change intercept to estimate tissue isotonic concentration.
  • Assess plant-tissue osmosis methods using controls, scatter, trends and outliers.
  • Choose isotonic transplant storage solutions to prevent net water movement.

Covered syllabus points

D2.3.1—Solvation with waterD2.3.2—Water movementD2.3.3—Osmosis into/out of cellsD2.3.4—Changes in plant tissueD2.3.5—Effects on cells without wallD2.3.6—Effects on cells with wallD2.3.7—Medical applications

Exam points

  • Explain how water polarity and hydrogen bonding attract ions and polar solutes in aqueous solution.

Question 1

[Maximum number: 3]

The figure represents a water molecule.

Figure for Question 1 — IB Biology SL

Water has important solvent properties. Explain these properties using an example to illustrate your answer.

Exam points

  • State that osmosis requires a partially permeable membrane and a solute or water concentration gradient.
  • Explain that water moves through the membrane toward the solution with higher solute concentration.
  • Distinguish osmosis from evaporation by identifying membrane-mediated water movement in cells or tissues.

Question 1

[Maximum number: 1]

Which process(es) occur(s) by osmosis?

I. Uptake of water by cells in the wall of the intestine
II. Loss of water from a plant cell in a hypertonic environment
III. Evaporation of water from sweat on the skin surface

A

I only

B

I and II only

C

II and III only

D

I, II and III

Exam points

  • Explain how matching osmolarity between cells and their surroundings prevents net osmosis.
  • Predict whether water enters or leaves animal cells by comparing the surrounding solution’s osmolarity with the cell interior.

Question 1

[Maximum number: 3]

The electron micrograph shows the surface membrane of an animal cell.

Figure for Question 1 — IB Biology SL

Explain the reason that animal cells and tissues under investigation must be maintained in solutions with the same osmolarity.

Exam points

  • Use percentage-mass changes to calculate plant-tissue osmolarity from the zero-change point of a graph.
  • Read graph trends relative to the isotonic point to identify hypotonic solutions and count potato cylinders that lose mass.
  • Explain different isotonic points for plant tissues by referring to their internal dissolved solutes or growth and storage conditions.
  • Explain why percentage mass change allows fair comparison when tissue samples have different starting masses.
  • Evaluate plant-tissue graph reliability using trend, scatter, error bars or outlier evidence.
  • Describe controls for a plant-tissue osmolarity experiment, such as sample size, solution exposure time, temperature and drying method.

Question 1

[Maximum number: 3]

An experiment was carried out to determine the change in mass due to osmosis. Potato cubes were soaked in different concentrations of sodium chloride for 12 hours. The table shows the mean data for the mass of potato before and after the 12-hour period.

Table for Question 1 — IB Biology SL

Question (a)

(a)

Explain how the data may be used to determine the solute concentration of the potato tissue.

[ 2 ]

Question (b)

(b)

Estimate the solute concentration of the potato tissue.

[ 1 ]

Exam points

  • Predict water entry, swelling and lysis when a wall-less animal cell is placed in a hypotonic or distilled-water solution.
  • Infer hypertonic or hypotonic exposure from animal-cell morphology and explain water loss, shrinkage or crenation.
  • Explain how freshwater protists use contractile vacuoles to expel osmotic water and maintain internal water balance.

Question 1

[Maximum number: 1]

Red blood cells were transferred from solution 1 to solution 2,3 or 4 . The images show the effects on these cells.

Figure for Question 1 — IB Biology SL

What can be concluded about the concentrations of the solutions?

A

Solution 1 is hypotonic to solution 4.

B

Solution 2 is hypotonic to solution 1.

C

Solution 3 is hypertonic to solution 4.

D

Solution 4 is hypertonic to solution 2.

Exam points

  • Explain osmosis through plant-cell membranes and predict water entry, turgor or cell-wall support in a hypotonic solution.
  • Explain how hypertonic conditions cause walled plant cells to lose water, become flaccid and plasmolyse.

Question 1

[Maximum number: 1]

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

Figure for Question 1 — IB Biology SL

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

A

Cells would gain mass.

B

Cells would take in water by osmosis and swell.

C

Cells would burst open, releasing their content.

D

Cell membranes would detach from walls at some points.

Exam points

  • Select an isotonic solution with the same osmolarity as transplant-organ tissue to prevent net water movement and osmotic damage.

Question 1

[Maximum number: 1]

A human organ is being prepared for transplant. In what type of solution must it be bathed?

A

A hypertonic solution

B

A hypotonic solution

C

Pure water containing no solutes

D

A solution with the same osmolarity as the organ tissue