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D2.3.2—Water movement

Water moves by osmosis through partially permeable membranes from lower solute concentration toward higher solute concentration in cells and biological systems.

Syllabus
First assessment 2025
Objective
D2.3.2
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper1
Typical marks1

Common command terms

  • Explain
  • Outline

Scoring notes

Common mistake
Saying solute moves by osmosis instead of water.

Recent exam appearances

May 2021Paper1 ["HL"] · TZ13[ 1 ]D2.3.2—Water movement
May 2018Paper1 ["HL"] · TZ132[ 1 ]D2.3.2—Water movement
November 2016Paper1 ["HL"] · TZ032[ 1 ]D2.3.2—Water movement
Practice this objective

Coverage 2016–2021 · Updated 16 Jul 2026

Water Moves from Hypotonic to Hypertonic Solution

Across a partially permeable membrane, net water movement is from the less concentrated solution toward the more concentrated solution.

Comparison term Solute concentration relative to the other solution Expected net water movement
Hypotonic Lower Away from this solution
Hypertonic Higher Toward this solution
Isotonic Equal effective concentration No net movement

If solution A is 0.10 mol dm⁻³ sucrose and solution B is 0.40 mol dm⁻³, A is hypotonic to B and net water movement is from A to B if water can cross but sucrose cannot.

At SL, express the comparison using solute concentration—not 'high water concentration'. Tonicity is relative and depends on solutes that do not freely cross the membrane.

Water movement

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, multiple choice, commonly using Explain / Outline.

Command terms

Explain / Outline

What earns marks

Build the answer around this relationship: Osmosis requires a partially permeable membrane.

Watch for

Saying solute moves by osmosis instead of water.

Representative question

Question 1

[Maximum number: 2]

Outline the conditions necessary for osmosis to occur.

Core Osmosis Effects

Water forms hydration shells around ions and polar solutes; hydrogen bonding and charge attraction reduce free water movement. Water moves by osmosis across partially permeable membranes from hypotonic/lower solute solutions toward hypertonic/higher solute solutions. Osmosis direction depends on internal and external solute concentration; isotonic conditions have dynamic water movement but no net osmosis. Plant tissue changes mass or length in sucrose solutions; percentage change graphs estimate isotonic or osmotic concentration. Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions; freshwater protists use contractile vacuoles to expel excess water. Plant cells become turgid in hypotonic solutions as vacuoles swell; hypertonic solutions cause flaccidity and plasmolysis from water loss. Isotonic saline prevents harmful water gain or loss in body cells; IV fluids and transplant organ baths must match tissue osmotic concentration.

Concept essentials

  • Osmosis requires a partially permeable membrane.
  • Water moves toward the side with higher solute concentration.
  • A solute concentration difference or osmolarity gradient is required.
  • Root water uptake can occur by osmosis.