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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
SL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–2

Common command terms

  • Explain
  • Outline

Scoring notes

Common mistake
Saying solute moves by osmosis instead of water.

Recent exam appearances

November 2024Paper3 ["SL"] · TZ11(a)[ 2 ]D2.3.2—Water movement
May 2023Paper3 ["SL"] · TZ21(b)(ii)[ 1 ]D2.3.2—Water movement
May 2019Paper1 ["SL"] · TZ14[ 1 ]D2.3.2—Water movement
May 2015Paper1 ["SL"] · TZ27[ 1 ]D2.3.2—Water movement

Coverage 2015–2024 · Updated 16 Jul 2026

Osmosis Requires Water, a Gradient and a Selective Membrane

Osmosis is the net movement of water across a selectively permeable membrane from the side with lower solute concentration to the side with higher solute concentration.

The definition requires all three ideas:

  • the moving substance is water
  • the two sides differ in solute concentration
  • the membrane allows water to cross but restricts at least some solute

Say that water moves by osmosis. Osmosis is the process, not a verb performed by water.

Water crosses a partially permeable membrane from a hypotonic solution toward a hypertonic solution.

Tonicity Compares an External Solution with a Cell

Tonicity is relative: a solution is described as hypotonic, isotonic or hypertonic to a particular cell or reference solution.

External solution relative to cytoplasm External solute concentration Net water movement
hypotonic lower into the cell
isotonic equal none overall
hypertonic higher out of the cell

To predict direction, compare solute concentration on the two sides first; then point the net water movement toward the side with the higher solute concentration.

Hypotonic does not mean that the cell contains little water. It describes the external solution's solute concentration relative to the cell.

Predict Osmosis from Solute Interaction to Net Movement

more dissolved particles → more water associated in hydration shells → lower freedom and movement tendency of water → net water movement across a selective membrane toward that side

For any cell diagram:

  • identify inside and outside
  • compare solute concentration
  • confirm that water can cross the membrane
  • draw net water movement toward the higher solute concentration
  • if concentrations are equal, state dynamic equilibrium
Use Avoid
water moves by osmosis water osmosises
no net osmosis no water movement
external solution is hypertonic to the cell the cell is hypertonic, without a reference

Water movement

[ 2 ]

Outline the conditions necessary for osmosis to occur.

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.