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D2.3.3—Osmosis into/out of cells

Osmosis into or out of cells depends on relative osmolarity, with isotonic conditions causing no net water movement across membranes.

Syllabus
First assessment 2025
Objective
D2.3.3
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper3
Typical marks3

Common command terms

  • Explain

Recent exam appearances

May 2023Paper3 ["SL"] · TZ12(e)[ 3 ]D2.3.3—Osmosis into/out of cells

Coverage 2023–2023 · Updated 16 Jul 2026

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.

Isotonic Solutions Are in Dynamic Equilibrium

In an isotonic condition, water crosses the membrane in both directions at equal rates, so there is no net osmosis and no sustained change in cell volume.

Molecular level Whole-cell result
water molecules continue moving randomly both ways inflow equals outflow
individual crossings still occur average cell volume remains stable

“No net movement” does not mean “no movement.” It means that the two opposing rates balance.

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

Osmosis into/out of cells

[ 3 ]

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

Concept essentials

  • Hypotonic external solutions cause net water entry into cells.
  • Hypertonic external solutions cause net water loss from cells.
  • Isotonic conditions produce no net osmosis.
  • Animal tissues are maintained in matching osmolarity to preserve cell structure.