Solvation and osmosis

Cells (Continuity and change) · Water potential · note 1 of 3

Spec D2.3.1, D2.3.2, D2.3.3Download all Water potential notes (PDF)Download all notes (PDF)5 pages

Solvation and osmosisSpec D2.3.1, D2.3.2, D2.3.3

In short

Osmosis is the net movement of water across a partially permeable membrane from a less concentrated solution to a more concentrated solution. Solutes dissolve because water forms hydrogen bonds with polar molecules and is attracted to positive and negative ions. In an isotonic environment there is dynamic equilibrium, not zero water movement.

Solvation

Solvation is the process of a solute dissolving, with each solute particle surrounded by solvent molecules. Water is a good solvent because its molecules are polar: the oxygen end is slightly negative (δ−) and the hydrogen ends are slightly positive (δ+).

Positively charged ions (for example Na⁺) attract the δ− oxygen ends of water molecules.
Negatively charged ions (for example Cl⁻) attract the δ+ hydrogen ends of water molecules.
Polar molecules such as glucose form hydrogen bonds with water molecules.

Each ion or molecule becomes surrounded by a shell of water molecules, so it separates from the others and dissolves.

Solvation: a water molecule with δ− on the oxygen and δ+ on each hydrogen; a sodium ion Na⁺ surrounded by water molecules with their oxygen (δ−) ends facing in; a chloride ion Cl⁻ surrounded by water molecules with a hydrogen (δ+) end facing in; and a glucose molecule whose OH groups form hydrogen bonds (dashed lines) with water molecules. — Solvation: polar water molecules surround cations, anions and polar molecules such as glucose, so they dissolve.

Direction of water movement

Osmosis is the net movement of water through a partially permeable membrane. Water moves from the less concentrated solution to the more concentrated solution, meaning from lower to higher solute concentration. Water molecules bound to solutes move less freely, so more water molecules cross from the side with fewer solutes.

Hypertonic: Having a higher solute concentration than another solution.
Hypotonic: Having a lower solute concentration than another solution.
Isotonic: Having the same solute concentration as another solution.

Osmosis into and out of cells

Predicting net movement of water
Environment of the cell	Net movement of water
Hypotonic (lower solute concentration than the cytoplasm)	Into the cell
Hypertonic (higher solute concentration than the cytoplasm)	Out of the cell
Isotonic	No net movement: water moves in and out at equal rates (dynamic equilibrium)

Common mistake:

Describe direction in terms of solute concentration, not water concentration. And in an isotonic solution water still moves; the rates in and out are just equal.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between hypertonic, hypotonic and isotonic?

These terms compare the solute concentrations of two solutions. A hypertonic solution has a higher solute concentration than the other, a hypotonic solution has a lower one, and isotonic solutions have the same solute concentration. Water moves by osmosis from the hypotonic solution to the hypertonic solution.

What happens to animal cells in a hypotonic solution?

Animal cells in a hypotonic solution gain water by osmosis, because the solution has a lower solute concentration than the cytoplasm. With no cell wall to resist, the cell swells and may burst. Red blood cells bursting is called haemolysis. In a hypertonic solution they shrink and crenate instead.

Why do plant cells not burst in pure water?

Plant cells do not burst because of their cell wall. As water enters by osmosis, the cytoplasm and vacuole push against the wall and turgor pressure develops. The strong wall resists further expansion, so the cell becomes turgid. Without a wall, an animal cell would swell and burst.

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