Tonicity, osmolarity and solutions

The big idea: Whether water moves into or out of a cell depends on how concentrated the solution outside is compared with the inside.

Osmolarity measures that concentration — it is the total amount of dissolved solute particles in a solution.

Tonicity is the comparison between two solutions, and it tells you which way water will move by osmosis. The three tonicities are hypotonic, isotonic and hypertonic.

Solute

A substance (such as salt or sugar) that is dissolved in a liquid.

Osmolarity

The total concentration of solute particles in a solution. A higher osmolarity means more solute particles per unit volume — a more concentrated solution.

Osmosis

The net movement of water across a partially permeable membrane, from a more dilute solution (lower osmolarity) toward a more concentrated one (higher osmolarity).

Hypotonic solution

A solution with a LOWER osmolarity than the cell (more dilute). Water moves into the cell.

Isotonic solution

A solution with the SAME osmolarity as the cell. There is no net movement of water.

Hypertonic solution

A solution with a HIGHER osmolarity than the cell (more concentrated). Water moves out of the cell.

Tonicity is always a comparison: A solution is never 'hypertonic' on its own — it is hypertonic to something else.

The prefixes are relative: hypo- = less (lower osmolarity), iso- = equal, hyper- = more (higher osmolarity).

Always ask: more or less concentrated than what?

Osmolarity matters because water always moves by osmosis toward the solution with the higher osmolarity — toward the side that is more concentrated in solute.

So once you know the tonicity of the outside solution, you can predict exactly which way water will move and what the cell will do.

The one rule to remember: Water moves toward the higher osmolarity (toward the more concentrated solution).

Put another way, water moves down its own concentration gradient — from where there is more water (dilute) to where there is less water (concentrated).

Hypotonic outside → cell swells: If the outside solution is hypotonic (lower osmolarity, more dilute than the cell), the inside of the cell is the more concentrated side.

Water moves into the cell, so the cell gains water and swells — and without a wall it may burst.

Hypertonic outside → cell shrinks: If the outside solution is hypertonic (higher osmolarity, more concentrated than the cell), the outside is now the more concentrated side.

Water moves out of the cell, so the cell loses water and shrinks.

Isotonic outside → no net change: If the outside solution is isotonic (equal osmolarity), neither side is more concentrated.

Water still crosses the membrane, but it moves equally both ways, so there is no net movement and the cell stays the same size.

A memory hook: Hyper = high solute → water hurries out of the cell (it shrinks).

Hypo = low solute outside → water heads in (the cell swells).

Water always chases the salt — it moves toward the higher osmolarity.

How this is tested: A 1-mark Define question asks for the meaning of osmolarity — answer with the total concentration of solute particles in a solution (the more solute, the higher the osmolarity).

Membrane and osmosis topics are a Paper 3 / Paper 1B data favourite: you are given a graph or table of tissue mass (or length) after soaking in a solution and asked to explain the result. The reasoning is always the same — mass lost = water lost = hypertonic solution; mass gained = water gained = hypotonic; no change = isotonic.

On Paper 1A a multiple-choice item may give two solutions and ask you to deduce which is hypertonic to the other from their osmolarity.

✓ Why this scores the mark: A 1-mark Define needs the core meaning, not an example.

'How much solute is dissolved' / 'the concentration of solute particles' is the scoring idea. Saying only 'how salty something is' is too vague to be sure of the mark.