Hypotonic against hypertonic against isotonic solutions: the difference, what happens to an animal cell and a plant cell in each (swell and burst, shrink and crenate, turgid, plasmolysed), the osmosis diagram, the saline and IV fluid examples, the potato experiment, and a printable sheet with practice

"Hypotonic vs hypertonic" is searched about 10,000 times a month in the United States and 1,500 in Canada, by biology students from Grade 7 to AP and by nursing students, who meet the same three words in intravenous fluid charts. The three terms describe one thing, osmosis, seen from three starting points, and once the direction of the water is clear everything else (swelling, shrinking, turgid, plasmolysed, saline, sports drinks) follows. This page gives the difference, the comparison table, the diagram, the memory trick, what happens to animal and plant cells, the medical and everyday examples, the potato experiment that every GCSE and NGSS class does, and a printable sheet with practice.

Download the tonicity sheet with practice (PDF) (A4, the table, the diagrams and the examples on page 1, the potato experiment and the practice set with answers on page 2, free).

First, osmosis

Osmosis is the movement of water across a partially permeable membrane from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution). The membrane lets water molecules through but not the dissolved solute (salt, sugar). The cell membrane is such a membrane, so every cell is constantly gaining or losing water depending on what surrounds it.

The three words describe the surrounding solution compared with the cell's contents:

• Hypotonic: the solution has a lower solute concentration than the cell (more water). Water moves into the cell.
• Hypertonic: the solution has a higher solute concentration than the cell (less water). Water moves out of the cell.
• Isotonic: the same solute concentration. Water moves in and out equally; no net change.

The trick: hypo is under (a hypodermic needle goes under the skin), hyper is over (hyperactive). A hypotonic solution is "under" in solute, so the cell is the saltier side and water flows toward it, into the cell. And the word always describes the solution, not the cell.

The comparison table

What happens to an animal cell

An animal cell has only a thin membrane around it.

• In a hypotonic solution (a red blood cell dropped into pure water), water floods in, the cell swells and bursts within minutes. This is why intravenous fluids are never plain water.
• In an isotonic solution (0.9 percent saline) the cell keeps its shape, which is why that concentration is used for drips, wound washes and contact lens solution.
• In a hypertonic solution (3 percent saline, or syrup) water leaves, the cell shrinks and its surface puckers: crenation.

What happens to a plant cell

A plant cell has a rigid cell wall outside its membrane, which changes the outcome.

• In a hypotonic solution water enters the vacuole, the cell swells until the wall resists, and the cell is turgid. Turgor pressure is what holds leaves and stems up; well-watered plants are turgid.
• In an isotonic solution the cell is flaccid: not shrunken, but not pressing on its wall either. A plant with flaccid cells droops.
• In a hypertonic solution water leaves the vacuole, the membrane peels away from the wall, and the cell is plasmolysed. Salting a slug, salt-damaged roadside plants and weedkiller-by-salt all work this way. Plasmolysis is reversible if water is restored quickly.

The medical and everyday examples

The potato experiment

The required practical in GCSE biology and a staple of NGSS middle-school life science:

1. Cut potato cylinders of equal length and mass; blot and weigh each.
2. Place one in each of a range of sugar (or salt) solutions, from pure water to 1 mol/dm³, for the same time.
3. Blot and reweigh. Calculate the percentage change in mass: (final − initial) ÷ initial × 100.
4. Plot percentage change against concentration. Cylinders in dilute solutions gain mass (water in: hypotonic to the potato cells); in strong solutions they lose mass (hypertonic). Where the line crosses zero, the solution is isotonic with the potato, which gives the concentration of the cell sap (usually about 0.25 to 0.3 mol/dm³ sucrose).

Marks go on blotting before weighing (surface water is not cell water), using percentage change (cylinders start at different masses) and reading the isotonic point off the graph.

The mistakes that cost marks

• Applying the word to the cell instead of the solution ("the cell is hypotonic"). The solution is hypotonic to the cell.
• Comparing water instead of solute. The definitions are in solute concentration; water moves toward higher solute.
• Saying water moves "to where there is less water" without saying across a partially permeable membrane. Osmosis needs the membrane in the answer.
• Saying a plant cell bursts in pure water. The wall stops it: turgid, not burst.
• Saying nothing moves in an isotonic solution. Water moves both ways at equal rates; the net movement is zero.
• Confusing diffusion (any particle, down its own gradient) with osmosis (water only, across a membrane).

Where it sits in each curriculum

Practice (answers in the PDF)

1. A red blood cell is placed in distilled water. Describe and explain what happens.
2. A plant cell is placed in a concentrated salt solution. Name the state it reaches and describe the membrane.
3. Why is 0.9 percent saline used for intravenous drips rather than pure water?
4. In a potato experiment, a cylinder of 2.40 g becomes 2.64 g. Calculate the percentage change and say whether the solution was hypotonic, isotonic or hypertonic to the potato.
5. Why does salting a slice of aubergine draw out water?
6. Two solutions, A at 0.2 mol/dm³ sucrose and B at 0.6 mol/dm³, are separated by a partially permeable membrane. Which way does water move, and which solution is hypertonic to the other?

Download the sheet and the practice set (PDF)

Where to get help

A 1-on-1 online biology class with a tutor who teaches NGSS middle and high-school life science, GCSE and A-level biology, CBSE Class 9 to 12 and the Australian Years 8 to 12 every week: osmosis taught with a diagram and a potato before a definition, so the three words attach to a picture of water moving. The biology page lists the courses; the free 30-minute first class is a real lesson on this week's topic. Related pages: living and non-living things and adaptations in animals and plants.

Questions parents ask

See how we teach this, 1-on-1 online →