The big idea: Water moves across a cell's membrane by osmosis — it travels from where water is more concentrated (a dilute solution) to where it is less concentrated (a concentrated solution).
An animal cell has no cell wall — only a thin, flexible plasma membrane holds it together.
So whether water enters or leaves, and what happens to the cell, depends entirely on the concentration of the solution outside compared with the inside of the cell.
- Osmosis
- The net movement of water across a partially permeable membrane, from a more dilute solution to a more concentrated one.
- Tonicity
- A comparison of the solute concentration of the solution OUTSIDE a cell with the concentration INSIDE it — described as hypotonic, isotonic or hypertonic.
- Hypotonic solution
- A solution that is MORE DILUTE than the inside of the cell (it has a lower solute concentration). Water moves into the cell.
- Isotonic solution
- A solution with the SAME solute concentration as the inside of the cell. There is no net movement of water.
- Hypertonic solution
- A solution that is MORE CONCENTRATED than the inside of the cell (it has a higher solute concentration). Water moves out of the cell.
No wall = no protection: A plant or bacterial cell has a strong cell wall outside its membrane that resists swelling.
An animal cell has no wall, so it cannot resist water entering or leaving — it will burst if too much water enters, or shrivel if too much leaves. This is exactly why this topic is about cells without a wall.
Picture a red blood cell (or a cheek cell) dropped into three different solutions. In each one, water moves by osmosis until it has nowhere left to go — and because there is no wall, the cell changes shape as a result.
Work through them in order: dilute outside, equal outside, concentrated outside.
An animal cell (no wall) in three solutions: in a hypotonic solution water enters and the cell swells and may burst (lysis); in an isotonic solution there is no net movement; in a hypertonic solution water leaves and the cell shrinks (crenation).
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Hypotonic — water enters, the cell bursts: In a hypotonic solution (more dilute outside), there is more water outside than inside, so water moves into the cell by osmosis.
The cell swells. Because there is no wall to resist it, the swelling continues until the membrane bursts — this bursting is called lysis (in red blood cells it is haemolysis).
Pure (distilled) water is the most extreme hypotonic solution, so cells placed in it burst very quickly.
Isotonic — nothing changes: In an isotonic solution, the concentration outside equals the concentration inside.
Water still moves both ways, but at the same rate — there is no net movement, so the cell keeps its normal shape and size.
This is why the blood plasma surrounding red blood cells, and the fluid given in a medical drip, are kept isotonic to the cells.
Hypertonic — water leaves, the cell shrivels: In a hypertonic solution (more concentrated outside), there is less water outside than inside, so water moves out of the cell by osmosis.
The cell shrinks, and its surface becomes wrinkled or spiky — this shrivelling is called crenation.
A strong salt or sugar solution is hypertonic to a cell, which is why it draws water out.
Hypotonic (dilute outside)
- More water outside the cell
- Water moves INTO the cell
- Cell swells and may burst (lysis)
- No wall to resist the swelling
Hypertonic (concentrated outside)
- Less water outside the cell
- Water moves OUT of the cell
- Cell shrinks and wrinkles (crenation)
- Membrane pulls inward — no wall to hold the shape
Osmoregulation — how single cells survive fresh water: A single-celled organism such as Paramecium lives in fresh water, which is hypotonic to it. Water is constantly entering by osmosis — so without a wall, it should burst.
It survives using a contractile vacuole: a structure that collects the excess water and pumps it back out of the cell. This control of water balance is called osmoregulation.
In the 'functions of life', the contractile vacuole carries out excretion / homeostasis — removing the excess water the cell takes in.
A memory hook: HypO = water in, cell blOws up. HypER = water exits, cell shrivels.
And read it backwards in data questions: a burst cell means the solution was hypotonic; a shrunken cell means it was hypertonic.
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How this is tested: On Paper 3 a 1-mark Describe question commonly asks what happens to a cheek cell or red blood cell placed in a concentrated (hypertonic) salt solution — the answer is that water leaves by osmosis and the cell shrinks (crenates).
On Paper 1A and Paper 2 this micro is a data-reasoning favourite: you are shown micrographs of human blood cells and asked to deduce the tonicity of the solution from the cells' appearance (swollen or burst -> hypotonic; unchanged -> isotonic; shrunken -> hypertonic).
A 1-mark Suggest question may ask why no cells are visible after cells are placed in distilled water — they have burst (lysed).
IB-style question — describe the effect of a hypertonic solution
A red blood cell is placed in a concentrated salt solution. Describe what happens to the cell and explain why. [3]
How to score all three marks
- Name the water movement. The salt solution is hypertonic (more concentrated than the cell), so water moves out of the cell by osmosis.
- Describe the visible effect. The cell shrinks and its surface becomes wrinkled — this is called crenation.
- Explain the cause. Water leaves because there is a higher water concentration inside the cell than in the concentrated solution outside, so water moves down its gradient. (Mark 1: water leaves / moves out by osmosis. Mark 2: cell shrinks / crenates. Mark 3: because the outside is hypertonic / more concentrated.)
Final answer
Water moves out of the cell by osmosis because the solution is hypertonic (more concentrated outside); the cell shrinks and becomes wrinkled — crenation.
✓ Why this scores full marks: It gives the three required points for a 3-mark answer: the direction of water movement (out), the named effect (shrinks / crenation), and the reason (hypertonic outside).
A common slip is to say the cell 'gets smaller' without naming crenation or saying water moved out by osmosis.
| Solution outside the cell | Net water movement | Effect on the animal cell |
|---|---|---|
| Hypotonic (more dilute than the cell) | Water moves INTO the cell | Cell swells and may burst — this bursting is called lysis (cytolysis / haemolysis in red blood cells) |
| Isotonic (equal to the cell) | No net movement of water | Cell stays the same — its normal shape and size are kept |
| Hypertonic (more concentrated than the cell) | Water moves OUT of the cell | Cell shrinks and the surface looks wrinkled — this shrivelling is called crenation |