Osmosis Diagram
Osmosis moves water across a membrane from a dilute solution to a concentrated one. The diagram shows what that does to a red blood cell in each type of solution.
| Solution outside | Net water movement | Animal cell | Plant cell |
|---|---|---|---|
| Hypotonic (dilute) | In | Swells, may burst (lysis) | Turgid: firm, wall stops it bursting |
| Isotonic (equal) | No net movement | Normal shape | Flaccid: soft, limp |
| Hypertonic (concentrated) | Out | Shrinks and crinkles (crenation) | Plasmolysed: membrane pulls off the wall |
How to work out which way water moves
- Compare the solute concentration outside the cell with the inside.
- Water moves towards the side with more solute (less water), never the other way.
- If the outside is more concentrated (hypertonic), water leaves the cell; if it is more dilute (hypotonic), water enters.
- Then apply the cell type: animal cells burst or shrivel; plant cells become turgid or plasmolysed.
Worked example: potato cylinders
A 5.0 g potato cylinder left in 0.8 mol/dm³ sugar solution weighs 4.4 g the next day: a change of (4.4 − 5.0) ÷ 5.0 × 100 = −12%. The solution was hypertonic to the potato cells, so they lost water by osmosis. A cylinder in pure water gains mass instead, and where the line of % change crosses zero is the concentration that is isotonic with the potato.
Watch water cross a membrane particle by particle in the Diffusion & Osmosis simulation, read what happens to a red blood cell in salt water, or label every organelle on the animal cell diagram.
Frequently asked questions
What is osmosis?
Osmosis is the net movement of water molecules across a partially permeable membrane, from a dilute solution (more water) to a concentrated solution (less water). It is a special case of diffusion and needs no energy from the cell.
What is the difference between hypotonic, isotonic and hypertonic?
They compare the solution outside the cell with the inside. Hypotonic: the outside has fewer solutes, so water moves in. Isotonic: equal concentration, no net movement. Hypertonic: the outside has more solutes, so water moves out.
What happens to a red blood cell in pure water?
Pure water is hypotonic, so water rushes in by osmosis. Animal cells have no cell wall, so the red blood cell swells and bursts (haemolysis).
Why don’t plant cells burst in water?
The cell wall pushes back as the vacuole fills. The cell becomes turgid (firm) instead of bursting. In a hypertonic solution the membrane pulls away from the wall instead: the cell is plasmolysed.
