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What happens to a red blood cell in salt water depends on how salty the water is. Put a red blood cell in salt water that matches blood plasma (about 0.9% salt) and nothing much changes. Make the water saltier and the cell loses water and shrivels. Use pure water and it swells until it bursts. All three come down to osmosis, and once you see how it works you'll notice it everywhere, from hospital drips to limp lettuce.

Osmosis in one sentence

Osmosis is the net movement of water across a partially permeable membrane, from a more dilute solution to a more concentrated one.

The membrane around a red blood cell lets water through easily but blocks dissolved salt. Water molecules cross in both directions all the time. When one side is more dilute, it has more "free" water molecules, so more of them cross from that side than come back. The result is a steady flow of water one way until the two sides balance, or until something gives.

It helps to keep this separate from diffusion, which is any kind of particle spreading out from where it's crowded to where it isn't. Osmosis is the special case where water moves through a membrane that won't let the dissolved substance follow.

Hypotonic, hypertonic and isotonic solutions

These three words describe the solution outside the cell, compared with the inside:

Outside solution	Compared with the cell	Water moves	Red blood cell
Hypotonic (e.g. pure water)	more dilute	into the cell	swells, may burst
Isotonic (0.9% salt)	the same	in and out equally	stays normal
Hypertonic (e.g. 3% salt)	more concentrated	out of the cell	shrivels

An easy way to remember them: hypo means "under", so there's less dissolved stuff outside and water piles in. Hyper means "over", so there's more dissolved stuff outside and water gets pulled out. Iso means "equal".

What happens to a red blood cell in pure water?

Pure water is as hypotonic as it gets. Water floods into the cell, and a red blood cell has nothing to hold it back. It's wrapped in nothing but a thin, flexible membrane. The cell puffs up from its usual dimpled disc into a sphere, and if the water keeps coming, the membrane tears. Biologists call this haemolysis, or simply lysis.

And in very salty water?

Now flip it around. In a strong salt solution the outside is hypertonic, so water leaves the cell faster than it comes in. The cell shrinks and its surface crinkles into a spiky shape. This is called crenation, and you can see it under a light microscope when a drop of strong salt solution is added to blood.

Why do hospitals use saline instead of water?

When a patient needs fluid through a drip, putting pure water straight into a vein would be dangerous. Their red blood cells would take in water and could burst. So hospitals use normal saline: 0.9% sodium chloride, which is isotonic with blood plasma. Water moves in and out of the blood cells equally, and they keep their shape.

The same idea explains why drinking seawater makes thirst worse. Seawater is roughly 3.5% salt, about four times saltier than your blood. Your kidneys can't make urine that salty, so getting rid of the salt costs you more water than you drank.

Plant cells: why they don't burst

Plant cells go through the same osmosis, but they have one big advantage: a tough cell wall outside the membrane. (If you're not sure what else separates them, here's how animal and plant cells compare.)

In pure water, water moves into the plant cell's large central vacuole, and the cell swells until the membrane presses hard against the wall. The wall pushes back, so the cell can't burst. A cell in this firm, pumped-up state is called turgid, and turgid cells are what hold up soft stems and leaves. Leave tired lettuce in a bowl of cold water for half an hour and it crisps up again for exactly this reason.

Plasmolysis in onion cells

Put a plant cell in strong salt water and the opposite happens. Water leaves the vacuole, the contents shrink, and the membrane peels away from the wall, leaving gaps in the corners. That's plasmolysis. A wilting plant is on its way there: its cells have lost enough water that they no longer press firmly against their walls.

Plasmolysis in onion cells is a classic school practical, because the skin inside an onion layer is a single sheet of cells, thin enough to see straight through. Red onion works best, since its coloured vacuoles make the shrinking easy to watch when you add salt solution. Swap the salt solution for pure water and the cells fill out again.

Frequently asked questions

What is the difference between osmosis and diffusion?

Diffusion is the spreading of any particles from where they are more concentrated to where they are less concentrated, such as perfume spreading through a room. Osmosis is a special case of diffusion: the movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Neither needs energy from the cell.

Why does a red blood cell burst in water but a plant cell doesn't?

A red blood cell has only a thin, flexible membrane, so when water floods in nothing stops it swelling until it tears. A plant cell has a strong cellulose cell wall outside its membrane. As the cell fills, the wall pushes back with equal pressure, so the cell becomes firm (turgid) instead of bursting.

What does isotonic mean?

An isotonic solution has the same concentration of dissolved particles as the inside of the cell. Water moves in and out at the same rate, so the cell neither swells nor shrinks. For human red blood cells that is about 0.9% sodium chloride, the strength of the saline used in hospital drips.

Try it yourself

In the Diffusion and Osmosis lab, switch to the cells view and try this:

• Start with the red blood cell and slide the outside down to 0% salt (pure water). Watch it swell, then burst.
• Start again and push the salt above 0.9%. The cell shrivels, and the panel tells you it's crenated.
• Find the setting where water moves in and out equally. That's isotonic.
• Switch to the onion cell and repeat. In pure water it turns turgid but never bursts. In salty water it plasmolyses.

To see where the vacuole and cell wall actually sit, open the 3D Cell Explorer and compare the animal and plant cells side by side.