Osmosis and the cell membrane

Water goes through, sugar does not

The plasma membrane of a cell is selectively permeable. Small molecules, like water, pass through it. Larger ones, like sugar or salt ions, pass slowly or not at all.

Put such a membrane between two solutions: pure water on one side, water with sugar on the other. The sugar cannot pass, but the water can. What happens?

Osmosis

Osmosis is the movement of water through a selectively permeable membrane. The net movement is from the side with less dissolved substance, so more water, to the side with more dissolved substance, so less water. Osmosis is a special kind of diffusion: the diffusion of water.

Three kinds of solution

Compare the solution outside a cell with the liquid inside it.

Plant cells

A plant cell has a cell wall. In a hypotonic solution, water enters and the vacuole swells. The cell presses against its strong wall, so it does not burst; it becomes firm, or turgid. A plant stands up straight because its cells are turgid.

In a hypertonic solution, water leaves the cell. The membrane and cytoplasm shrink and pull away from the wall. This is plasmolysis, and a plant that loses water this way wilts. An animal cell has no wall, so in pure water it can swell and burst.

You should find that in pure water an animal cell swells until it bursts, while a plant cell only presses against its wall. In a strong solution both shrink, and the plant cell's membrane pulls away from its wall.

Everyday examples

• A raisin swells in water, because water enters by osmosis.
• A wilted lettuce becomes crisp in water.
• Salt on cut fruit draws water out of it.
• Too much fertiliser makes the soil water more concentrated than the root cells, so water leaves the roots and the plant wilts.

An experiment with potato

Cut a potato into cylinders of the same size and find the mass of each. Put each in a different salt solution for one hour, then dry it and find the mass again. A cylinder in pure water gets heavier; one in a strong solution gets lighter. Between the two is a solution where the mass does not change: it has the same concentration as the potato cells.

Try it in Python

This program finds the percent change in mass for each solution, then finds where the change is zero by drawing a straight line between the two nearest points.

Practise with new numbers

Common mistakes

• Getting the direction backwards. Water moves towards the side with more dissolved substance.
• Thinking the dissolved substance moves. In osmosis it is the water that moves.
• Saying a plant cell bursts in pure water. Its wall stops it; it becomes turgid.
• Forgetting the sign of the percent change. A loss is negative.

Remember these

• Osmosis is the net movement of water through a selectively permeable membrane, from less dissolved substance to more.
• In a hypotonic solution a cell swells; in an isotonic one it stays the same; in a hypertonic one it shrinks.
• A plant cell does not burst because of its cell wall. In a hypertonic solution it shows plasmolysis.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A cell is placed in a hypertonic solution. Does it gain water or lose water?

2. A potato cylinder has a mass of 8.0 g. After one hour in a solution its mass is 8.4 g. What is the percent change?

3. Why does a plant cell in pure water not burst?

4. What is plasmolysis?