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Home > Biology > Investigating cells > Cells and diffusion


Cells and diffusion


Osmosis is the special case of diffusion involving water molecules. Water molecules move from areas of high water concentration to areas of low water concentration through a selectively permeable membrane.

A selectively permeable membrane allows small, soluble molecules to pass through it, but prevents large insoluble molecules from passing through.

Take a look at the following slideshow:

Greater movement of water from pure water to the dilute solution

Beaker containing pure water on one side and dilute solution on other, separated by selectively permeable memebrane. Greater movement of water from pure water to dilute solution

If a selectively permeable membrane separates the two solutions, water moves through it in both directions at the same time. However, more water leaves a dilute solution (high water concentration) and passes into a more concentrated solution (low water concentration) than enters it. Although the water appears to move across the membrane in one direction, it is in fact moving in both directions but more one way than the other. When the concentration of water is the same on both sides of the membrane, the movement of water will be the same in both directions.

Pure water has the highest water concentration possible. As more salt or sugar is dissolved the water concentration decreases. A concentration gradient [concentration gradient: A difference in concentration between two areas next to each other. Particles will move down the concentration gradient from an area of high concentration to an area of low concentration. is where there is a high water concentration in one area and a lower water concentration in another.

Using these words and information we can now summarise osmosis with its definition.

Osmosis is the movement of water molecules through a selectively permeable membrane from higher water concentration to a lower water concentration - or - down a concentration gradient.

Animal cells are surrounded only by the membrane and may swell up and burst if too much water enters by osmosis. Plant cells have a strong cell wall outside the membrane and this wall prevents them from swelling up too much. They become stiff and hard like a well inflated football. The cell is turgid. Animal cells just shrivel up when they lose water by osmosis. Plant cells shrink a little, but the tough cell wall keeps its shape when the membrane inside shrinks away from it so the cell becomes limp and floppy like a football with no air in it. The cell is plasmolysed.


To gain a better understanding of osmosis, here’s a new way of looking at it, with the help of scientist and rapper Jon Chase, some volunteers, and a large net …

Video: Osmosis

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To find out more about both diffusion and osmosis, listen to the Naked Scientists explain how things get in and out of cells.

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Magnified cells

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Video clip exploring plant and animal cells


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