Osmosis is the diffusion of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration through a selectively permeable membrane.
A dilute solution contains a high concentration of water molecules while a concentrated solution contains a low concentration of water molecules.
The slideshow shows an example of osmosis showing the direction of movement of water between two different concentrations of sugar solutions:
Water molecules move from solution one to solution two.
When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. There will be no net movement of water molecules. In theory, the level of solution two will rise, but this will be opposed by gravity and will be dependent on the width of the container.
Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar.
Plant cells have a strong cellulose cell wall outside the cell membrane. The cell wall is fully permeable to all molecules and supports the cell and stops it bursting when it gains water by osmosis.
If plant cells are placed in solutions of increasing solute concentration:
Plants would be exposed to higher concentrations of solutes if there was less water in the soil - for instance, if plants were not watered, or plants in drought conditions. Plant cells would then lose water by osmosis.
Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.
For example, red blood cells:
In animals, the concentration of body fluids - blood plasma and tissue fluid - must be kept within strict limits – if cells lose or gain too much water by osmosis, they do not function efficiently.