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 partially permeable membrane.
A dilute solution contains a high concentration of water molecules and is said to have a high water potential, while a concentrated solution contains a low concentration of water molecules and so has a low water potential.
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.
The cell membrane is partially permeable.
Water will move into and out of cells by osmosis.
Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist, root hair cells, will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.
If plant cells are placed in solutions of increasing solute concentration:
Fully turgid cells support the stems of non-woody plants.
Highly concentrated solution
In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.
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.
Aquatic, freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also 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: