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:
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.
Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar.
Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist then 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.
Under normal circumstances, the cells in all organisms live in a safe equilibrium. They have enough water to live but not too much. They also have the correct amount of salts and sugars. If individual cells are placed in different solutions, we can see how important it is that equilibrium is maintained in living organisms.
In a more concentrated solution, the cell contents lose water by osmosis and the cells shrink. The cell membrance starts to pull away from the cell wall. The cell becomes flaccid.
Highly concentrated solution
In a very concentrated solution, the cell membrane pulls away from the cell wall completely; this is plasmolysis.
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: