Osmosis
Definition: movement of water from a weak solution to a more concentrated one through a semi-permeable until equilibrium is reached.
What does this mean?
A weak solution is one which has few substances dissolved in it. Pure water is obviously as weak as a solution gets. Water moves from a weak solution to more concentrated one. A semi-permeable membrane separates the two solutions. This prevents the dissolved solutions from mixing, so only the water can move. This will continue until the solutions on either side of the membrane are the same concentration.
E.g. imagine a plant cell in water. The cytoplasm is a concentrated solution and so water will enter the cell by osmosis. This will make the cell swell up as its volume increases. This continues until the cell wall prevents the cell from expanding more. This state is called tugor and the cell is said to be turgid. This gives a great deal of support to the plant.
If red blood cells were placed in water they would take in water by osmosis, swelling up until they burst.
The reverse will happen if the cells were placed in a solution that was more concentrated than the cytoplasm. Water would leave the cell by osmosis and enter the surrounding more concentrated solution. This means that the cell contents would become smaller as it loses liquid.
When this happens to a plant cell the cell membrane pulls away from the cell wall. This causes the plant cell to lose its support. This is called plasmolysis. If too many cells in a plant are plasmolysed the plant will wilt.
