Define the term 'water potential' and describe the difference between isotonic, hypotonic and hypertonic solutions. Suggest the different effects on cells placed in the different solutions.

Water potential basically means how likely it is for water molecules to diffuse out of or into a solution.Pure water (i.e. water with no solutes) has a water potential value of 0.As you add solutes (i.e. sugar or salt) to water, the water potential value is lowered. This therefore means that the water potential value for any solution is always negative (as the value is lowered from the starting value of 0).So, the more solutes there are dissolved in water, the lower (or more negative) the water potential value is.HIGHER CONCENTRATION OF SOLUTES IN WATER = MORE NEGATIVE WATER POTENTIAL VALUE.The term 'isotonic' is where two solutions have the same water potential value. Placing cells in an isotonic solution will not have an effect on the cells - the cells will neither lose nor gain water as the cells have the same water potential value as the solution.The term 'hypertonic' is where a solution has a lower water potential value than the cells - it has a higher concentration of solutes. Water will therefore diffuse out of the cells via osmosis. This can cause cells to shrink. In plant cells, it causes something called plasmolysis to occur where the loss of water from the cell causes the cytoplasm to come away from the cell wall.The term 'hypotonic' is where a solution has a higher water potential value than the cells - it has a lower concentration of solutes. Water will therefore diffuse into cells via osmosis. This causes the cells to swell and if they swell too much they can burst - cytolysis. Plant cells have a strong cell wall, which prevents them from bursting so when they swell, the cells become rigid instead.