A student cut a piece of potato into 10 discs, each 1mm thick and with a 10mm diameter. He then placed them in a beaker of deionised water. After a couple of days, they had increased in size- both thickness and diameter. Explain why this is the case.

The cells composing the potato tissue contain fluid, or cytoplasm. This contains dissolved salts and these ions attract water molecules; cations, or positive ions, attract the partially negatively charged oxygen atom in the water molecule, while negative ions attract the positively charged hydrogen atoms. This means that the water molecules are not free within the solution but are weakly bound to the salt ions. This decrease in the number of free water molecules is measurable, and can be described as a decrease in the water potential of the solution. If a situation arises where two solutions are separated only by a thin membrane, water molecules will try to distribute themselves between the two solutions so they are balanced, moving from a solution of lower concentration - and a higher water potential - to a solution of higher concentration - lower water potential. In this case, water molecules will move from the deionised store in the beaker into the potato cells, causing them to increase in volume and the potato discs to swell.