# Water potential of potato tuber cells - the weighing method.

Ryan Magee 13Q        Biology Coursework        AS Level

Water potential of potato tuber cells – the weighing method

The Investigation of Osmosis

The investigation was to find the differences in water molecules moving from an area of high concentration to an area of low concentration using potato tubes and varying the concentration of the solutions. The way in which water molecules do this is called osmosis. Osmosis is the movement of water through a semi-permeable cellulose wall and cell membrane along a water potential gradient.

From previous research I predict that as the concentration of the solution outside the potato tubes is increased there will be a increase in the number of water molecules moving from the solution into the potato tubes, and also then as the concentration of the solution decreases there will be an increase in the number of water molecules moving from the potato tubes. Using this investigation we will also be able to work out the water potential of the potato tuber cells. Water potential is the potential that water molecules in a solution have of diffusing out of a cell as a result of their free energy. The symbol used to denote water potential is the Greek letter psi or ϕ. The SI unit that is used is the same as that used for pressure (kPa). Water always flows passively from an area with high water potential to an area of low water potential. It may move from a less negative (eg -650 kPa) to an even more negative (eg -950 kPa) water potential.

Distilled water has the greatest water potential of 0. As the concentration of a solution increases the more negative the water pressure becomes e.g. 0.5M sucrose solution will have a water potential of –1450kPa and 1M sucrose solution will have a water potential of –3500kPa. There are two other forces involved in osmosis these are the solute potential (ϕs) and pressure potential (ϕp). Solute potential is kinetic energy in a solution, which allows it to collide with the membrane and thus exert a pressure. The greater the concentration of solutes the more negative is the water. Pressure potential then is the pressure exerted on the cell wall by the cell membrane. In most plant cells the pressure potential is positive.

The relationship between water potential, solute potential and pressure potential is shown in the equation below:

ϕcell          =             ϕs             +              ϕp

Water                         Solute                           Pressure

Potential                      Potential                         Potential

In the case that the external solution has more negative water potential than the internal solution water is drawn out of the cell by osmosis and the protoplast shrinks. As it does so pressure potential decreases. A point is reached where the protoplast no longer presses on the cell wall, and hence the pressure potential falls to 0. This point is called Incipient Plasmolysis and the cell ...