Determining the Water Potential of Potato Tuber Cells

Determining the Water Potential of Potato Tuber Cells

I will carry out an investigation that will enable me to determine the

water potential of the tested potato tuber cells.

Water Potential is the measurement of the tendency of water molecules

to move from one place to another. (Ridge 1991) Water always moves

down the water potential gradient, therefore moving from an area of

higher water potential to an area of lower water potential.

Equilibrium is reached when the water potential in one region is equal

to the water potential in another region. For example, if a plant cell

(like the potato tuber cells) is in equilibrium with an external

solution of such a concentration that there is no net gain or loss of

water then the water potential of the external solution will be equal

to the water potential of the cell. (Roberts 1991)

By convention, the water potential of pure water is set at zero.

Knowing that solutes make the water potential of solutions lower,

solutes make solutions negative. Solute potential is the amount that

the solutes lower the water potential of a solution.

Pressure potential is especially important in plant cells. If a plant,

for example the potato tuber cells, is placed in pure water (or a

dilute solution), the water (or solution) has a higher water potential

than the plant cell. This causes the movement of water to the cell due

to the higher water potential in the cell. Water enters a cell through

the partially (semi) permeable membrane by osmosis.

Osmosis is the movement of water molecules from a region of higher

water potential to a region of lower water potential through a

partially permeable membrane.

The diagram below shows the water potential changes in a plant cell in

a solution of different water potentials (Cambridge 2000)

The different changes shown in the diagrams are caused because all

plant cells, unlike animal cells, have a cell wall.

A plant cell wall is extremely inelastic. This property allows very

little water to enter a plant cell - preventing the cell from