Water Potential Investigation

Water Potential Investigation

Aim: To determine the water potential of potato cells and sweet potato

cells during the process of osmosis. (Only water molecules can pass

through by the process of osmosis)

Osmosis is the movement of water particles from a low region of water

concentration to a high region of water concentration through a

partially permeable membrane. A partially permeable membrane is known

as the barrier which allows certain substances to move across, such as

water molecules which move through freely, but prevents the movement

of other substances, like sucrose.

[physioweb.med.uvm.edu/bodyfluids/osmosis.htm

& www.purchon.com/biology/osmosis.htm]

The cell wall of a plant cell is made up of insoluble cellulose fibres

which are embedded in a matrix of pectin and other materials. The

fibres are very strong and often arranged in layers in which they are

orientated in different directions. The composite structure provides

an extreme resistance from the extension or the compression of the

cell. For instance when too much water is taken up, the cell wall

prevents the cell from expanding too far and bursting by allowing the

cell to become turgid. However the cell wall is completely permeable

and thus it has no role in controlling what can enter or leave the

cell. [Mary Jones & Geoff Jones, Student Support Materials for

Edexcel, AS Biology Unit 1: Molecules and Cells, Collins, 2000]

The cell membrane is made up of two types of molecules known as lipids

and proteins which are set in a very specific arrangement of layers

known as a phospholipid bilayer. These bilayers are partially

permeable since only selected molecules can pass through. The

permeability of the membrane is maintained by phospholipids and

proteins. Organic molecules which are fat soluble can only pass

through the lipid bilayer alone. Consequently the cell membrane

prevents the passage of water soluble molecules like sucrose

Provided that the molecules are water soluble, the membrane proteins

can allow these molecules into and out of the cell by forming

permanent or temporary pores.