Investigation to find the water potential of a plant cell.

Introduction: -

Theory:

Plant cells are made up of a phospholipid bilayer (fig. 1). There are hydrophilic heads, which face the liquid medium in and around the cells. A hydrophobic tail attached to the head, which face inwards. These tails are non-polar. Non-polar molecules like oxygen can diffuse through the membrane easily. Water, despite being a polar molecule, can also diffuse rapidly across the membrane because it is so small. Big polar molecules and ions can not pass straight through the membrane, e.g. Na+, Cl- and glucose. These molecules can only pass the membrane through protein channels, which only allow specific molecules in.

(fig. 1) Diagram of cell membane.

7nm phospholipid

width bi-layer.

Water molecules posses kinetic energy in that, when together in liquid or gas form, they move about very rapidly in random directions. When water is surrounded by a membrane the water molecules will tend to hit it as they move randomly, and in doing so, they will generate pressure (fig.2). This pressure is known as water potential. The more times the water molecules hit the membrane in a unit of time then the higher the pressure, i.e. the higher the water potential.

(fig. 2)

Diagram showing

Water potential.

One definition for water potential could be ‘the tendency for water molecules to move from one place to another- with or without a net movement’. Water always moves from an area of high water potential to an area of low water potential, meaning it always moves down its water potential gradient. There will be no net movement of water molecules when equilibrium is reached, meaning that the water potential in one area is the same as in the other area. The symbol for water potential is ψ, the Greek letter psi, and this is measured in units of pressure, kilopascals (Kpa).

The highest water potential is distilled (pure) water, measured at 0. If any solute e.g. sucrose, is placed in the distilled water, it will lower the ‘purity’ of the water and, therefore, lower the water potential. This means that the water potential will be less than zero, thus a solution with any solutes in it, will have a negative water potential. Solute potential, ψs is the amount that a solute lowers the water potential, meaning that solute potential is always negative. In (fig. 3a), it can be seen that two solutions are separated by a partially permeable membrane. In solution A, there are lots of water molecules but not so many solute molecules. In solution B, there are not so many water molecules but there are lots of solute molecules. Because the solute molecules are too big to pass the partially permeable membrane, the water molecules move via osmosis. Because solution B has a higher solute to water ratio than solution A, solution B has a lower water potential. And because water moves down its water potential gradient, from a region of high water potential to a region of lower water potential, the net movement of the water molecules will be from A to B. In (fig.3b), the solutions have reached an equilibrium where there will be no net movement across the partially permeable membrane

(fig. 3a) Showing the movement of water molecules in a

solution across a partially permeable membrane.

Patially permeable membrane

Water molecule

Solute molecule

(fig. 3b) The equilibrium of the two solutions.

Pressure potential (ψp) is another factor that affects water potential. Plant cells have a very strong cell wall made up of cellulose. This means that if ...