Determine the Water Potential of Potato tissue.

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Determine the Water Potential of Potato tissue

The aim of this experiment is to determine the water potential of potato tissue, using different concentrations of sucrose solution.

If plant cells are placed in pure water, water will initially move into the cells. After a period of time the cells will become turgid. Turgor pressure is the pressure exerted against the cell wall by contents of the cell. At first most water movement is into the cell. As the turgor pressure increases water will begin to diffuse out of the cell at a greater rate, eventually equilibrium will be reached and water will enter and leave the cell at the same rate. This stage is used to find the water potential of a particular cell.

Potato cells contain polysaccharides starch and glycogen they are good for storage. The potato cell is surrounded by plasma membrane it is a fluid mosaic model, which is a mosaic of phospholipids and proteins moving around they are not solid. This is why plant cell can become turgid and flaccid because their walls (plasma membrane) can stretch.  

The plasma membrane is a selectively permeable barrier between the cell and the extra cellular environment. Water enters in the cell through phospholipids.

Water moves into and out of the cell by osmosis. This is a passive process which does not require any energy.

  • Water always moves from high water potential to low water potential.
  • Water potential is a measure of the tendency of water to move from high free energy to lower free energy.
  • Distilled water in an open beaker has a water potential of 0(zero).
  • The addition of solute decreases water potential.
  • The addition of pressure increases water potential.

Diagram showing direction of water moving at different stages

More water

More water molecules                                             Less water molecules

Less water molecules                                              More water molecules

            Equal                                                                      Equal

Solvent molecules don’t move in and out of the cell because their particles are too big, only water molecules can diffuse in and out of the cell.

Water potential is a measure of the ability of water molecules to move from one region to another. The more water molecules there are per volume of the cell the more likely that by random movement they will collide with the cell's plasma membrane, and travel out of it.

Pure water has a water potential of 0. As all solutions have less water molecules per volume than pure water they have a lower water potential therefore all solutions have negative water potentials. The movement of water molecules is not totally random it is fixed.

Pressure potential helps plant cells from exploding when it is turgid.

Water potential, solute potential and pressure potential are related and linked by this formula:

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Water potential (W) = Solute potential ( S) + Pressure potential ( P)

In this experiment we have to determine the water potential of potato tissue. Water potential equals to solute potential because pressure potential is zero. This is because there is no pressure added or taken away in this experiment, pressure does not vary in this experiment we are leaving it natural. Therefore we now don’t need to use the formula above for this reaction we will only need to find solute potential which will equal to water potential.

Water potential (W) = Solute potential ( S)

In a free standing solution ...

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