# To determine the water potential of a potato tuber cell using varying salt solution.

Sneha Patel        Page         5/6/2007

## Aim

To determine the water potential of a potato tuber cell using varying salt solution.

## Introduction

Osmosis is the diffusion of water molecules from a region where it has higher water potential to a region where it has lower water potential through a partially permeable membrane. As osmosis is a type of diffusion the same things that affect diffusion have an effect on osmosis some of theses things are:

• The concentration gradient – the more the difference in molecules on one side of the membrane compared to the other, the greater the number of molecules passing through the membrane and therefore the faster the rate of diffusion.
• The surface area – the larger the area the quicker the rate of diffusion
• The size of the diffusing particles – the smaller the particle the quicker the rate and polar molecules diffuse faster than non-polar ones.
• The temperature – the higher the temperature the more kinetic energy the particles have and so the faster they move.

From the diagram we can see the process of osmosis in a simple expression. On the right side there is pure water, which has the maximum water potential of 0. Water potential is the pressure created by water. As you can see from the diagram the pure water is pushing its way through the semi permeable membrane at a high pressure. This is its water potential. Water potential is measured in kilopascals (kPa) and is represented by the letter Ψ.

## Prediction

I predict that the lower the concentration of salt solution the higher the water potential will be and therefore the higher the % change in mass will be, I predict this because osmosis is the diffusion of water molecules from a region where it has higher water potential to a region where it has lower water potential through a partially permeable membrane. Therefore the distilled water will have the highest water potential and the 1.0 mole of salt solution will have the lowest water potential. This means that the potato cell that is left in the distilled water should gain the most weight according to the definition of osmosis.

There are some things that need to be considered, water potential is affected by solute potential and pressure potential.

When solute molecules are dissolved into water, the concentration of water molecules is reduced, and hence so is the water potential. The solute potential is devised by the amount of solute in the experiment in this case it would be the salt. Solute potential is always in negative numbers as the only positive solute potential is that of pure water and that does not include any solutes. Its symbol is Ψs.

Pressure potential has the symbol Ψp. When water enters plant cells by osmosis, pressure can build up inside the cell, this increases the pressure potential. Pressure potentials are caused by resistance of the tissues to water flow.

To calculate water potential this formula is used:

Water potential Ψ = solute potential Ψs + pressure potential Ψp

The differences in pressure potential of potato cells placed in different concentrations of salt solution, will cause the cell to react in different ways. The diagram below depicts what happens to the cell:

Figure 2 Plasmolysis and water potential in a plant cell

A As you can see from the first picture, the plant cell has been placed in a high concentration of water for example pure water. The water is entering the cell causing a huge amount of pressure thus causing the cell to swell and become turgid. When the cell becomes turgid the protoplast (the outer cell surface membrane, cytoplasm and the tonoplast) starts to swell and is pushed against the cellulose cell wall because of the pressure of water entering the cell. Eventually the pressure increases to a point where the cell resists any more water entering the cell. The protoplast is staying pressed against the cell wall and this point the cell has become turgid. Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis.

B The middle picture depicts what should occur when the water potential inside and outside of the cell is at equilibrium. At this point the protoplast is no longer being pressed against the cell wall and is beginning to pull away. At this stage the pressure potential equals 0. The plant cells are placed in a solution, which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity, this stage is known as incipient plasmolysis.

C Finally the final picture illustrates what happens to the cell when the water potential outside the cell is lower then inside the cell. Water leaves the cell and as it leaves the protoplast completely pulls away from the cell wall. This causes the cell to shrink and we say it has become flaccid or plasmolysed.

I predict from my preliminary results that my graph showing the relationship between concentration of salt solution and % change in mass will look something like this:

There are 3 basic stages to this graph and these are co ordinate to the stages shown in the Figure 2.

A (between 0.00 and 0.30 mole) = where the cell has become turgid because it has been placed in a high water potential solution. Water is entering the cell  because the water potential outside the cell is higher then inside the cell, and as we know water moves from a region of high potential to a region of low water potential. Here the cell will gain mass according to the definition of osmosis. I predict that for the potato cells placed in between the concentrations of 0.00 and 0.40, the cells will gain mass and have a high water potential.

B (between 0.30 and 0.50 mole) = the amount of water potential outside the cell is equal to that of inside the cell therefore the cell will not lose or gain wait, this point is called incipient plasmolysis. When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. (the pressure potential is equal to zero so the Ψ = Ψs therefore there is no movement of water)

C (between 0.50 and 1.00 mole) = the cell has become flaccid or ...

#### Here's what a teacher thought of this essay

***** An excellent account with a very high level of detail and correct use of A level biological terminology throughout.