The endeavour of this investigation is to ascertain if there is any effect of water potential changes on potato tissue.

Khaled Hamid Page

Aim:

Background knowledge:

There are four key aspects of natural science which are pertinent to this investigation and must be discussed prior to conducting the experimentation. These are:

Osmosis
Water potential
Solute potential
Pressure potential

I will also discuss how the different potentials affect cells in general

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. It is regarded as a special kind of dissemination in which water molecules are the only molecules diffusing owing to the presence of a partially permeable membrane which does not allow the passage of solute particles. Water molecules move from a region of their high water potential (a dilute solution) to a region of their low water potential (a more concentrated solution), through a partially permeable membrane. It is thought that this process may occur more rapidly than can be accounted for by simple diffusion, and that mass flow is involved because the membrane is also partially permeable to some solute molecules.

Water potential, w, '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 w of 0. As all solutions have less water molecules per volume than pure water they have a lower w; therefore all solutions have negative water potentials. The movement of water molecules is not totally random. The net movement of water molecules is always from a region of high water potential to one of lower water potential. They move down a water potential gradient until equilibrium is reached. Equilibrium is reached when the water potentials on both sides of the plasma membrane are the same. Water potential, w, is equal to the solute potential, s, plus the pressure potential, p. Therefore: w = s + p

Solute potential is the amount the solute molecules reduce the w, by reducing the number of water molecules per volume. As an increase in solute always reduces the w, the value for s is always negative.

Pressure potential is the amount added to water potential by pressure. By increasing the pressure in a cell its water potential is increased as the force of the water molecules on the plasma membrane is increased, making it more likely for them to travel out of the cell. The pressure potential increases the Yw of a cell, making it closer to zero, therefore less negative and giving p a positive value.

The plasma membrane that bounds a cell is partially permeable; it allows only small molecules, such as water, to pass through. Celeriac as a plant tissue also has a cell wall. This has a rigid lattice structure but is freely permeable.

Process of Plasmolysis and turgidity

Cells can be found in three broad categories of saturation; fully plasmolised, flaccid or turgid. Fully plasmolised is when the level of water in the cell is so low that the cell can not recover. This state is not seen is nature as the plant will normally die before this state is reached. Flaccid cells are when the cell has lost so much water that the cell is beginning to shrink (this is not possible in plants as the cell wall is too strong, but the inner membrane will begin to pull away from the cell wall and the plant will wilt).

Turgid is when the cell has plenty of water. (see fig below)

Summary

Water can move between cells (unligninified) freely as the cell walls are permeable. However its movement is controlled by a number of factors which are given names; solute potential (), Pressure potential (+ Matrix potential =0 normally) () and water potential ().

Solute potential (): this refers to the amount of substance dissolved in the cytoplasm. This effects water movement because water always moves from a less concentrated medium to a more concentrated medium.

Pressure potential (): this refers to the pressure exerted by the cell walls. In a plant, the cell will fill with water until the inner membrane is pushing on the outer membrane to the same force as the wall is pushing back; they then are equal forces and cancel out. Therefore there is no more movement of water.

Water potential (): this is a figure worked out from the equation; water potential = solute potential + pressure potential (). The figure represents the tendency of a cell to give out water.

Preliminary experiment

I conducted a preliminary experiment on how water potential affected onion cells. The results were not very reliable but it gave the basic idea that as the morality of the solute increases the number of incipient plasmolysed cells decreases because the onion cell has a high water potential than the solute solution which increases in sucrose morality and therefore the water molecules are reducing in the external solution. The experiment involved checking the onion cell under a microscope to find the number of incipient plasmolysed cells in varying sucrose concentration. This obviously meant the experiment was relying on human capabilities to ensure accuracy.

The number of onion cells used was 100, therefore if 50% of the cells were showing incipient plasmolysed cells, then this meant the water potential in the onion cell was also equal to the water potential of the solution. The results showed that equilibrium was between 0.25

Results from preliminary experiment

Molarity: No. of cells showing incipient plasmolysis: Result:

0.00M 0 turgidity

0.25M 4 turgidity

0.40M 60 near equilibrium

0.50M 20 plasmolysis

0.80M 5 plasmolysis

Evaluation:

This investigation was not totally reliable because it was prone to inaccuracies due to human in capabilities. The trends were as expected from the background knowledge and were similar to the secondary source tables results in the Appendix

Advantages:

The main advantage was that the experiment showed a very brief result of what is expected when varying the sucrose concentration
The experiment proved to a degree that my investigation should work accordingly with a plausible method
The experiment showed magnification of the expected activity in the cells and evidence of osmosis itself

Disadvantages:

difficulty preparing the slides
human eye was not a reliable source for counting the cells as the microscope was not clear due to poor resolution of the light microscope
there was a tendency to estimate the number of cells so therefore the figures were not totally precise or accurate
the experiment was only conducted once. Repeats would have ascertained any trends or findings

The preliminary experiment may not be of much use to my investigation due to the fact the cells/tissues being used were that of an onion cell which is totally different to that of a potato cell/tissue. Therefore there could be major differences, i.e. onion cells may be able to allow more water molecules to enter the cell due to different size/structure membrane, therefore the water potential maybe different between the two.

Secondary preliminary experiment to determine shape of potato and the molarities required for my investigation:

Pilot study

A pilot experiment was conducted in which three different shaped potato tissue pieces were placed into different sucrose solutions for 20 minutes. The sucrose solutions used were 0.0, 0.25, 0.5, 0.75 and 1.0 mol dm-3, the potato pieces which had been cut into 1.5cm cubes; 4cm lengths of cylinders made with a cork borer and cuboids of dimensions 1.5 by 1 by 0.3cm, these three different shape chips had different masses to begin with: 3.6, 1.25 and 1.07g respectively.

20ml ...