An Investigation to Determine the Water Potential of a potato tuber

Background

This experiment involves the process of osmosis. Osmosis is the process by which water moves across a membrane is known as osmosis and is described as the diffusion of water through a selectively permeable membrane The direction and amount of movement of water is dependant on the water potential. The water potential is based on the number and concentration of water molecules a solution has. For example, distilled water has the highest water potential, as it is the purest state of water. A highly concentrated solute solution has a low water potential. The highest water potential value (which distilled water has) is 0. Any dissolved solute-solution has a negative water potential value. Water potential is measure in the units Pascal. Water moves from a region of high water potential to a region of low water potential. Therefore, it moves down the water potential gradient.

In this experiment we will be trying to determine the water potential of a potato tuber.

Osmosis in this respect works on a cellular level. The cells that we are looking at are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other. In areas not exposed to light, as in a potato, food storage in the form of starch grains is the main function (Where light is present, e.g. in a leaf, photosynthesis is the main function). The amount of water present in these cells results in them having certain water potentials.

If we place these tissues in different concentration sucrose solution we can see the if water has gone in or out of the cells. If the environment is hypotonic (having a lower concentration of solute than the cell) the net movement of water was into the cell, it would become turgid, the cytoplasm and vacuole pushed up against the cell wall, making the cell stiffer and wider and also result in the whole cell to increase in mass. Water moving out of the cell will have the opposite effect and so the cell will become flaccid. The solution surrounding the cell is known as hypertonic (having a higher concentration of solute than the cell). This explains why bacterial cells dehydrate and die in concentrated salt water and why foods can be stored in a salt solution without spoiling.

Water will continue to move across the membrane until equilibrium is reached i.e. the water potential is the same either side of the membrane. This is called the isotonic point (at which water neither leaves nor does it enter the cell). This however is not the case for plant cells. Although the plant cell will change in mass according to whether it gains water or loses water, its shape is kept the same. This is due to the cell wall in the plant cell resist the pressure from either side of the wall. This means in a hypotonic environment, water moves into the cell until pressure inside the cell rises and 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. In a hypertonic environment, the cell wall prevents the cell from collapsing completely by having strong collagen walls to keep the shape of the cell.

However, water does move across the cell wall, in which case we would see a change in mass in the potato. If they have increased in mass, it means that the outer solution is less concentrated than the cytoplasm in the cell of the potato and therefore water has moved into the cell from the outer solution. A decrease in mass means the opposite. The solution we will be using is sucrose solution. We will be preparing the solutions ourselves and we will be making the solutions from different amounts of 1 molar solution of sucrose (how this is done will be shown in the method).

Preliminary work

We have carried out a trial run of the experiment to get ideas of what kind of dependent variables we would be noting and the types of independent variables that would affect the experiment.

The independent variable we are changing to acquire our results is the changing of the surrounding sucrose solution’s concentration. The values chosen for this experiment will range from 0.0 molar to 1.0 molar solutions and will include 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 solutions. These values will allow me to get a range of results and also to allow ...