The second graph plotted with the use of a table showing solute potentials of given sucrose solutions at 20 ºC.
Interpretation of Results:
Graph 1 shows the percentage gain of mass of the potato tissue when five potato cylinders were placed in five different concentrations of sucrose. The graphline is a steep straight line sloping downwards with a negative gradient. There is a linear relationship between the percentage mass gain and the sucrose concentration. When the molarity of the sucrose solution increases the percentage mass gained by the potato cylinder decreases. And when the concentration of the sucrose decreased the percentage mass gained increased. The reason behind this is all to do with water potential.
Water Potential, Ψ, is the potential that a solution has of absorbing water molecules by osmosis. The SI unit used is the same as, pressure (kPa). Pure water has a Ψ of 0. Testube 1- 0M of sucrose, had water potential of 0. The solution could not absorb any more water by osmosis. The cell in the testube was placed in a region with less negative water potential. Therefore, by osmosis the water molecules from the pure water entered the potato cells, as it was a region of higher water potential. Thus the potato tissue in 0M sucrose gained mass. My graph proves this because when the sucrose concentration (and water potential) was 0 the potato tissue gained the most mass. This movement of water from the pure water to the potato tissue occurred because osmosis continues until it has reached an equilibrium point (when water potential on both sides of the plasmamembrane is equal).
In a solution the more water molecules, there are per volume of the cell, the more likely that by osmotic movement (net movement of water molecules from an area of high water potential to an area of lower water potential) they will collide with the plasmamembrane of the cell and travel out of it. The solutions in testubes 2-5: 0.25-1M sucrose had less water molecules per volume than pure water; they had lower water potentials than pure water; and had negative water potentials. Referring to graph 1, I found this out as the potato tissue in 1M sucrose had lost the most mass. The 1M sucrose concentration had the most negative water potential than the rest of the solutions in the testubes. It had the least molecules of water per volume. To reach its equilibrium, osmotic movement occurred. The solution was of more negative water potential than in the potato cells and furthermore there was movement of water molecules from inside the potato tissue into the solution. Therefore, the potato tissue in 1M sucrose lost mass.
Water potential is represented by the equation:
Ψ = ΨS + ΨP
Water potential = Solute potential + Pressure potential
“The water potentials of the cytoplasm inside a cell and the solution bathing the cell will determine whether water diffuses into or out of the cell” Advanced Biology. So, depending on what the concentration of sucrose the potato cells are placed in, will decide how much osmotic uptake or loss of water molecules. This you will notice when referring to my graph. In other words the solute potential (sucrose concentration) and the pressure potential (the inflow of water opposed by the plasmamembrane), added together will contribute to whatever the water potential is.
As the sucrose concentration increased, the solutions were of more negative water potential and had a greater ability to absorb water molecules from inside of the potato tissue. What’s more, the potato tissue lost more mass as the molecules diffused out of the cell and into the solution. This you can see from the negative slope of the graph.
When the sucrose concentration in the boiling tubes was lower, the solutions were of less negative water potential. They therefore had the least ability of absorbing water molecules by osmosis. The potato cells in high concentrations of sucrose gained mass because the cells had a higher water potential and greater capacity to absorb water molecules from the solutions. This is noticeable on my graph because when the concentrations of sucrose were low the percentage mass gain was high.
If a cell is placed in a solution, which has a lower pressure potential, there will be a net movement of water molecules out of the cell. The plasmamembrane and cytoplasm will start shrinking away from the cell wall. This is referred to as plasmolysis. When half of the cells have been plasmolysed that is known as incipient plasmolysis. Then the pressure potential will be 0 and Ψ = ΨS. When incipient plasmolysis occurs both sides of the plasmamembrane have equal water potential then there will be no mass change.
You can find out the point of incipient plasmolysis by referring to the graph. The point when the graphline intercepts the x-axis is the point of no gain/loss of potato tissue mass. If you refer to graph 1 this occurs when the sucrose concentration is 0.45. This is the point of incipient plasmolysis of the potato cells. Referring to Graph 2, it shows at what sucrose concentrations the solute potential is equal to the water potential. If you look down the graph at sucrose concentration 0.45, the solute potential is –1280 therefore the water potential is –1280 of the potato cells.
Evaluation:
I followed my method, which was fairly simple. The results I gathered followed a sharp straight line. All the points were on or close to the graphline drawn. However there was one anomalous result that I obtained and that was at 0.5M sucrose concentration. This may be due to many reasons.
In my experiment the sucrose solutions were just poured into the boiling tubes according to how much was required. This meant that the distribution of the sucrose molecules may have been uneven particularly in that boiling tube containing the sucrose solution with molarity of 0.5M. Uneven distribution may have caused some degree of inaccuracy. In future methods to be used perhaps, the solutions could be stirred to even out the dispersion of the sucrose molecules.
In the method, that we used the temperature was an uncontrolled variable. The experiment was carried out at room temperature, which does vary. The effect that temperature has on water potential is that it increases the free energy of the molecules in the solution and furthermore increasing the movement of molecules from one area to another. In my experiment, an increase in temperature may have occurred which may have caused more movement of molecules from one region to the other. However, an increase in temperature would have affected all of the samples and not just the sample containing a potato cylinder at concentration of sucrose at 0.5M. But in future for maximum accuracy, the experiment could be carried out at a constant temperature of 20 ºC rather than at room temperature.
To improve the investigation followed in future more replicates of the experiment could be carried out. In that way, any anomalous result would not affect the final results as much and would not cause as much inaccuracy.