With this in mind I expect that as the molarity of the glucose solution increases the potato mass will increase by small increments until it reaches its isotonic point, then the mass will begin to decrease because the solution has a higher concentration of glucose solutes than inside the cell.
I also think that because research shows that new potatoes contain more glucose there will be a greater increase in mass than the old potatoes until it reaches the isotonic point, I predict this because new potatoes will have a higher water potential so more water will move into it.
Apparatus
- 12 lids and specimen tubes
- Ceramic tile
- Cork Borer
- Measuring Cylinder
- Ruler
- Scales
- Scalpel
Procedure
- The following molar quantities of glucose solution will be obtained, 0.2, 0.4, 0.7, 0.9, 1.0 and distilled water.
- Using a measuring cylinder 10ml of each molar solution will be measured twice and put in specimen tubes, one for the ‘new’ potato and one for the ‘old’ potato.
- Using a cork borer cut cylinders from a new potato and from an old potato.
- Cut these cylinders to 6 lengths of 10mm, then cut each length into 5 discs of approximately the same size.
- Individually weigh and record each batch of 5 discs then emerse them into the designated glucose solution.
- Once this duration is completed remove the potato from the solution
- Dry off the excess liquid from the discs using paper towel and reweigh recording the mass.
Variables
The independent variable is the factor which is manipulated, In this experiment it is the concentration of the glucose solution. I chose this for the independent variable because it will be relatively easy to measure different concentrations accurately and successfully.
The dependant variable is what is affected by the independent variable, it is also measurable. The mass of the potato will be the dependant variable because it is affected by the change in concentration (IV) it will be measured using a set of scales.
Fair Test
To ensure my experiment is fair, All variables excluding the independent variable must be kept constant to ensure that the results obtained are as accurate and reliable as possible.
These variables include:
-
Temperature - Altering temperature affects the kinetic energy of the molecules, thus affecting the diffusion rate, My experiment will be conducted at room temperature of approximately 22°C.
- The length and diameter of the potato, namely 10mm
- The volume of glucose solution will be kept consistent at 10ml.
- The same old and new potato will be used for samples
- All potatoes will be left in the solution for the duration of 48 hours, they must have the same exposure for osmosis to take place.
Risk Assessment
- Care must be taken when using the scalpel
- Care should be taken when using any chemical in the laboratory
- Care should be used with glassware
- Hands should be washed before and after lab-work
- Use the ceramic tile when cutting the potato.
Results
This table shows the results of the experiment involving the new potato.
This table shows the results of the experiment involving the old potato
Using the graphs plotted overleaf, the water potential of both the old and new potato can be calculated. By reading of the isotonic points of both varieties of potato, (this is when there is no change in mass) This concentration can be read off on the calibration graph to tell us the water potential.
The isotonic point of old potatoes is 0.95 (mol dm )
The isotonic point of new potatoes is 0.26 (mol dm )
By reading these figures off the calibration graph, the water potentials can be calculated:
The water potential (ψ) of old potatoes is 0.225 (mpa)
The water potential (ψ) of new potatoes is 0.7 (mpa)
Results Analysis
The graph shows that there is a similar pattern for the change in mass for both the old and new potatoes. This being that in distilled water the greatest increases in mass is observed. As the glucose solution becomes more concentrated the increase in original mass rises by smaller increments until no change in mass is observed, after this point the original mass decreases quite rapidly until at around 0.7mol dm the decrease in mass becomes less drastic.
The general trend is that the greatest increase in mass occurs with distilled water for both old and new potatoes. As the concentration increases, the original mass increases by smaller increments until its reaches its isotonic point. This is an equilibrium state where the water potential of the solution is equal to that of the cell and there is no movement of particles – therefore no change in mass. There is no movement of the water particles, because water moves to regions of higher negativity, if the water potential of the glucose solution is isotonic therefore equal to that of the potato no movement will occur.
As the concentration continues to increase past the isotonic point, the original mass then starts to decrease. This is because there are more solutes present in the glucose solution - The solution is hypertonic, it has more solutes dissolved than the potato and therefore has a more negative water potential. Thus the water particles move from within the potato cell into the solution, observed as a decrease in potato mass.
The greatest increase in mass occurs when the potato is put in the distilled water. The reason behind this is because pure water has a potential of zero because there are no solutes (dissolved molecules) present.
Because water moves into regions of higher negativity molecules will obviously move into the potato cell because water is at zero which is the maximum value so none can move into the water.
This is the general trend for both varieties of potatoes, However what are the individual differences between old and new potatoes?
In distilled water, it is the new potatoes that increase in mass more than the old. The new potatoes increase by 7% more than the old potato; Previous research showed that new potatoes are sweeter because they have glucose energy stores, whereas old potatoes have their energy trapped in storage polysaccharides such as starch. This is simply because it has finished growing and starch is a more efficient storage component because of its secondary structure and insolubility. So therefore because the new potato has a higher value of dissolved solutes it will have a highly negative water potential, therefore more water will move into it so a greater increase in potato mass will be recorded.
Using the calibration graph the water potential of both the old and new potato cells has been calculated to allow for comparison.
The ψ of old potatoes in 0.225 mpa
The ψ of new potatoes is 0.7 mpa
The above figures show that new potatoes have significantly higher water potential than old potatoes.
From the conclusions that I have drawn from my results I can conclude that my hypotheses can be accepted that new potatoes have a higher water potential than old because of their composition consisting of higher concentration of glucose energy stores rather than starch.
It also supports the claim that yes potatoes sold as ‘new’ potatoes are actually newly grown potatoes and not smaller sized previously grown potatoes.
Evaluation
Overall the experiment was carried out efficiently and successfully, no real problems occurred. The results obtained were as accurate as could be expected because there are some limitations of accuracy with this experiment. For example the experiment has limited accuracy due to human error and also due to the equipment, measuring the glucose using a measuring cylinder or cutting the potato to size using a ruler has a limited accuracy of ± 0.1mm.
Also it is impossible to tell how much each individual potato piece was dried with paper towel, if some weren’t dried as thoroughly as other this would increase their final weight and could lead to inaccurate misleading results.
However no obvious anomalous results were obtained that could not be fitted into the pattern in some way.
Improvements
- To make the experiment as accurate as possible it needs to be repeated more times, (3 for example) for each concentration. This makes any anomalous results obvious and also for an average to be calculated.
- A greater range of molarities could be investigated, ideally over smaller increments. This would allow for a more precise graph curve to be drawn,
- Ideally all samples should be taken from the same part of the potato to avoid any differences in cell texture and composition.
Bibliography
- Advanced Biology - Principles and Applications C J Clegg & D G Mackean
- Advanced Biology – Simpkins & Williams
Ruth Higgins
