Modifications:
Due to the areas of weakness highlighted by the preliminary experiment I have decided to make a few modifications to make the real experiment more accurate therefore increasing the chances of a successful experiment.
Firstly I have decided that instead of measuring the change in length of the potato tuber cells, I have decided to measure the mass. This is because it allows for all increases in size of the cells and also the scales can measure to a much more accurate degree than a ruler.
Secondly I will attempt to take all cores from the same potato; this will require a much larger potato than the original ones. This would ensure that all the water potentials are the same. Another possible way of doing this however would be to soak all cores in distilled water for the same period to try to even out the water potentials.
Also the real experiment will use bungs, this is to stop any of the sucrose solution evaporating and therefore preventing the concentration being affected.
Finally I would increase the period of time the cores are immersed in the sucrose solution. This would allow more time for the effect to occur with more significant, noticeable results.
References:
Diagram showing net movement of water molecules :
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/Membranes/bauerp/mem3.gif
Real Experiment
Variables:
Tables Showing Variables within the Real Experiment
Fig. 3
There are two types of variable. Independent and Dependant. They are basically the variables which either must stay the same (dependant) or must vary (independent). In this case and Independent variable would be the concentration of the sugar solution. A Dependant variable would be the mass of the potato cylinders.
Apparatus:
Table Showing the Apparatus used in the Real Experiment
Fig. 4
Apparatus Diagram:
Fig.5
Risk Assessment:
- Take due care with equipment, when carrying glass test tubes, scalpels when chopping…etc
- Always wear safety goggles to protect eyes.
- Attend to any spillages quickly to avoid slipping.
Method:
- All equipment was set up so experiment could be conducted quickly efficiently and safely. (see fig.2)
- First of all the borer and large potato was used to attain the potato cylinders.
- 21 cylinders were made, three for each molarities of sucrose solution and three for distilled water.
- All 21 cylinders were then put into a large beaker and immersed in distilled water
- This is to try to equal the water potential of the potato cylinders
- All cylinders were left in distilled water for 24hours
- After 24hours all cylinders were removed from the distilled water
- The Scalpel was then used to cut all cylinders to approximately the same size.
- The potato cylinders were then weighed and the mass was recorded.
- An average size for each set of cylinders being immersed in each solution was recorded. (For example: the average size of the three potato cylinders being immersed in 0.1mol of sucrose solution was taken and the same for 0.2mol…etc)
- The solutions were then measured in the measuring cylinder.
- 20ml of each solution was put into separate boiling tubes.
- The boiling tubes were rested in the boiling tube rack so they did not move.
- Three potato cylinders were then added to each boiling tube.
- The potato cylinders were then left for 1hour 30minutes
- After 1hr 30mins the potato cylinders were removed from the solution
- The new masses were then measured using the weighing scales and the results recorded.
Results:
Results Table Showing Concentration, Mass at the start and end, Average masses at the start and end.
Fig. 6
Results:
To be able to plot these points accurately on a graph the results must be used to calculate the change in mass per gram.
Change in mass per gram = Change in mass/ Mass at Start
The average mass at the beginning and end of the experiment will be used to calculate the change in mass per gram. Also this will produce only one result allowing the graph to be produced using one point.
After calculating the average change in mass per gram the percentage change in mass per gram can be calculated.
Change in mass per gram X 100 = Percentage change in mass per gram
Table Showing Calculations and Change in mass per gram for each Sucrose concentration and the Percentage Change in mass per gram for each sucrose concentration
Fig. 7
The graph showing these results can now be plotted. Plotting Change in mass per gram and Percentage change in mass per gram of the potato tuber cells. Against the sucrose concentrations, in two different graphs.
Graphs:
After looking at the graphs it is possible to distinguish an approximate water potential of the potato tuber cells. The point at which the line crosses the x-axis shows the point at which there was no change in mass. By reading the figure and using this table:
Table Showing Molarities Against Water Potential
Fig .10
The water potential can be established reading the x value which represents the molarities and using this value to match up with the relevant water potential value.
After looking at the graphs and using the above technique the water potential of the potato tuber cells is approximately -860kPa.
Analysis
Conclusion:
After analysing the experiment I conclude that it was a success. The tables show no particularly anomalous results. Also the results followed the basic principles of osmosis. When the concentration of sucrose was low the results showed an increase in mass of the potato tuber cells. When at a medium concentration there was little or no change in mass of the potato tuber cells. Finally when the concentration of sucrose was high the results showed a decrease in mass of the potato tuber cells.
By using the mass at the end and the mean average (the sum of all required values divided by the number of values, this was the average taken throughout the experiment) mass at the end I can prove there were no particularly anomalous results by showing that all repeats were +/- 5% of the average.
Table Showing Mass and Average Mass of the Potato Tuber Cells at the End of the Experiment And +/- 5% Values for the Average
I added on 5%: Average Mass At End X 1.05
I took away 5%: Average Mass At End X 0.95
All results were within +/- 5% showing that there were no anomalous results.
Discussion:
The results for this experiment suggest that the water potential of potato tuber cells follow the principals of osmosis and are accurate to predictions. As the concentration was low the mass increased, it levelled off when the concentration was a medium value, when the concentration was high the mass decreased. This relates to my prediction precisely. My prediction states: Based on my background information I predict that in a low concentration of sucrose solution the potato tuber cells will increase in mass. In solutions with high concentrations of sucrose the potato tuber cells will decrease in mass. I believe this because in solutions of low sucrose concentration there is a high water potential. This means water will move from the solution to the potato tuber cells. In solutions of high sucrose concentration the water potential is low, therefore the water will move from the potato tuber cell to the sucrose solution. Meaning it will decrease in mass in solutions of high sucrose concentration.
This is also the case for each concentration. At 0.0mol (distilled water) the concentration is lowest. This means that it has the more water molecules in relation to the potato tuber cells, therefore the net movement of water molecules will be into the potato tuber cells making it increase in mass.
At 0.1mol the concentration is still very low, therefore the solution will have a higher water potential than the potato tuber cells. This is because the potato tuber cell has higher concentration of sucrose within the cells than is in the solution. This means the net movement of water molecules will still be into the potato tuber cells. However, 0.1mol has a lower water potential than 0.0mol. This means that although it will increase in mass it won’t be as much as in 0.0mol.
At 0.2mol the water potential is still higher due to the fewer number of sucrose molecules in comparison the amount in the potato tuber cells, so again net movement of water molecules is into the cell making it increase in mass. Again though 0.2mol has a lower water potential than 0.1mol and 0.0mol. This means that the change in mass will be less at this concentration compared to the 0.0mol and 0.1mol.
At 0.3mol the water potential of the solution and the potato tuber cells is the same. So the amount of sucrose molecules is the same for both the solution and the potato tuber cells. This means there is not net movement of water molecules meaning no change in mass. This point identifies the water potential of potato tuber cells.
At 0.4mol the water potential of the solution is less than that of the potato tuber cells. The sucrose concentration is higher in the solution than in the potato tuber cells. This therefore means a net movement of water molecules out of the potato tuber cells and into the solution. This will make the potato tuber cells decrease in mass.
At 0.5mol the water potential of the solution is lower once again. In this solution there are a higher number of sucrose molecules than in 0.4mol solution. This therefore means an increased net movement of water molecules out of the potato tuber cells into the solution. This means a greater decrease in mass than in the 0.4mol experiment.
Finally at 0.6mol the water potential of the solution is much lower than that of the potato tuber cells. It has more sucrose in the solution than any other concentrations of sucrose solution. This means there is the highest net movement of water molecules out of the potato tuber cells meaning the greatest decrease in mass.
These results follow the background information at the beginning of this investigation. The water molecules have diffused through a partially permeable membrane from a region of high water potential to a region of low concentration.
Evaluation
General Comment:
The results of this experiment show that it has been a success. There do not appear to be any anomalous results. Meaning repeats are relatively close together considering the level of measurement. Also the results follow the predicted trend suggesting that it has been a successful investigation.
Limitations:
Even though precautions have been taken there may have been some possible limitations within the experiment. This may have caused some inaccuracy within results.
Table Showing Limitations Within the Experiment
Main Source of Error:
The main source of error within this experiment is removing the excess water. I believe this because the weighing scales measure to such a degree than any change due to lack or excess of water would effect the results.
Improvements:
Table showing possible Improvements in the Experiment
Reliability of Evidence:
The reliability of the evidence is very good. The results show that the net movement of water molecules are from a region of high water potential to a region of low water potential. This is the same conclusions made by other published studies, for example the Biology 1 book. Published by the press syndicate of the university of Cambridge and endorsed by OCR.
Validity of Conclusions:
My conclusion states: After analysing the experiment I conclude that it was a success. The tables show no particularly anomalous results. Also the results followed the basic principles of osmosis. When the concentration of sucrose was low the results showed an increase in mass of the potato tuber cells, they then levelled off at a medium concentration. Finally when the concentration of sucrose was high the results showed a decrease in mass of the potato tuber cells.
Although the limitations may have affected results I believe this would have been very slight. Even the main source of error being the ability to remove excess water would have had little significance on the overall results. I believe this because precautions and improvements were made after the preliminary stage to make results more accurate. Making the limitations less relevant. The results coincide with other published studies and predictions made. For these reasons I believe the conclusions are valid and the investigation was a success.