Discover how potato pieces are affected when placed in varying concentrations of sucrose solution and to find the concentration of cell sap in a potato piece.

Evaluation:

I have found that although my results show that osmosis has occurred because the amount of sucrose solution has increased, there is not a wide enough range of data to allow me to be certain of the accuracy of the results. Therefore, for the real experiment, a wider range of concentrations could be investigated, or the experiment will be repeated at least another 2 times. Because I only measured the amount of solution increase, it does not provide me with enough evidence to support a prediction and is not reliable enough to say that they all started with 30 ml of solution. Therefore, in future I will work out the mass change which makes it easier to obtain good results. I did not measure the potato beforehand, therefore the sizes were judged subjectively by eye. This is not accurate enough and may have allowed for mistakes to occur. In the next experiment, the potato chunks will be measured exactly the same so it is fair. Either this or a cork borer can be used to cut out potato cylinders to certain heights. When measuring the heights of potato after the experiment, I can use a calliper which is far more accurate.

Experiment:

Apparatus:

Potato

Knife

Cork borer

Ruler

Measuring cylinder

Distilled water

Sucrose solution (varying concentrations)

Weighing scales

Test tubes

Test tube rack

As I found out from the preliminary, solutions of concentrations 0.1, 0.5 and 1 Molar are not sufficient in range. Therefore, I will be using 0.2, 0.4, 0.6 and 0.8 Molar sucrose solutions. These go up in equal stages and there is a wider range for accuracy.

Method:

1. Get potato, knife, cork borer and ruler.
2. Pierce into potato with cork borer.
3. Use ruler to measure 3cm lengths.
4. Repeat until there are 5 equal length potato cylinders.
5. Use scales to measure each mass.
6. Record in table.
7. Get test tubes, distilled water, sucrose solution and measuring cylinder.
8. Fill each test tube with 50ml of distilled water, 0.2M, 0.4M, 0.6M and 0.8M sucrose solution.
9. Place in test tube rack.
10. Repeat twice to be able to calculate averages to ensure reliability.
11. Leave for 2 days.
12. Retrieve potato cylinders.
13. Dry with paper towels.
14. Weigh mass with scales.
15. Record in table.

Fair Test:

This is an extremely important part of any investigation. If an experiment is not fair, it may lead to wrong conclusions being formed based on inaccurate results. To ensure this certain aspects of the investigation will stay constant while one factor varies, in this case, the concentration of sucrose solution. The start mass of the potato cylinders must be exactly the same. If any are smaller then there is a smaller surface area for osmotic action to take place. Measuring of the length will be done very carefully and the mass will be measured using a very sensitive set of scales. To ensure that there is fairness in the weight of mass, the same scales will be used because measurement may vary between different ones. The amount of solution in the test tubes must remain equal to prevent more or less water particles being available and measuring at the end of the investigation to be erred. All the test tubes will be left for the same amount of time so no more or less osmosis can occur. By using a cork borer, the width of the cylinders is precise and exactly the same so surface areas are equal. I must make sure that my hands are clean and dry when handling the potato cylinders in case of contamination, which would affect the results.

Safety:

Accidents can easily be prevented if the following precautions are taken:-

1. Extreme care must be taken when using the knife to cut the potatoes.
2. None of the solutions must be drunk in case of possible damage.

Results:

Averages:

Graph:

Because averaged results are more accurate, these are the figures I will be working with to draw my graph.

A Line Graph to Show Changes in Mass of Potato Cylinders

Conclusion:

The results that I obtained support my hypothesis. I found that the potato cells took in and let out water particles at varying degrees depending on how concentrated the solution was. All three sets of data follow the same pattern of mass loss and gain as concentrations got higher. However, my prediction that the amount of mass gain or loss would be directly proportional to change in concentration was incorrect. When 50 millilitres of 0.4M sucrose solution was doubled to become 50 millilitres of 0.8M sucrose solution, the decrease in the mass of the potato cylinder was not doubled,     -0.162 to become -0.872. This is a decrease of over 538 %. However, if I calculate the mass change from 0.2M to 0.4M sucrose solutions I find that there is a decrease of 570%. This therefore indicates to me, that although the mass does not change in direct proportion, it does change in indirect proportion.

My graph shows a curved line of best fit which slopes downwards as the concentrations get higher. This proves the trend that the higher the concentration, the more water particles are lost from the potato cylinders. The gradient of the line is not constant. This is because the two ends which curve slightly are at the two extremes, completely water with distilled water, and then a very high concentration of sucrose solution. At the end with a very high concentration the cells have to become as flaccid as possible so the change in mass of each molar concentration becomes close together with each increase.

At the distilled water end, the line seems to tail off. There appears to be no further water gain which suggests that a gain of 0.146 grams is close to the maximum a potato cylinder of this size can let in. Most osmosis occurred at 0.8 M sucrose solution where 8.72 grams of mass was lost. Here is also seems to not be able to support any further water loss, suggesting the cell is nearly fully plasmolysed. Between 0.2M and 0.8M the points of the graph are more spread out indicating there was a larger change of mass at those concentrations, thus proving my hypothesis further.

From the graph I can make an estimate at the concentration of cell sap in the potato. This is the isotonic point at which the line of best fit crosses the x-axis, in this case approximately 0.27M. As discussed previously, this is the point where no change of mass occurs as both the solution and potato have the same molar concentration. This tells me that the concentration of the potato cell sap is approximately 0.27M.

This experiment was successful as I was able to use the data collected to predict the isotonic point. However, it is not 100% reliable when used as evidence or as basis for another hypothesis due to many inaccuracies in the actual experiment conducted.

Evaluation:

I think that my experiment was quite successful on the whole. My graph did not show any anomalous points which indicate a greater degree of accuracy in the experiment. However, because my line of best fit was curved and was drawn from an average of the results collected, it is more difficult to find anomalies within the data plotted.  Therefore, I need to look at my raw data in order to discover a clearer view of the anomalies in the results of my experiment.

There is quite a broad range between all the figures, especially in the percentage change. The ranges are;

1) Distilled water – 4.251

2) 0.2M – 2.88

3) 0.4M – 3.766

4) 0.6M – 4.903

5) 0.8M – 5.588

These suggest that the results for the 0.2M sucrose solution were most accurate, while the results for the 0.8M solution have most variance and therefore less reliability. Because there are obvious fluctuations in the raw data, the standard deviation of them can be calculated which provides a much more truthful representation of how exact the results were in relation to each other.

Formula Used:

The results show that the 0.2M sucrose solution was closest to the mean while the 0.8m solution was the furthest. However, the smaller the standard deviation, the fewer values are accounted for, therefore indicating that there are most outliers. I can calculate the amount of outliers in the raw data by using the standard deviation. 1 standard deviation accounts for 68% of the data, 2 standard deviations is 95%, and 3 standard deviations is 99%. Therefore;

My raw data has on average 10.74% outliers, hence making it 89.26% accurate. The evidence is satisfactory as it allowed me to make a conclusion about the concentration of the potato cell sap. However, because the start masses were different, it made the raw data I had less reliable because the larger the surface area of the potato, the more osmosis can occur at one time. This did not follow my plan for a fair test. Other reasons for the fluctuations in the results are that the time which is timed each experiment for may have varied as delays may have occurred in either stopping or starting. Longer experiments would have had longer for osmotic action to occur, which means they would have had a higher percentage change. The range of concentrations plotted on the graph means that there are gaps in which I had to guess the rough amount to plot. This means that the curve plotted may have been entirely wrong as the values in between those investigated could have distorted the graph so much that the gradient of the line was different. The measuring of each solution and liquid was not completely precise as human error occurred therefore there would have been differing amounts of sucrose solution and water in each test tube. The more solution there was the more osmosis is likely to occur; therefore making it more probable that the end mass is greater. The cutting up of the potato cylinders may not have been exact, as human error may have taken place when measuring the length, and also when using the knife to cut it. The knife may have been held at a slant and therefore less potato was included in the experiment. This means that less osmosis is likely to happen. However, because the percentage change was calculated, this would not have made too large a difference.

The time I allowed for the experiment was much longer than needed but it let me see the effect of osmosis over a longer period of time than say just a minute or two, which makes the experiment more realistic. By having such a long time, osmotic action was given enough time to be fully completed, which takes into consideration more than an experiment of a timed few minutes. In the future, when repeating this investigation, I could also find the saturation point of the potato cylinders as an extension experiment to see whether that had an effect on this experiment. This is the state when no more water can be absorbed or lost by the potato cylinder.

I think that the range of concentrations was adequate but maybe not sufficient enough to be able to draw an absolutely correct line of best fit on the graph. In the next experiment, I will use concentrations of 0.1M, 0.3M, 0.5M, 0.7M and 0.9M as well so that there are no gaps between points where I had to basically guess, which would have made the line of best fit less reliable, therefore providing less evidence of the isotonic point as that was not directly on a point of concentration I had experimented with. This would help when finding the isotonic point as it would be much more accurate and less of an assumption. It would also make anomalies more apparent so that I can be more certain that the data is correct and accurate.

To make this experiment more exact then each concentration and each individual test tube could have been tested at a time so that the time it takes to get them out of the solution, dry them and weigh them is cut down. However, if this happens then they must also be put in later to ensure that the potato cylinders have been in immersed in the solution for the same amount of time. If the cylinders are not then some get less time to absorb and let out water particles so the change is less. During the experiment, human error may have caused further inaccuracies when weighing and drying the potato pieces. The scales may not have been set to 0 prior to the experiment. This would not have made a difference to the overall conclusions as it would be consistent throughout. However, it is impossible to account for different scales being used. This distorts the raw data.

When cutting the potatoes into equal lengths I found that often they were cut lopsided so some cylinders got more potato than others meaning some have more surface area to absorb water than others, so there is a larger or smaller change in mass compared to other sized cylinders, again, affecting reliability. To improve on this a machine could be found to cut the potato into pieces with no angle at the ends.

Using a measuring cylinder to measure the amount of distilled water or sucrose solution may not have been very accurate because sometimes I was in a rush and may not have looked very carefully at the amount. Therefore this increases the amount of miscalculation, hence increasing the dependability of the experiment. Also, when the liquid is poured into a beaker there is always approximately a millilitre remaining. However, because this would happen each time, it is not particularly worrying. To ensure accurate measurements, a burette could have been used.

Bibliography:

Letts Biology Study Guide