Investigating the effect of sucrose concentration on osmosis in potato cells

• Temperature: The temperature of a solution partly determines the amount of energy the molecules within the solution have. If each molecule has more energy, it will change that into more kinetic energy, so it moves faster. Therefore, as the particles are travelling faster, more would cross the membrane in the same amount of time, increasing the percentage change in mass due to osmosis. I will control this by conducting all the experiments in the same environment and in the space of two hours, so the temperature is unlikely to be significantly varied during the entire investigation.
• Type of Potato: There are several varieties of the potato, and each one has slightly different cells. This means that the cell membranes of some potato cells may be more permeable than others. This means that different amounts of water may be able to pass through different membranes, so the amount of osmosis that takes place will vary. The concentrations of cell sap in different potatoes may be different as well, which alters the concentration gradient I will only use one type of potato for all my experiments to avoid this problem.

Using these variables, I have come up with a complete set of all the measurements I will make and use, which are:

• 5x 2mm disks of potato tissue per solution, which I will cut using the largest cork borer available and a scalpel, and I will measure with a ruler.
• 20cm3 of sucrose solution measured using a measuring cylinder, in the following concentrations: 0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1M. I have chosen these concentrations because they are wide-ranged and equal distances part, which makes it easier to plot them on a graph and identify trends. It will also be easier to find, using the graph, the approximate concentration when the percentage change in mass crosses the x-axis, when the potato begins to lose, rather than gain mass, so where the concentration of the sucrose solution is equal to that of the inside of the potato cells. This will allow me to work out the water potential of the potato cells as well.
• I will measure the percentage change in mass to find out how concentration affects osmosis. I will do this by measuring the mass using a balance before and after the potato tissue goes in the sucrose solution and working out the percentage change.
• I will leave the potatoes in the solution for 20 minutes using a stopwatch.
• I will repeat each experiment 3 times to ensure that my results are accurate.

Method

From this, I can make a detailed method. The apparatus I will need are:

• A Cork Borer: to bore cylinders of potato tissue out of the potato;
• A scalpel: to cut the cylinders into discs;
• A ruler: to ensure that all discs are the same size;
• A white tile: to use as a surface to cut on;
• Petri Dishes: to store potato discs in;
• Measuring cylinders: to measure out the correct volumes of sucrose solution and water so that I can achieve the desired concentrations;
• A Graduated Pipette: to ensure that the concentrations are completely accurate;
• 6 x 150ml beakers: to conduct the experiments in;
• A top-pan balance: to measure the mass of the potatoes before and after osmosis.

My method will be this:

1. I will calculate the volumes of water and 1M sucrose solution I need to make 20 cm3 of sucrose solution at the correct concentrations. Then I will measure these out using a measuring cylinder and put them into a beaker.
2. I will cut out 2mm discs of potato using a cork borer, a scalpel and a ruler. I will need 90 discs in total: 5 per solution x 3 for repeats x 6 for each concentration = 90. I will store them in a Petri dish to prevent them from drying.
3. I will find the mass of the 5 discs used for each experiment collectively.
4. Then I will put the discs into the beaker of solution and leave them for 20 minutes.
5. After this time, I will remove the discs, lightly dry them and find their mass again.
6. I will work out the percentage change in mass by dividing the change in mass by the original mass and multiplying by 100.
7. I will repeat this three times for each concentration to ensure accuracy.
8. I will then repeat the entire experiment for each of the six concentrations to be used.

Prediction

        Using knowledge from previous experiments and other work, I predict that, in the lowest concentration (0M), the potato will gain the most mass and, as concentration increases, the amount of mass it gains will decrease until it begins to lose mass, and it will lose the most mass in the highest concentration (1M). I predict this because osmosis happens with the concentration gradient, that is, from a high water potential to a low water potential. So when the sucrose concentration is lowest, the water potential is highest, higher than the water potential inside the potato cells, so water will move into the potato by osmosis, increasing its mass. In the same way, when the sucrose concentration is highest, the water potential is lowest; lower than the water potential inside the potato cells, so water will move out of the potato by osmosis, decreasing its mass. There will be a sucrose concentration at which the potato will keep the same mass, as the water potential of the sucrose solution at that concentration will be the same as the water potential inside the potato. At a higher concentration than this, the potato will lose mass and at a lower concentration it will gain mass. This allows us to find the water potential inside the potato cells, which I predict will be found at around 0.5M sucrose concentration, as the potato is not likely to be that turgid or flaccid, and this is the middle concentration. So, because of this, these are my predictions for each concentration:

• 0M: The potato will gain a lot of mass
• 0.2M: The potato will gain mass, but not as much as in 0M solution
• 0.4M: The potato will gain a very small amount of mass
• 0.6M: The potato will lose a small amount of mass
• 0.8M: The potato will lose more mass than in 0.6M solution
• 1M: The potato will lose the most mass

        I think that I have planned enough so that my results will be reliable enough to support my prediction. My prediction is based on scientific knowledge; it is not just made up, so it is likely to be correct. I have based much of my method on previous successful experiments, so there is no reason why it will not be successful again. I have examined all possible variables, and have looked at the best ways to counter them, so my results should be accurate as well. In summary, I am hoping for a very successful experiment.

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Results Table showing change in mass of potato cells after being left in sucrose solution of different concentrations

(Results in grey are anomalous results that were discounted before averages were formed, results in red are results that are repeats for the anomalous results; all results to 2 decimal places))

This is a selection of my calculations, to show how I worked out the percentage change in mass and average:

Result 1 – Concentration 0.2 M

Start Mass: 4.02g        End Mass: 4.42g        Percentage change in mass: 4.12 – 4.02 = 0.1        (0.1 / 4.02) x 100 = 2.49%

Result 2 – Concentration 0.2 M

Start Mass: 3.62g        End Mass: 3.75g        Percentage change in mass: 3.75 – 3.62 = 0.13        (0.13 / 3.62) x 100 = 3.59%

Result 3 – Concentration 0.2 M

Start Mass: 3.85g        End Mass: 3.97g        Percentage change in mass: 3.97 – 3.85 = 0.12        (0.12 / 3.85) x 100 = 3.11%

Average Percentage Change in Mass – Concentration 0.2 M

2.49 + 3.59 + 3.11 = 9.19                9.19 / 3 = 3.06 %

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Analysis

        I can conclude from my results that the potatoes gain less, and then lose more mass as the concentration of solution increases. To look at this more closely, I will look at each concentration:

1. M (Distilled Water) Solution

In all experiments, the potato cells in this solution gained mass, on average 6.87%, highlighted in blue on the table and graph. This happened because water molecules moved across the partially permeable cell membrane by osmosis. It moved both ways, but because the concentration of water in the distilled water was higher than that inside the potato cells, more water molecules moved into the potato than moved out, so the potato cells gained mass. In other words, the water potential of the solution was higher and the concentration gradient was high, and osmosis happens from a high to a low water potential with the concentration gradient.

2. M Sucrose Solution

In all experiments, the potato cells in this solution gained mass, on average 3.06%, highlighted in yellow on the table and graph. This happened because water moved across the partially permeable cell membrane by osmosis. It moved both ways, but because the concentration of water in the solution was higher than that inside the potato cells, more water molecules moved into the potato than moved out, so the potato cells gained mass. But the potato did not gain as much mass as in distilled water, because the water potential of the 0.2 M solution is lower than that of the distilled water, so not as many more water molecules moved across from solution to cell than from cell to solution, so the concentration gradient is smaller.

4. M Sucrose Solution

The average gain in mass for the potato cells in this solution was 0.09%, but the potato cells lost mass in one experiment and gained in the other two. This shows that the concentration of the potato must be close to 0.4 M, and indeed the average graph shows the concentration inside the potato cells to be 0.405 M (where it crosses the x-axis, highlighted in green). This means that there are almost the same amount of water molecules compared to other molecules inside the potato cells and this solution, so the same number of water molecules moved across the partially permeable membrane by osmosis each way, so the mass stayed almost the same.

6. M Sucrose Solution

In this solution, all potato tissues lost mass with an average loss of -5.07%, shown in pink in my results and graph. This happened because the water potential of the solution was lower than that of the cell sap, and osmosis happens from a high water potential to a low water potential across a partially permeabnle membrane, which is the cell membrane. Water molecules still moved from the solution to the cells, but in this experiment, more water molecules moved out of the cell into the solution as the concentration of water was higher in the cells, so the cells lost mass.

8. M Sucrose Solution

In this solution, all potato tissues lost mass with an average loss of -5.89%, shown in purple in my results and graph. This happened because the water potential of the solution was lower than that of the cell sap, and osmosis happens from a high water potential to a low water potential across a partially permeabnle membrane, which is the cell membrane. Water molecules still moved from the solution to the cells, but in this experiment, more water molecules moved out of the cell into the solution as the concentration of water was higher in the cells, so the cells lost mass. It lost more mass than the tissue in 0.6 M solution because its water potential was even lower, so even fewer of its water molecules moved into the potato cells, whilst the same number moved out of the potato cells, so the water potential is larger again.

1. M Sucrose Solution

In this solution, all potato tissues lost mass with an average loss of –9.86%, shown in black in my results and graph. This happened because the water potential of the solution was lower than that of the cell sap, and osmosis happens from a high water potential to a low water potential across a partially permeabnle membrane, which is the cell membrane. Water molecules still moved from the solution to the cells, but in this experiment, more water molecules moved out of the cell into the solution as the concentration of water was higher in the cells, so the cells lost mass. It lost more mass than the tissue in 0.6 M and 0.8 M solution because its water potential was even lower, so even fewer of its water molecules moved into the potato cells, whilst the same number moved out of the potato cells.        

        I can analyse my results by looking at the graphs as well. All four graphs show strong negative correlation, which is expected. This shows me that the potato gained less mass as concentration increased, up to a point, when it began to lose mass and it continued to lose more mass as concentration increased.

        Now I am going to look at the extent to which these results support my prediction. My prediction had two main points: that, in the lowest concentration (0M), the potato will gain the most mass and, as concentration increases, the amount of mass it gains will decrease until it begins to lose mass, and it will lose the most mass in the highest concentration (1M); and that the water potential inside the potato cells will be found at around 0.5M sucrose concentration. I will look at the first point first. This has certainly been proved to be correct – the highest gain in mass (excluding obvious anomalies) was found in repeats 1 and 3 of 0.0 M solution – both 6.94%, as shown in my results and graph in light blue. The greatest loss in mass was found at the highest concentration (1.0 M) in the first repeat: -10.23%. Between this, the amount of mass gained began to decrease until it began to lose mass, which increased until it reached the maximum, as I said in my prediction. So this part was obviously proven correct, which is probably due in part to precautions I planned to take and carried out, such as doing repeats and looking at all potential variables. The second part of my prediction, relating to the concentration inside the potato cells, was not proved to quite an extent. It was still a fairly good prediction, as according to the average, it was only 0.915 M away. On graph 1, it is shown as being almost exactly 0.4 M, on graph 2 it is 0.42 M, on graph three it is 0.39 M. So my prediction was a little high, possibly because it was not as well backed up with scientific knowledge as the first part of the prediction. So my prediction is supported well by my results, but it can only be supported for the specifications of my experiment, so my results do not prove that the prediction is correct for different concentrations or different tissues, for example.

Evaluation

        Overall, I believe that my investigation was a success because I found reliable results that support my prediction to a good extent. However, it was not perfect, for several reasons, which I shall look into now.

        There were many problems with the procedure that could have made my results less reliable. One major problem was with the drying of the potatoes before they were re-measured. The potatoes had to be dried to remove excess water, so that only the water inside the potato cells was measured. However, it was impossible to keep the amount of drying the same for each experiment. If the potatoes weren’t dried enough, they would have too much moisture on their surface, which didn’t take place in osmosis. If they were dried too much, water would be sucked out of the potatoes into the paper towels by osmosis, lowering mass and therefore disrupting the experiment.

        Another problem was that many different potatoes were used. Obviously, as we needed 90 discs of potato, we couldn’t use one potato, but this meant that some of the different potatoes used will have had different water potentials inside their cell and therefore varied water potential gradients in each experiment, so the experiment was not completely fair. I tried to counter this by mixing up the potatoes once they were cut, but it will still have been slightly varied.

        Other problems included the temperature of the room, which probably increased during two hours of working there; and inaccuracies with cutting, as it was not always possible to keep the discs exactly the same length. An increase of temperature gives water molecules more kinetic energy, meaning that they move faster across the partially permeable membrane, so more can move across in a certain time, so osmosis happens faster. The temperature will have probably increased over the two hours due to people’s body heat, causing some later results to be slightly higher than earlier ones, such as repeat 4 for a concentration of 0.4 M, which had a percentage change in mass of 0.29%, when the other results were closer to –0.29%. It did not make much difference though. Differences in size of potato discs causes a difference in surface area, which means that more of the potato cells are in direct contact with the solution, so osmosis happens faster at a larger surface area.

        All this said, most of my results fitted the pattern well. There were only four results that were omitted before the averages were made, and most of the results I decided to keep are very close to the line of best fit. Two of my anomalous results: 0.6 M solution repeat 1, and 0.4 M solution repeat 2, are extremely out at -27.16 and 28.48 respectively, as shown on my table (I could not fit them on my graph) in grey. I do not think that these could have been wrong for any other reason than human error in measuring and recording their mass, as they are such outliers.

        The other two anomalies that were omitted before averages were 0.8 M solution repeat 2: -3.44; and 1.0 M solution repeat 2: -6.55, shown in dark red. These results both show a lower than expected change in mass. This could have been because they had a smaller surface area, which means that less of the solution comes into contact with the potato cells, so osmosis happens slower. Inaccurate cutting could have decreased the surface area. Another possible reason is that these potatoes could have been dried more than the others, removing more water from the surface, so their mass would have decreased for the second measurement.

        There is a slight anomaly with the averages too. The average result for 0.6 M is higher than expected, at -5.07, whereas the line of best fit would suggest it was closer to -3 – it is shown on the graph and table in pink. This could have been because the solution used each time for 0.6 M may have been slightly stronger, which my conclusion shows to increase the rate of osmosis. Or, as by that stage of removing them at timed intervals it will have been getting a bit hectic, they could have all been removed a little later than they should have been from the solution, which would cause osmosis to happen for a little longer, in this case, causing its mass to decrease more.

        Because of these anomalies, there are several improvements that could be made to my experiment. One of these would be to do more repeats, to make my experiment even more accurate. If I had more time, I would have done at least five repeats for each concentration of solution. Also, osmosis may not have finished when the allotted time, 20 minutes was up. I could have instead looked at the amount of time taken to reach a constant mass, when osmosis has finished in each experiment.

        Another problem was that, in the more concentrated sucrose solutions, the discs were less dense than the solution, so they floated. This decreased the amount of osmosis that could take place, because it decreased the available surface area of the potato tissue. To counter this, I could have used something like wire mesh to keep them in the middle of the solution, not affecting their surface area as much.

        If I were to do a further experiment, I think I would look at more closely the concentration inside the cell sap. I know it was around 0.4 M, so if experimented between 0.3 M and 0.5 M sucrose concentration, I could find out more accurately where it was. My experiment would work in the same way as my previous one, except the concentrations would be 0.3 M, 0.35 M, 0.4 M, 0.45 M and 0.5 M, with one in distilled water (0 M) as a control. I would expect to once again find the water potential of the cell sap around 0.4 M.

        I think that my results are reliable enough to support my conclusion for the substances used, ass there were very few anomalies and I repeated enough times so that I had three valid results for each concentration. However, they cannot support my conclusion for different concentrations of solution, such as stronger concentrations than 1 M. It also cannot support my conclusion for tissue other than potato or solutions other than sucrose.

        In summary, I have had a successful experiment in which I have managed to conclude that, in potato cells in sucrose solution, potatoes gain mass at solution concentrations lower than 0.405 M and lose mass at higher concentrations through osmosis.

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