What is the effect of solutions with different concentrations on water balance in potato cells

What is the effect of solutions with different concentrations on water balance in potato cells?

Initial plan

I am going to investigate the effect of different concentrations of sucrose solutions on water balance in potato cells.

Hypothesis

From my knowledge of osmosis theory I predict that more dilute sucrose solutions will cause the greatest gain in mass in the potatoes because they will take on most water.

Osmosis theory

Overall flow of water molecules

Factors that could affect the water balance

The concentration of sucrose in the solution which the cell is placed in
The volume of solution
The number of cells (mass of potato)
The surface area of the cells (surface area of potato)
The type of cell

The factor that I am going to change (the independent variable) will be the concentration of sucrose in the solution which the potato is in. I will measure the affect of this on the change in mass of the potato (the dependent variable). I will control the other factors (control variables) that I have listed so that they are constant for every experiment to make sure that they do not affect the dependent variable. I will use a fixed volume of solution for every experiment, a fixed size of potato chip and a fixed shape of potato chip. It would be hard to control the mass of the potato chips so that they are all exactly the same so I will weigh the chips before and afterwards and work out the percentage change in mass, so the original mass should not affect the results. The cells will all be potato cells and I will try to make them as similar as possible by removing the skin and trying to use pieces with no blemishes.

Apparatus

Potato slice
Scalpel (selected because it is very sharp and so will cut the potatoes cleanly and precisely)
Different concentrations of solutions (I selected the solutions: 0 ml sucrose with 30 ml water; 10 ml sucrose with 20 ml sucrose; 20 ml sucrose with 10 ml water; 30 ml sucrose with 0 ml water. I chose these concentrations because 30 ml is the optimum amount for a boiling tubes, and four concentrations should have been enough to see a trend in the data)
Balance (selected because it can give very accurate readings; to 2 decimal places)
Boiling tubes (selected because they are wider than test tubes and so will be better suited to put a potato chip in)
Forceps (used to extract the chips, so that they do not have to be handled)

Method

Cut the potato slice into regular chips of 3cm by 1cm with the scalpel
Weigh each chip on the balance and record the masses
Put one chip in each different solution
Leave to soak overnight
Weigh each piece on the balance again and record the masses against the original masses

Results

Ideas and modifications from my preliminary work

In my preliminary experiment I discovered that boiling tubes were hard to work with because I could not fit the forceps down them to get the potato chips out. I decided that plastic cups would be ...

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Method

Cut the potato slice into regular chips of 3cm by 1cm with the scalpel
Weigh each chip on the balance and record the masses
Put one chip in each different solution
Leave to soak overnight
Weigh each piece on the balance again and record the masses against the original masses

Results

Ideas and modifications from my preliminary work

In my preliminary experiment I discovered that boiling tubes were hard to work with because I could not fit the forceps down them to get the potato chips out. I decided that plastic cups would be more suitable to do the experiment in because they could hold enough solution to submerge more than one chip, and the chips could be easily removed.

120cm3 of solution was found to be a good volume to put in the plastic cups because the chips were totally submerged. I decided that I should cover the cups with Petri dishes so that the solution does not evaporate and affect the results.

In my preliminary experiment I discovered that it was hard to cut the potato into equal shapes, so I decided to use a potato-cutting instrument that sliced the potato into prisms with equal width and height, so I only had to cut them into equal lengths.

I decided to use an increased number of different solutions so that I had more results to draw a conclusion from. I also decided to do four repeats of each concentration so that the results are more reliable and I can spot any outliers. So that I could tell the repeats apart, I decided to stick different coloured pins into each one.

I decided that to do so many experiments I needed to work in a bigger group and this meant that we had to come up with routine ways of doing things such as drying the potatoes so that the results are reliable.

I decided to use means of the four results for each concentration because this would give the best estimate of the true value. I decided to use four repeats because the mean will be more accurate then if I just used two.

Collecting data

Apparatus

Potato
Potato-cutting instrument
Different concentrations of solution: 0cm3 sucrose with 120cm3 water; 24cm3 sucrose with 96cm3 water; 48cm3 sucrose with 72cm3 water; 60cm3 sucrose with 60cm3 water; 72cm3 sucrose with 48cm3 water; 96cm3 sucrose with 24cm3 water; 120cm3 sucrose with 0cm3 water. (These concentrations are at six regular intervals between 0cm3 sucrose, and 120cm3 sucrose, with an extra one at 60cm3 sucrose because I wanted to include the middle value.)
Balance
Plastic cups
Petri dishes
Paper towels (to dry the chips with so the excess solution does not affect the results)

Method

The same method was used as for the preliminary experiment, but the modifications outlined above were observed.

Results

The two results that are bold and underlined are so far outside the range of the other results that they are considered to be outliers. I will not include them in my calculations of the mean, as they would distort the results.

Calculation of means and ranges

Interpreting my data

The graph (page 6) that I have drawn shows a negative correlation: as the volume of sucrose in the 120cm3 solution increases, the mean percentage change in mass decreases. The line of best fit is straight which suggests that the relationship between the volume of sucrose in the solution and the mean percentage change in mass remains constant.

From the graph I could draw the conclusion that the more sucrose there is in proportion to water in the solution, the more mass is lost, and the more water there is in proportion to sucrose in the solution, the more mass is gained. The intercept on the x-axis is 40cm3 of sucrose in a 120cm3 solution. From this I can conclude that the ratio of water to sucrose within the cell is 3:1, because it must be the same as the ratio outside, as there is an equal flow of water in and out so no mass is gained or lost.

This conclusion is supported by osmosis theory. If there is more water in the solution then it will be more dilute than the solution inside the cell, so overall the flow of water will be into the cells as the molecules move about randomly and spread out. This means that the cell would gain in mass because it would gain water.

In the converse situation, if there is more sucrose in the solution then it will be more concentrated than the solution inside the cell, so overall the flow of water will be out of the cell as the molecules move about randomly and spread out. This means that the cell would lose water, and so lose mass.

Evaluation of my data

I collected 28 results, which seems a sufficient number to draw a sound conclusion from. The results seem to be accurate because the error bars are relatively small and all except one show a real difference because only the last two results have means within the range of the other concentration.

All the results seem to fit the trend/pattern except for the last concentration which seems to be a little further from the best fit line than any of the others. This result may not fit the pattern because the potato may not be able to lose any more water. To check my conclusion I could repeat the last set of experiments to check that no mistake was made in the procedure.

My conclusion explains most of my results except for the two outliers that I omitted from the graph because they were too far apart from the range of the other results. These outliers could have occurred because a mistake was made in the process of the experiment or simply because the potato was blemished, or in some way different to the other chips, because there is bound to be variation in biological material.

Evaluating my methods

The range of concentrations that I used was large, so it should be enough to support a firm conclusion. I also used four repeats in every concentration, which should have made the results more reliable.

I used a potato-cutting instrument to cut the potatoes so that they were all equal in size, which should have made my results more accurate by controlling the surface area of the potatoes so that it did not affect the results.

The volume of solution was also controlled so that it did not affect the results. However, the mass of the potato chips was too hard to keep constant so I had to calculate the percentage change in mass so that the original mass did not affect the results. The fact that all the potato chips were made the same size should have roughly controlled the masses so that they were not too varied.

I used a balance to weigh the potatoes, which gave very accurate readings to two decimal places. This gave me precise quantitative data, which should have made my results more accurate and reliable.

To improve my method I could make sure that the potatoes all went in and came out of the solution at the same time, because the fact that some were in for a few minutes longer could have affected the results, although probably only slightly.

I could also do more repeats because this would have made my conclusion more reliable. I could experiment with a greater number of different concentrations of sucrose solution to confirm the trend in the data.

I could carry out the whole experiment myself as working in a large group could have brought about variations in the method, which might have had a slight effect on the results. If I did it myself I could make sure that everything is done in exactly the same way for every experiment, which would make the data more reliable.

Evaluating my conclusion

I am pretty confident in my conclusion because, as I have previously discussed, my results seem to be fairly accurate and reliable, which gives me more confidence in the conclusion drawn from them. The seemingly anomalous result for the final concentration does, however, cast a slight doubt on the reliability of the conclusion, so I would like to repeat the experiment for that concentration to verify the conclusion.

To boost my confidence in my conclusion I could repeat the whole experiment with the improvements that I outlined earlier, but for now I believe that my conclusion is fairly accurate and I would say that I have confidence in the general conclusion, if not every result.