In this Sc-1 experiment, we are going to put potato chips into different concentrations of sucrose solution, contained in high-density polystyrene cups, to find out how the concentrations of solutions affect the uptake of water by plants.

Richard Lai Biology Sc1 Coursework 05/05/2003

4th Form Fitz WJC

Planning

These are what we are given for this experiment:

Potato chips
1x tray
Max. 15x plastic cups
Sucrose solution (1M)
Beakers, cylinders and syringes
Knives
Chip extruder

These are the procedures of my experiment:

Cut chips into identical shapes;
Mark details onto my plastic cups;
Make solutions with different concentration of sucrose;
Weigh chips;
Place chips into the cups;
Take readings at set occasions;
Plot results onto a graph;
Draw conclusion.

Background knowledge & Prediction

This experiment will require some basic knowledge on osmosis – the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. As a prediction, using some knowledge based on what I know about osmosis: the higher the concentration of sucrose in the solution is, the lower the mass of chips will get, since water in the vacuoles of the potato cells will move into the solution where the concentration of water is lower; the lower the concentration of sucrose, the higher the mass of chips will get as the water from the solution moves into the vacuoles of the potato cells where the concentration of water is lower.

This has been proved by three previous experiments that we have done in the earlier months: the egg and syrup experiment – the egg shrunk in the golden syrup because the water moved from the inside of the egg to the syrup where there was a lower concentration of water; the visking tubing experiment – water entered the visking tubing filled with sugar solution where there was a lower concentration of water; and the red onion cells experiment – the red onion tissue in the water became turgid because there was a lower concentration of water in the red onion cells.

All these experiments have been done with both sugar solution and just water, to prove that the results are not just by fluke.

Method and Achieving Accurate Results

These bullet points are referring to the procedure steps listed above.

The chips that I am going to use are going to be cut into identical shapes. In a perfect world I would choose to have identical masses instead, but the mass of the chips can always change if left exposed to the air by, for example, evaporation. Also, different types of potatoes may also differ from the amount of water that they can absorb, so I will make sure that the chips I am going to use belong to the same potato specie. The same can also be affected by the potatoes’ age.

My method of cutting the chips is to use the chips extruder to make chips in identical cross-section areas, and then they will be lined up and using a ruler to make a 90˚ angle straight cut, so that they would be cuboids with identical cross-section areas and lengths. The potato skin will be cut off since it may be permeable to water, therefore may affect my experiment.

The chips will then be piled up in line on top of each other. This is to check whether the shapes of the chips are the same as the others. I think I won’t need to set a standard size for the chips: as long as they are in identical shapes and size, then they would give the right results; but I’ve decided to cut the chips down to 2cm long, since I find that most of the chips can be cut into two or three chips that are 2cm long, which saves using a lot of chips (I will be using 3 chips for each cup, and I will be having 14 cups, which makes 42 chips in total!).

I will have to make sure that each of the 14 cups is labelled with my name, form, teacher’s name, set number (see 3.) and the concentration % on them, so that the cups will not get mixed up with the others (by doing so can cause a great chaos to my results!).
I will be setting 7 different concentration levels for this experiment: 0%, 20%, 40%, 50%, 60%, 80% and 100%. This should be enough for gaining accurate results. I have also decided the constant volume of solution in each cup to start off as 140ml. This has been tested and it completely covered three potato chips in a cup.

To make the solutions accurately, I am going to use a syringe and a cylinder to measure the amount of sucrose solution and water that I am going to mix together with. The amount of sucrose will depend on the concentration of the sucrose that I want, e.g. if I want a 40% of sucrose concentration in the solution, then I will add 56ml – which is 40% of 140ml – of sucrose solution and 84ml of water into the cup. This will be a solution with a sucrose concentration of 40%. In this procedure, I will make as little ...

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I will be doing a repeat of this at the same time; therefore I have named the two sets as “Set 1” and “Set 2” so that I won’t get confused with both sets.

Before weighing, I MUST zero the balance; otherwise I might get a set of bizarre results.
When placing chips (x3) into the cups, I will make sure that no spillage or splash is made; or else, I will have to refill the cup and get new pieces of potato chips in order to maintain the accuracy of the results. Also, I should do this quickly to every cup so that the delay between each other is minimised, i.e. they would all be starting at approximately the same time.
The mass of the chips will be measured on 5 occasions: after 0 hours, after 1 hour, after 3 hours, after 5 hours, and after 24 hours. This will make sure that I have taken a recording of the maximum mass that the potato chips can gain. The masses will then be converted into mass change % by dividing the gain or loss in mass by the original mass and then times 100. This can be either before or after taking the average.

We will be doing the actual experiment on a Monday, and our lesson ends at 12:30pm, so if I take my first reading at 12:15pm, then I can take my second reading at 1:15pm (in lunch break); third reading at 3:15pm (break between a double Chemistry lesson); fourth reading at 5:15pm (after school); and last reading at 12:15pm in the next day (between games session).

I have decided to take an average of the mass (g) of three chips for every concentration of sucrose solution being measured.

In order to achieve more accurate results, before weighing the chips, I will have to wipe off excess surface water on the chips, since the surface water is not absorbed by the chips.

I shall have to record the mass of the chips from different sets of apparatuses before and after the experiment, and then compare the results – maybe by plotting the results into a graph, with the concentration (%) of sucrose solution on the X-axis, and the mass difference % on the Y-axis. The final graph should look something like this, according to my prediction:

The results are going to be recorded onto a table first:

The potato chips should not be used after they have been in the solution for over 24 hours, as they will start to decay. This is crucial because after 24 hours the mass of the chips may have a sudden change – either increase or decrease, which can greatly affect my results.

Results

Set 1

Set 2

Mean of Set 1 and Set 2

Analysing

In my “mass change % against concentration of sucrose solution” graphs, I have found out that the chips in sucrose solutions with a concentration below about 40% ended up having increased masses; chips in sucrose solutions with a concentration above about 40% ended up having decreased masses. The graph shows that the mass change % decreases as the concentration of sucrose solution increases, i.e. the chips gains less mass or loses more mass as the concentration of sucrose solution increases.

The most significant decrease of mass change % is in the solutions with a concentration of between around 35% and 65%, and then the decrease rate of mass change % starts to drop at a concentration of 80%. Maybe this is when the chips have lost most of their water contents. I did not consider about this at the beginning of this experiment, and I thought the graph would be a straight line travelling diagonally downwards, according to my prediction: the higher the concentration of sucrose in the solution is, the lower the mass of chips will get, since water in the vacuoles of the potato cells will move into the solution where the concentration of water is lower; the lower the concentration of sucrose, the higher the mass of chips will get as the water from the solution moves into the vacuoles of the potato cells where the concentration of water is lower; but the final graphs has shown that I have missed something in my prediction; it will always come to a point where the chips will lose most of their water contents, and therefore their masses stop decreasing; the mass left will mostly belong to the chips’ plasmolysed cells. Still, the graphs shows that as the concentration of the sucrose solution increases, the masses of the chips decreases, but it will not decrease forever.

I have decided to do graphs focusing on the mass of the chips against time (not mentioned in my planning), in order to aid my prediction and my “mass change % against concentration of sucrose solution” graphs.

In my “average mass of chips against time” graphs, I found out that chips in sucrose solutions with a concentration below 40% ended up having their masses increased; the chips that have been put in sucrose solution with 40% concentration did not make much difference on their masses; the chips that have been put in solutions with concentrations above 40% ended up having their masses decreased. The graphs go steeper downwards as the concentration increases, showing that the mass decreases as the concentration of the sucrose solution increases.

The graph showing the mass of chips in the sucrose solution with a 40% concentration did not change much over the 24 hours. The mass stayed around at its original state. This is suggesting that maybe the concentration of water in the chips was the same as the solution’s, which is 60%. This makes sense since the definition of osmosis is the movement of water travelling from a high water concentration to a low water concentration, so looking back at the graphs, the sucrose solutions with a concentration below 40% have a water concentration of 60% and above, which is higher than the water concentration of the chips, therefore the water from the solution will move into the chips, where the concentration of water is lower; the sucrose solutions with a concentration of above 40% will have a water concentration of below 60%, which is lower than the water concentration of the chips, therefore the water from the chips will move into the solution, where the water concentration is lower. This, again, supports my prediction: the higher the concentration of sucrose in the solution is, the lower the mass of chips will get, since water in the vacuoles of the potato cells will move into the solution where the concentration of water is lower; the lower the concentration of sucrose, the higher the mass of chips will get as the water from the solution moves into the vacuoles of the potato cells where the concentration of water is lower.

Evaluation

Overall I think my investigation on the effect of concentration on water uptake of plant tissue have been successful, since the results obtained from the experiment support my prediction. All the results showed that higher concentration of the sucrose solution decreases the mass of the potato chips.

The first two mass change % against solution concentration graphs are slightly different from each other: the graph of set 1 was more like a straight line, when the second one shows a gradual decrease on the mass change at the end, which makes more sense since the chips should have run out of most of their water content; but after taking the mean from both graphs, the new graph fixed graph 1’s problem of lacking the curve to show that the mass of the chips does not decrease forever. After all, only water is involved in the occurrence of osmosis, not the potato cells, so the chips should have mass left even if the concentration of the sucrose solution is over 1M, and the mass would belong to the cells of the potato chips. This is why the final graph makes sense, and it also supports my prediction: the higher the concentration of sucrose in the solution is, the lower the mass of chips will get. It has also added into my prediction the fact that the mass of the chips does not decrease forever.

I didn’t like the graphs that the computer produced. In the set 2 graph the second last plot should be an anomaly. Maybe I have not wiped off the water on the surface properly, therefore decreased the mass loss when it should have lost more mass. I have hand-drawn the graph and I think the graph ought to have ignored the second last point, according to the pattern of the other plots (see attached graph). The good news is I have taken the mean from two graphs, so the plots are brought closer to the line of best fit; but if I had more time I would have investigated on chips soaked in solutions with a concentration between 60% and 100% so that the graphs can be better drawn.

Graph 1’s problem can be solved by continuing the experiment with higher concentrations of sucrose solution, so that we can see a clearer pattern of the graph levelling off.

I am fairly happy with my “mean of average mass of chips against time” graphs. They all show a good pattern of increase and decrease of mass over time. The only thing that I would like to do is to find out what happens between the readings after 5 hours from start and after 24 hours from start, since I will then be able to tell from the graphs if the mass of the chips have already reached the limit of mass change during the period of five hours after start and 24 hours after start. This, of course, is only possible in a perfect world, where we are not required to do anything else, including going to lessons and eating, but just taking the readings of the mass of the chips. I can possibly do another experiment on a Sunday when I don’t need to go to any lessons, and when the laboratory is accessible for us to work in, but this, again, is only possible in a perfect world, because you’ll never know if you are actually going to be doing nothing at all on a Sunday; you might suddenly be needed for an important event (since there is often one on a Sunday, for me anyway). It is possible to get friends to take the readings for me, but their procedure of taking the readings may be different from mine, or they may even be doing the wrong thing, and therefore ending up taking inaccurate or even wrong results; so I think I will have to stick to the graphs that I have at the moment. They all support my predictions anyway, plus they are plotted with means of a number of readings, so they should be very accurate.

At the beginning of the experiment, I found it quite hard to cut the chips into the same shape and size. All their cross-section areas are identical, since they are extruded from a cutting device that had identical extruders. It was only the length that mattered. I was only given rulers and a blade. I had to cut the chips carefully to achieve clean and straight cuts, so that their sizes are roughly identical.

If I could make a cutter – like the ones used for making biscuits – that can allow me to cut the chips into identical shapes, then life would be much easier, and my results may be more accurate, but only if I had time. The little cutter could be made out of a thin, rectangular piece of aluminium foil. All I have to do is to fold up the foil so that becomes a cuboid without two sides, and it will be folded into the length that I wanted the chips to have. I can then just cut out the chips by pressing the cutter onto each chip.

This, though, could be quite dangerous, since the aluminium foil can be quite sharp on the edges, but I’m sure I would be able to find ways to get around this problem.

Apart from that, I am fairly pleased with my results, especially with the repeats that I have done and the mean of the results that I have taken. If I have more time I would do more repeats – maybe two or three more, and also I will extend the concentration of sucrose solutions to 150% so that the pattern of the graph can be seen clearer. I’m also pleased about how this investigation has reminded me of something that I have missed in my prediction, and now I’ve kept it in my mind.

What I am really interested after this investigation is what the water concentration is in the potato chips. In this experiment I found out that the mass of the chips that has been in the sucrose solution with a concentration of 40% didn’t change much over the 24 hours period. The mass change % against solution concentration graphs showed that the mass change of the chips stayed unchanged when they are soaked in solutions that have a concentration of around 35% to 40%, so if I do this whole experiment again, but this time with sucrose solutions with concentrations of between 35% and 40%, starting from 35%, then 36%, 37% 38%, etc. This should then give a graph showing a more precise value of the water concentration of the potato chips. On a mass change against concentration graph, the water concentration of the chips should have the same value of ‘X’ of the coordinate on where it is intercepting the X axis (‘X’,0), so the water concentration of the chips should be ‘X’%. This would be quite interesting to be found out.