Prediction: I predict that the results will form a graph somewhat like this:
I think the graph will look like this because of the theory of osmosis explained in the introduction. The lower sucrose concentrations will increase the mass of the potato because water will move in to the cell sap. The water will move into the cell sap because the concentration in the cell sap is higher, so it will try to average the two concentrations. Somewhere in the middle of the concentrations experimented with, there will the sucrose content of the cell sap itself. At this point there will be no mass change because there will be no water movement, this is the potato’s osmotic potential. This point is marked on the prediction graph by a circle. After the point where the line crosses the x-axis, it will drop into the loss of mass. This will happen in the concentrations above the concentration of the cell sap because water will move out of the cell sap. This will cause a mass loss.
We would want to choose a range of concentrations that included reading before and after the osmotic potential. We want this so we get even results either sides of the x-axis and so a more accurate graph, creating a more accurate perception of the osmotic potential of the potato.
Fair Test: We will need to keep certain variables the same in this experiment, as to not affect the conditions. This must be done to insure that any difference in the data we collect is because of the concentration and not something else. We will keep the following variables the same:
- Surface Area; as explained in the introduction, higher surface areas can increase the rate of water movement.
- Temperature; higher temperatures will speed up the molecules, making them move faster across the semi-permeable membrane, as will colder make them move slower.
- Amount of solution; to insure that there is no difference in the amount of water that can travel.
- Size of beaker; all chips must fully immersed in the same size beaker or test tube.
- Always dry the potato before weighing it; the excess water left on the potato after it is taken from the water can increase the mass, this must be dried off.
We will change only the one following variable:
- Concentration of sucrose solution; this is the aim of the coursework, to see how this affects the total mass change.
Safe Test: This test does not contain any hot liquids or acids, so goggles are not compulsory. Any spillage should be cleared up immediately and anything in the eye should be removed or washed out, but will not cause any damage. There should be no running as with any experiment, and anything not needed for the experiment should not be around the work area.
Preliminary Work:
Method:
- Cut potato chip to correct size and weight, record these in table. Measure out 25cm³ of the sucrose solution (whichever concentration using).
- Drop potato in solution and start timer.
- At whatever time period you chose, take out potato and weigh, record new weight. Always dry potato before weighing it.
Experiment 1: To find a good immersion time.
I will look at the time taken to get a noticeable difference in the mass change of identical potato chips in 25cm³ of pure water and 80% sucrose solution. I chose these two as the two extremes here I should see the most mass change for each time bracket. I will chose the immersion times where I get definite masses.
Results.
I will use 20 minutes for my immersion time because it is where both extremes have changed a lot, but are changing at a slower rate. Between 20 and 25 minutes there is little change.
Experiment 2: To work a suitable range of concentrations.
To work out a suitable range of concentrations to give results above and below the concentration of the cell sap. To find out the concentration of the cell sap I will use a vast range of concentrations to find out which shows little change. Then I will use a graph to predict where there will be no mass change. I can then choose my range to give a spread of results above and below this.
If you take all results in to consideration then the osmotic potential would be around 20%. But if you left out the result for 80% sucrose, then the osmotic potential is closer to 6 or 7%. I will chose my range around these percentages. I chose the range of 1% - 10%, at 1% gaps. Hopefully this will give a range of results around the osmotic potential.
I carried out these preliminary experiments to work out:
- The amount of time to leave the potato in the sucrose. I chose 20 minutes from the table I made in experiment 1.
- The range of sucrose I will use. I did this by making a graph and looking at the point at which it crosses the x-axis. This was about 6/7%.
- I chose before hand that cylinder shaped potatoes were the best potato shape to use as they were the easiest to cut accurately with a corer.
- I will repeat all experiments and calculate means, to get accurate results.
Method: Apparatus:
- Potato
- Balance
- Beakers
- Measuring cylinder
- Stop clock
- Potato
- Corer (to create cylinder potato pieces)
- Paper towel
- Scalpel
Instructions:
- Cut a cube of potato with continuing height of 2cm with the scalpel.
- Core as many holes in that as you need to obtain exactly the same size potato pieces. Weigh these and record their weights.
- Pour 25cm³ of the require concentration of sucrose solution into a beaker, measured in the measuring cylinder for better accuracy.
- Drop potato cylinder into sucrose solution insuring they are fully immersed. Then start the timer promptly.
- After 20 minutes, remove the potato piece and dry off in paper towel. Once dry weigh on balance and record mass change from start of experiment.
Results: These are the results.
Conclusion: The graph above shows how the movement of water across a semi-permeable membrane differs with different concentrations of sucrose solutions that it is placed in. In the table the results below 5% increase in mass and above 5% decrease. But with the best-fit line in the graph, this change happens at 7%. The graph shows that the osmotic potential of a potato is 7% sucrose. This means that the solution in the cell sap is 7% sucrose, because that is the point where there is no water movement, or mass changes.
The diagram on the next page shows what happens when the concentrations on either side of a semi-permeable membrane are equal.
The preliminary work says that the osmotic potential is close to 5/6%. This is not very accurate, as all results are very spaced out and extreme. But still they were close to the graphs 7%, and bang on the 5-6% gained from the table, but the graph is always more accurate with a best-fit line.
The graph I made from my prediction is the same shape as the graph I got, with a similar gradient. They also both cross the x-axis in the middle. This sows that my prediction was correct.
Evaluation: My results were all close to the best-fit line. However if I added my other separate results (not the mean) then some would not be as close. This shows that doing more experiments and averaging them made my results more accurate. They were a couple of results that weren’t as bad as anomalous but were different. These could have been down to a number of reasons:
- Too long in the solution
- Too much solution
- Larger surface area
- Hotter (improbable because of room temperature, doesn’t change enough)
- Not being sufficiently dried.
- Not fully immersed
I think I did do enough repeats because my mean average did eliminate most odd results. I could have done more data points within my experiment had I been given more time, maybe adding half way marks between percentages. I think the immersion was adequate, but maybe more time would have lead to more accurate results, as long as they weren’t too long to be pointless.
To make the conclusion more accurate I could use more results, with a closer range or more repeats.
Another experiment to investigate osmosis is experiment 2.4 in ‘Biology for You’ by Gareth Williams. This experiment shows the action of osmosis not by mass change but by the water level rising up a thin tube.
Setting up the apparatus as below causes the action of osmosis.
The water moves across the visking tubing (which is a semi-permeable membrane) and into the strong solution inside that. The increase in water in this small space causes the water to rise up the glass tube. You could conduct an investigation to find the different rates of osmosis in different concentrations. By measuring how many cms the water travels in a set period of time you could calculate the rate for each concentration to see if there is a difference.
