Variables:
- Concentration of sucrose solution - as the concentration of sucrose increases, there will be more solute molecules and less water molecules, therefore the water potential will be lower, and so water will move out of the cells and the cells will lose mass, as the concentration of sucrose increases, the opposite will happen and the cells water will move into the cells making them gain mass. This is the variable that I will investigate; I will use a standard 1M sucrose solution and dilute it with distilled water to get different concentrations.
- Size of potato cylinders- the lengths and diameter of the potato cylinders will affect the surface area of the potato cylinders, a long thin potato cylinder will have a large surface are to volume ratio than a short fat cylinder, this will mean there is more area for water to move into and out of the cells so a long, thin cylinder will show the effects of osmosis in a short period of time. I will control this by using the same cork borer for all my cylinders (so they will all be the same diameter, and I shall cut them all to exactly 3 cm long.
- Species of potato- Different species of potato will have slightly different contents in their cell sap, this means the solutions within the cells will be different so there water potential will be slightly different, therefore so will their osmotic effects. I will control this by using the same potato to cut all my cylinders.
- Length of time the potatoes are left in the solutions- The longer the potatoes are left in the solutions, the longer time there will be for the net movement of water, so the movement of water will be greater, after a while the movement of water will end, as both the solutions have the same water potential, but if not left for long enough, the effects of osmosis may not be finished. I will control this by ensuring all the cylinders are put into their solutions at the same time, and are taken out at the same time. The potato cylinders will all be left in their solutions for 24 hours, which should be enough time for osmosis to have occurred fully, and there will be no net movement after that time.
- The amount of solution- the more solution there is the more solute and water molecules there are so the movement of molecules will occur quicker, therefore the net movement of water in osmosis will occur quicker. I will control this by using a measuring cylinder and ensuring all the solutions amount to 10cm .
- Temperature of the solutions- The higher the temperature of the molecules, the faster they will move so the movement of water molecules will increase, so the higher the temperature the higher the net movement of water. I will control this by leaving all the solutions in the same environment- the science lab, at room temperature.
Pilot run
For my pilot run I decided to investigate the best range of sucrose solutions to put my potato cylinders in, this will also give me a chance to ensure the equipment I will use is appropriate and that the lengths and diameters of the cylinders are suitable to measure the effects of osmosis.
Risk assessment:
Apparatus:
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Distilled water
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McCartney bottles and lids
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Sucrose solution
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Potato
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Syringes
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White tile
- Scalpel
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Ruler
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Balances
- Cork borer
- Tissues
Method:
- Find the cork borer with a diameter of 0.8 cm to cut out the potato chips
- Put a potato of a white tile to cut the potato- and NOT on your hand as the cork borers are very sharp and will easily cut your hand.
- Cut out six cylinders of potato
- Ensure there is no skin left on the potato cylinders
- Line the six cylinders up and cut the all to 3 cm long.
- Weigh each cylinder making sure you know which mass applies to which cylinder and then put the cylinders to the side
- Make the sucrose solutions; adding different amounts of 1M sucrose and water can make these. For the 1M sucrose solution measure out 10cm of the sucrose with no water, for the 0.8M measure out 8cm of sucrose and add 2cm of water to make it up to 10 cm. Do the same for the 0.6M, 0.4M, 0.2M and the 0M (which is just water)
- Put each solution into a McCartney bottle.
- Add a potato cylinder to each of the bottles (making sure you know which cylinder is in which solution, so the mass change can be calculated)
- Ensure the lids are put on the McCartney bottles to prevent any loss of water by evaporation.
- Label all the bottles and leave them overnight.
- Take the potato cylinders out of the solutions, making and observation of their appearance
- Blot the potato cylinders to remove excess liquid and then reweigh them.
- Record your results in a table and calculate the percentage change in mass using the equation: (Change in mass/ original mass) x100
Results:
Conclusion: My pilot run was very successful, the equipment I used and the size of the potato chips were appropriate. I choose this size of potato cylinder- 3cm length and 0.8cm diameter, because it was the variable I had investigated for my GCSE pilot run and, referring to my GCSE coursework I felt it was the size potato cylinders which would show the effect of osmosis best, my results were successful, so I shall use the same measurements for my actual experiment. The equipment I used was all appropriate and will be the equipment I use for my actual experiment. The range of concentrations I used for my pilot, which was the main aspect I was investigating, was, I feel, a little too wide, all the results but one, showed a negative change in mass, meaning they were all losing water, so the solutions were too concentrated. For my actual experiment I think I will cut my range of concentrations in half, and will experiment with using intermediate concentrations, such as 0.15 and 0.25, so that I can get a closer estimate of the potatoes water potential. For my real investigation I shall use solutions between 0 and 0.4 M sucrose, using intermediate values such as 0.15, so that I can get closer to finding the concentration at which no net movement occurs.
Actual Method
Apparatus:
How to make the sucrose solutions:
Using 1M sucrose as a starting point, the different solutions can be made. By diluting the 1M sucrose down with water, the molarity of the solution can be altered by the ratio of water to sucrose: Here is a dilutions table:
I will need at least 5 different concentrations to ensure my results are correct and that the concentrations go in the right order, therefore increasing the accuracy and I shall do 3 repeats to take into account any anomalous results, so if one result is very different it can be isolated, doing 3 repeats will allow me to take an average which will be more accurate.
Accuracy:
I have made my results as accurate as I can be using accurate measuring equipment, such as 10cm syringes, as they have the smallest scale so will therefore be able to be measured to a more accurate point, I also used a digital balance which measuring the mass to two decimal points. Distilled water was used to reduce the amount of impurities, which may affect the water potential, and reduce the accuracy of the results. To avoid muddling, I used a tray, which I labelled with the different concentrations, I put the cylinders under each of the labels with their corresponding mass, these were kept here until they went into the McCartney bottles, which had been labelled to ensure the right cylinders went into the right solution.
Method:
- Using a cork borer with a diameter of 0.8 cm cut 18 potato cylinders onto the white tile. Push the cylinders out of the cork borer and avoid handling as much as possible. 3 cylinders will be needed for each concentration to reduce the risk of anomalous results.
- Using a scalpel cut all the potato cylinders down to 3 cm and ensures there is no skin left on them, as the skin is less permeable and will affect the movement of water.
- Weigh each chip and record this on a table, ensure you know which mass goes with which chip so that when they are re-weighed the mass change can be calculated.
- Make up the sucrose solutions using the method above as guide for the different concentration.
- Pour the solutions into the McCartney bottles and label immediately to prevent any confusion later.
- Add the potato cylinders to the solutions, ensuring you know which cylinders are going into which solution, as three cylinders will be going into each McCartney bottle, colour code them so you can tell them apart and know which one weighs what.
- Replace the lids on the McCartney bottles to prevent the loss of water by evaporation, which would effect the sucrose concentration.
- After 24 hours, remove the cylinders from the solutions, blot with a tissue and reweigh, do one solution at a time to reduce confusion but don't take too long on each one. Record the results immediately
- Calculate the average change in mass and plot a graph of mass change against concentration.
- Find the point where the mass change is 0; find the concentration, which corresponds to this and that is the equivalent water potential. Using the graph of water potential against Sucrose concentration, (ref- see appendix) the concentration for no mass change can be converted into water potential (kPa). This will give you the water potential of potato cells.
