* Duration of the experiment – I am not sure how this will affect my results, therefore I will try to leave my actual experiment for longer to see if there were any changes noticed.
* The weighing scales – The weighing scales may not be exact, so I must leave a leeway of approx. 0.4/0.5 on each result due to the (perhaps!) inaccuracy of the scales, to produce more precise results.
Safety:
* I will be using a sharp knife, which could injure someone if not handled properly. Therefore I must not rush with the equipment and I must handle everything carefully.
* I will ensure the lab is safe around me as I am working; removing baggage and unnecessary books from my work area, pushing stools under the desks and stand as I do the experiment in the interest of safety.
* In order to keep the benches clear and clean I will lay paper towels around where I am working, because even though the substances are relatively harmless, I will be working with glass and sharp instruments, so it is safer to do so.
* I will also wear an apron to prevent any of the substances damaging my clothing as I work.
Fair Test:
* I will keep all the other variables the same and constant to make my experiment fair.
* I will keep the PH constant so that my results are not affected by a change in acidity.
* I will use the same amount of solution in each boiling tube, measuring different quantities of sucrose in ration to distilled water to create the correct molarity solutions.
* I will make sure the potato pieces are the same size and have approximately the same surface area to make sure that osmosis doesn’t occur more due to a larger surface area (see variables).
* I will make sure I use the same size of boiling tube and the same size corer to make sure the depth of the liquid and the surface area of the potato piece is approximately the same.
* The variable I will alter in my experiment is the concentration of sucrose.
* I will make sure the room temperature stays approximately the same and I will try not to vary it too much if it can be helped.
Accuracy:
* I will use a set of digital weighing scales to make sure that my results are as accurate as possible.
* I will use a measuring cylinder to measure out the liquids to make up the solution. If I had access to a more exact method of measuring out the solution, I would do so, but I only had limited equipment available.
* I used a ruler to measure each potato core to make sure it was the correct length, or as near as it was possible (never more than 1 mm out).
* My measurements will therefore be very precise. Below is the preliminary work I have carried out to determine some of the work above. I have used secondary sources in my scientific knowledge; a list is shown below;
Encarta Encyclopaedia ’98, Britannica Millennium Edition, DK Chronicles of Science, GCSE Science (Collins Study Guides), Various internet resources.
Preliminary Work:
The preliminary work allows you to make hypotheses and check that the test can be done completely fairly, with reasonable measurements. I also cut one piece of potato into mm strips, but this was very time consuming. However this did show that the surface area of the potato does make a difference to the results.
My first results were not as good as I had hoped on my first preliminary, so I altered my method and recorded far more accurate results. Here is a short list of the alterations I made:
* I realised I had to be more careful during the weighing process, because if we weighted the same piece twice, occasionally the results would be slightly different each time.
* I was going to do all the six different molarities with five repeats in different beakers, however I realised this would use up a lot of equipment and solution, so I decided to do all the five repeats in one beaker, and multiply the amount of solution by five so I would still have a fair test.
* I am still going to do five repeats for three different molarities, because that will produce more accurate results. I will still use the molarities in the range of 0.0 to 1.0M. I am taking several readings for all the results to allow me to find an average of results.
* I was going to use test tubes, but I realised the amount of solution I wanted to use would not fit into one test tube, having multiplied the amount of solution, so I used small beakers instead.
Preliminary Results:
Prediction:
Osmosis is the passage of water molecules from a weaker solution to a stronger solution, though a partially permeable membrane. In this case, the tin holes in the membrane of the potatoes will allow the water molecules to pass through in and out of the solution and the potato, depending on the concentration gradient of the two substances.
I predict that as I increase the concentration of sucrose then the amount of water moving in or out of the potato will change. A low concentration of sucrose in ratio to water will make the potato increase in length and mass.
I can support this prediction by looking back at my scientific knowledge. Water will move into the concentrated potato cells because of the high concentration gradient. I am expecting therefore to have a high water potential in the high concentration (of sucrose) solutions. I am expecting the opposite for the low concentration (of sucrose) solutions. In a very concentrated sucrose solution (high water potential) water will move out of the potato and into the sucrose solution. I therefore predict that the length and mass of the potato will decrease as the levels of sucrose increase.
Range:
I am going to put six pieces of potato at 2cm long in a beaker with 80cm3 of the sucrose/water solution. The levels of sucrose in ratio to water will be measured in moles, rising in 0.2 molarity each time. The range I will use is 0.0 (only distilled water), 0.2, 0.4, 0.6, 0.8 and 1.0 (only sucrose solution).
Precautions
*As was stated previously, I have to keep all the different non-variables the same, to make sure that none of them affected the experiment in any way.
*Whilst cutting the potato, extreme care was needed to make sure it did not harm anybody.
*The measurements of the solutions and the lengths and thickness of the potato had to be exact as to not change the outcome of the experiment.
* I had to ensure that each time I handled the potatoes my hands were clean and dry. This was to stop any contamination and made sure I didn’t pass on any excess water or sugar from my hands.
Apparatus:
* Cork borer – diameter of 8mm
* Distilled Water
* Sucrose solution – 1 molarity
* Measuring Cylinder
* Cutting tile
* Potato – of one type to make the experiment fair (see fair testing)
* 6 beakers
* Pen – to label the beakers
* Ruler – to measure the potato cores
* Scalpel – to cut out the potato cores to the correct size
* Paper towels – to help keep the work area clear and clean
* Digital scales – more accurate than a manual balance
* 2 Pipettes – one for the sucrose solution, one for the distilled water so that they do not become contaminated.
Diagram:
Diagram 1:
Diagram 2:
Molarity Table:
Method:
1. I took one average sized potato, checking it was hard and healthy.
2. Using a borer (size 8) I cut 30, 2cm lengths of the potato.
3. Taking 6 beakers, I labelled each one 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0.
4. Using the measuring cylinder, I measured out each of the correct levels of sucrose solution to distilled water (see molarity table above) and I placed each of the mixtures into the correct beakers.
5. Then I weighed all the potato chips on an electronic balance (see results) and recorded the results.
6. I placed 5 pieces of potato into each beaker and left them for approx 36 hrs.
7. After this time I drained out the solutions from the beakers and I carefully placed them in order of molarity on a paper towel.
8. I dried the potato chips gently and then weighed each potato piece and recorded the results.
9. As I had extra time I made a second experiment and also recorded those
Results:
These values clearly support my prediction, and even though there are some anomalous results, there is an overall negative trend across the whole set of results, proving them accurate.
Analysis of Results:
The sucrose concentration of the solution into which the potato tissue is placed affects to what degree it grows or shrinks. As you can see from the graph the results show a clear negative correlation, a very obvious inversely proportional trend. From this a conclusion can be drawn. When the water concentration is high, the potato gains water, as seen by the 0.0 molarity solution. When the water concentration is low, the potato loses weight and therefore decreases in mass, as seen in the 1.0 concentration solution. This proves my hypothesis correct. The results were quite widely ranged, as seen on the graph, with quite a few anomalous results, especially results no. 2. The results show that the line of best fit should be slightly curved, to accommodate for the 0.4 and 0.6 results. I have drawn the graph with % change, because it is more accurate. I have used the equation shown below to calculate the % change.
A = Starting weight of potato core
B = Final weight of potato core
% Increase = (B – A)
- All * by 100
As shown in my scientific knowledge, this experiment proves that the water potential in the potato cores ranging from 0.0 to 0.4 is high, decreasing gradually towards the 0.4 concentrations. The potato cores in the 0.6 to 1.0 solutions have the lowest water potential, gradually decreasing in potential towards the 1.0 concentration, and visa-versa. The potato cores in the 0.0 to 0.4 solutions can absorb more water, because water will move from an area of high concentration to an area of low concentration of water molecules. This also means that the 0.6 to 1.0 potato cores cannot absorb as much and therefore osmosis occurs in the opposite direction, the potato core losing water to the solution. This is shown on the graph by the points below the 0% line. The results fully support my prediction, except for the few anomalies on the graph. The possible reasons for the anomalies are outlined below, in the Evaluation and Conclusion sections.
Evaluation:
I believe we gained sufficient enough results, shown on the graph and the tables that conclude the experiment, and to prove my hypothesis. I obtained a good amount of fairly accurate results, from which I was able to make informative graphs. I believe I took enough repeat results for the number of concentrations I was using, and I believe the time I used was sufficient to prove that osmosis had occurred. My final results were very reliable, due to the precautions I took to make this a fair test. However there were some anomalous results. I believe these were caused by some of the reasons highlighted below and also the fact that I could not control any heat variations or the evaporation levels of the liquid within the lab. I could have controlled this by placing the beakers in a water bath kept at a constant temperature and by covering the beaker in cling film to prevent moisture escaping.
Even though there were anomalous results, and that I would have to repeat those results more to prove completely conclusively that the graph is completely correct, there is definitely a negative downward trend across the graph, proving my conclusion correct. I believe that my procedure was suitable for my experiment and I would improve it with the previous factors mentioned above and the list of factors mentioned below. I believe that overall my results were fairly accurate, and a firm conclusion can be drawn from them.
Looking at the overall experiment, I have thought of a number of things that would have helped to make the experiment more accurate.
- The potato cores all had different levels of moisture at the start of the experiment, meaning that some potato cores would need less osmosis to reach equilibrium within the same molarity solution. I could have regulated this by using the same potato all the way through the experiment.
- I could have used more solutions of different molarities, e.g. 0.1, 0.15, 0.2, 0.25 etc. This would have meant that I could have found out the isotonic point far more accurately than I did.
- The concentrations were measured out using a measuring cylinder, which was not completely accurate. The experiment would have been far more exact if there had been some form of exact measuring device, perhaps digital. Some solutions probably had a tiny amount more solution in the beaker than others due to human error.
- I will make sure the beaker’s size is kept the same size and that all the potato pieces are fully submerged to the same depth at the start of the experiment.
- I could have increased the time length of the experiment, to allow more osmosis to occur and to perhaps find the saturation point of the potato cells.
- When the potato pieces were dried after the experiment, to remove the surface liquid, it was not necessarily done the same with each potato piece. The experiment could have been more exact by creating a uniform drying method for each of the pieces.
- At the end of the experiment I noticed that the diameter of the centre of the potato increased or decreased according to the concentration of the solution. It would have helped to explain the results by measuring the centre diameter of each core before and after the experiment.
- The experiment would have been more exact if I had found a way of removing and measuring all the sores at the same time to reduce the time difference between weights being measured. This could have affected the results slightly. Otherwise it would have been better to do each core one by one and measure them after an exact space of time. However this is unpractical due to the probable slight variations in time and measuring of the substances due to human error.
- Using more molar solutions would have helped to obtain more accurate results and produced a far more accurate graph.
- I could have also varied the tissue type, for example, to use carrot, or apple or even animal cells, to prove that osmosis occurs in all living plant/animal cells. I could have also varied the potato’s age, to see if age affects the rate or levels of osmosis. I did try to keep the cells the same, but this was very difficult, as I had no way of telling their exact age. I also tried to use the cells from the same potato, however the potato didn’t have enough cells to cut enough cores from for all the results and therefore I was forced to use two different potatoes.
- I could have also altered the length and diameter of the cores. This however, would only change the results due to a larger or smaller surface area, and I would obtain the same sort of results as I did from the chopped core in my preliminaries. I could have also diced the potato core, like I did in my preliminaries, because that would increase the surface area.
- The scales I was using gradually became sticky due to all the potato cores, therefore more weight may have been added later because of the excess water and sugar.
- The potato pieces were all cut by hand with a ruler and scalpel, so the cuts were often not exact and often the potato cores were slightly longer or shorter than they should have been. For the next time, I would make some sort of template or machine to cut all the cores to exactly the same length.
- The anomalies in the graph may have been caused by the fact that the chips may not have been fully submerged in the solution, because the potato cores had a tendency to float on the surface, limiting the surface area, and therefore meaning that some of the cells were not in the solution and were not able to carry out osmosis. I could have made some sort of fabric sieve or cover over the pieces to make sure that they were properly submerged.
- I could repeat the experiment more to gain even more reliable results for each molarity.
Conclusion:
To conclude this experiment I can say that the movement of water through a semi-permeable membrane (the process of osmosis) is affected by the concentration of glucose solution, or by any other solution. The higher the concentration of glucose solution, the more the potato chip will lose its weight, width and length. All the evidence I have obtained supports my hypothesis. I have been able to collect sufficient results by repeating the experiment five times. However there are improvements that could be made (see Evaluation).
Because water molecules have a form of kinetic energy, they are always moving around in either a gaseous, solid or liquid state, randomly from one place to another. The greater the concentration of water molecules in a solution, the greater the total kinetic energy, and the higher the water potential will be. This means that as the concentration of glucose molecules increases in a solution, the concentration of water decreases, lessening the solution’s water potential, and decreasing a solution’s ability to move between solutions due to osmosis. Therefore as the concentration of glucose increases in each solution, the water in that solution is less able to move to the potato, causing water from the potato to move into the solution, decreasing the potato’s length, mass and width. An osmotic system is set up when a semi-permeable membrane is placed between two solutions. There are many examples in the biological world, in many plants and animals.
The plasma membranes of the cell decide the permeability of the membrane. A semi-permeable membrane occurs when some substances with small molecules can pass through, e.g. Water, and some substances cannot pass through the membrane because they have larger molecules, e.g. Sucrose. The permeability of a membrane varies with certain conditions, and can vary due to temperature, and even due to hormonal impulses, e.g. the loop of Henley in the kidney.
My investigation shows that, in concentrations above 0.6M, there appears to be no further water loss, suggesting that the cell is completely plasmolysed. However, it is important to realise that this is only an estimate because potato cells will not be uniform in their concentrations. I have enjoyed doing this experiment and I believe it has been very informative.
