Preliminary Method and how it informs the rest of my plan
In order to find out the best size of potato chip and optimum amount of time to leave the potato in the solutions I carried out a preliminary test using different sizes of potato chips and varying the amount of time I left them in.
Preliminary results go here
In the preliminary experiment I found out that the longer I left the potato chips in the solution, the greater the weight change and therefore the more accurate my results. I decided that one hour was the maximum amount of time I could leave the potato chips in solution for given the time available to me.
I also found that for the smaller potato chips, (give some idea of size here) there was a greater the variance in results between the potatoes which were left in for the same time in the same strength solution. This meant that the smaller the potato chip, the less reliable the experiment and the more prone to error my results were.
However, the large potato chips (size) had a lower surface area to volume ratio so osmosis couldn’t occur as rapidly; therefore I decided to use potato chips that were middling size so lots of osmosis would occur but also my results would be significant enough to draw conclusions from.
Diagram
Full apparatus list
- 18 boiling tubes
- Boiling tube rack
- Marker Pen
- 1 molar concentration sugar solution
- Distilled water
- 100ml measuring cylinder
- Pipette
- Stirring Rod
- Thermometer
- Cork Borer (5mm diameter)
- Tile Scalpel
- Large Potato
- Timer
- Paper towels
- Forceps
- 3 100ml beakers
- electronic top-pan balance
Method
First of all I took 18 boiling tubes, put them into the rack, labelled each one with the molar concentration of solution I was putting in it and numbered them 1, 2 and 3 for each strength of solution. This was to eliminate any possibility of me getting the potato chips confused with other ones.
I then filled the boiling tubes up to the required strength by mixing 1 molar solution of sucrose with distilled water. I made three lots of each of six solutions. I made 100ml of each solution. The solutions I made were 0, 0.2, 0.4, 0.6, 0.8, and 1.0 molar strengths of sucrose. To make these I mixed 0, 20, 40, 60, 80 and 100ml respectively of the 1M sucrose solution I was provided with and made it up to 100ml by adding distilled water.
I then got my cork borer and cut three potato chips from my potato. I cut them all to three centimetres long to try and get them roughly the same weight and surface area. I then weighed each one and recorded the weight. Then I started the stopwatch and put the chips each into boiling tubes 1, 2 and 3 of the first solution. I repeated this process of cutting measuring, weighing and recording of the potato chips for each of the other five solutions. I recorded the time on the stopwatch as they went into the solution.
I stirred each solution every five minutes to prevent osmosis only being able to occur between the potato and the part of solution in contact with the potato. I also checked the temperature every 15 minutes to make sure they were all the same.
When the stopwatch was approaching one hour I got paper towels ready and 3 beakers which I labelled 1, 2 and 3. Ten seconds before the stopwatch reached one hour I poured the first set of solutions into the beakers; 1 going into beaker 1, 2 going into beaker 2, etc. I did this because during the preliminary experiment I found it hard to take the potato chips out of the boiling tubes using the forceps and therefore could not take them all out at the same time. I chose one hour because I felt this was the longest I could leave the potato chips in for considering the times restrictions I had to do this experiment in. At the hour I took the potato chips out in the order I put them in (1, 2, 3) and placed them on the paper towels. I dabbed each one dry to get rid of excess solution on the outside of the potato chip before weighing them and recording the results.
To prevent cross-contamination I then got new paper towels and emptied and dried the beakers ready for the next set of solutions. When it got to one hour after I placed the second set of solutions in; I repeated this process as I did for the remaining four.
The reason I did each set of solutions one at a time is to stop weight change while the potato chips were left out on the side waiting to be weighed afterwards or put in before. By staggering the starts it greatly reduced the amount of time each potato chip had on the side not in the solutions. Ultimately this would have reduced the accuracy and significance of my results had I not staggered them out.
Results
I worked out the percentage weight change because by simply looking at the weight change the bigger pieces of potato will have larger weight changes when this would only be due to the fact they have more potato cells and therefore more vacuoles for osmosis to happen in.
Graphs
Analysis
From this graph we can see that when immersed in the distilled water it was the only time the potato chip gained weight. We can also work out the concentration of sucrose in the potato vacuoles by reading off the graph This is called the isotonic concentration. This concentration was 0.15M. This is less than I predicted. I expected there to be a higher concentration of sugar in the potato vacuoles. I expected it to be around 0.25-0.3 molar. My potato may have been low in sucrose levels. On further research this explanation is most likely because in a bigger potatoes the sugar has to be spread out across a much larger volume though the amount of sugar (sunlight) it receives is likely to be the same as that of a smaller potato. I used an extremely large potato in order to be able to get 18 samples out of it.
The graph starts out with a gradient of -37. I expected the gradient to start at around this level or perhaps a little less steep as the difference of sucrose concentration between the potato and the solution is very little positively or negatively. It then turns into a very steep gradient; this again is what I anticipated would happen. This is because the percentage change between 0.2 and 0.4 is so great. 0.4 is around 10 times higher is concentration when comparing them both with the concentration inside the potato. This means that the water potential gradient increases very steeply and thus lots more osmosis occurs at 0.4 than at 0.2. The gradient is -61 between these two points, easily the steepest on the graph.
After this the gradient slowly becomes less between 0.4 and 0.8 and by 0.8-1.0 the gradient is almost flat at only -5. This is happening because osmosis is already occurring at nearly all of the possible places it could occur so in order to increase the rate of osmosis you have to start changing other limiting factors such as temperature or surface area.
All the points are relatively near to the line of best fit. The line of best fit is a second order polynomial whereas my line is a third order polynomial. Due to the fact that my line is only very faintly a third order polynomial I thought it best that the line of best fit was a second order polynomial. This explains why point two is the only point a significant distance away from the line of best fit.
From graph two we can see that the percentage weight loss was least at the 1M sucrose solution. This suggests that the cells had been fully plasmolysed. There was no overlap of results except between 0.8 and1.0M meaning the results were accurate.
How the sucrose concentration affected the amount of osmosis between the solution and the potato chips.
It is obvious from my results that the greater the concentration of sucrose in the solution the greater the weight loss of the potato chip. Also, additional increases in concentration affect the amount of osmosis less than the lower levels. We can see this clearly from the graph showing that the greater the molar concentration of sucrose the greater the percentage weight loss of the potato chip.
How well my prediction has been confirmed
- My prediction has clearly been confirmed in that if you increase the concentration of sucrose in the solution the amount of osmotic activity between that and the potato chip will increase.
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My numerical prediction was accurate quite closely also; at 1M sucrose solution the weight loss was 28%, only 5% under what I predicted.
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I also predicted the shape of the graph would be a third order parabola which it was although not as clearly defined as I expected it to be.
- My prediction for the weight gain in distilled water was not so accurate. I have discussed the reasons for this and shown why this is likely to be due to the abnormally large potato I used for the experiment. I predicted a 15% weight gain but the experiment only produced a 5% increase in mass.
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I also predicted that the Sucrose levels in the vacuole would be slightly higher than 0.2 molar but they turned out to be under this value meaning that 0.2M concentration lost weight when I expected it to gain.
How results fit in with background knowledge
GO mum or get Dad to write something because I can’t and Dad is good at this sort of thing.
Experimental accuracy
- This experiment was quite an accurate one due to the simplicity of the measuring needed to done. One factor which could be significant in reducing the accuracy was the surface area of the potato chips. They were not all exactly the same. Especially the 0.8M set which are all considerably (10%) heavier than the other potato chips. This weight change probably only caused a 3 or 4% difference in my results which is why they are not noticeable on the graph as being out of place.
- Another inaccuracy could be the top-pan balance, although I used the same one throughout to try and reduce this inaccuracy they are probably accurate readings to within 2% or so but the fact I’m subtracting two results means that at worst my low percentage change results could be 25% inaccurate.
- My concentrations were measured out in a measuring cylinder and accurate to within 1%. I could have increased this accuracy using a burette if it was provided but they were not available.
- The timing was accurate to <0.1%.
- Another inaccuracy was the dabbing of the potato chips after being in solution. Although I tried to keep the dabbing uniform differences could easily account for a 3% inaccuracy in final weights.
- Despite all of these it was a very accurate experiment because the only advanced piece of measuring equipment needed was an accurate top-pan balance which I was provided with. All the other inaccuracies are negligible and were probably nowhere near as much as I stated they could be.
Significance of my results
Because of the accuracy of my experiment my results are easily repeated and cover a wide enough range of concentration to be very significant results.
A good way of measuring the significance is to see the variance in results between each reading of the same independent variable.
From this table we can see how low the variance between reading are strongly suggesting little error in measurements. The maximum measurement error is only 7.5% and minimum only 0.88%. These errors could be down to the different pieces of potato I used rather than inaccuracies in measurements.
Experimental reliability
The table above also shows the reliability of the experiment as well. Because the variance of readings is so low it suggests that the experiment is very reliable i.e. gets the same results again and again and my experiment wasn’t just a one off.
Procedure improvements
The best procedure improvement for this experiment would be to simply leave the potato chip immersed in the sucrose solution for a longer period of time.
It would also improve my experiment if the size and shape of the potato was exactly the same every time because surface area and mass affect rate of osmosis. I could use a cutter with two blades preset at 3cm apart to do this more accurately than I did so.
Since my experiment was investigating the rate of osmotic activity doing a control only dipping the potato chip in quickly would improve the accuracy. It would let me know whether just putting the potato in the solution maybe soaked some solution up. Because my results included this, the weight change may not all be due to osmosis occurring. I would subtract the weight change from dipping it in from my results to get the results due to osmosis.
Although this wouldn’t have affected the results much when I stirred it was impossible to do it in a totally uniform way. One way to avoid this slight inaccuracy would be not to stir any of the solutions at all.
I could have used a burette to get the concentration of solutions perfectly accurate and all the volumes exactly the same.
The volume of the solution was quite important because in a larger volume the water taken or given by the potato make little difference to the concentration of solution. If I did this experiment again I would do it in more solution so the molar concentration of the solution didn’t change over the course of the experiment.
Experimental extension
If I had the time I would do another experiment to increase the significance of mine.
- Compare different size potatoes sucrose concentration
- Compare different types of potatoes sucrose concentration e.g. King Edward against Maris Piper.
- Experiment how the surface area affects the rate of osmosis.
- Investigate the effect of mass on rate of osmosis.
I would do the same experiment but with different size potatoes to see if the sucrose concentrations were different depending on the mass. I would do this to see if my theory about why the sucrose concentration in my potato was correct.
To do this I would choose 4 potatoes each with different masses ranging from very light to very heavy. I would only measure 0, 0.2 and 0.4M concentration of sucrose because the idea is to find out what the sucrose concentration is inside the potato and it is highly unlikely to be over 0.4M. I would take three reading from each potato at each concentration. I would keep all other variable the same as I did in this experiment. I would leave them in for as close to two hours as possible though I may not have time for this long. I would record all the results with weights before and after being submersed in the solution. I would then draw a graph showing the percentage changes for each potato and read off the molar concentration of sucrose when the percentage change is 0%.