I find it difficult to estimate at what molarity that will be because I am unsure of the molarity of the 'strong sugar solution’ used in my first experiment. However, my second preliminary experiment helped me discover roughly where it is- somewhere between 0.3 molarity and 0.4 which means that I will take more readings from around that point to make my graph more accurate.
With regards to a graph I expect mine to look like the one below, with the line crossing the x-axis at x molarity which will be the molarity of sugar inside the potato. I think the two variables will be inversely proportional to each other because when the molarity is low the potato will gain weight, and when it is high it will lose weight.
Preliminary Experiment 2
For my second preliminary experiment I carried out the experiment I hope to do, to help decide upon my range of molarities and to aid my prediction. It also helped me to find out anything I was unsure of about the set up of the apparatus, and to discover the rough position of the concentration of the sugar inside the potato so I could take more readings from values around there to make my experiment more accurate.
Here are my results in a table and then in a graph:
Although the results appear slightly inaccurate, and the 0.5 result seems a definite anomaly this experiment has helped me with many things towards my experiment.
- I now know exactly how all the equipment works and will have no problem using it.
- I know how important it is to make sure all the potatoes are put in the solution, and removed at exactly the same time.
- I only left the potatoes in for 40 minutes, and my results don’t appear very accurate so I will definitely leave them in for 1 hour like in preliminary 1.
- I can see that the line crosses the x-axis at a molarity of around 0.4. On the graph it appears that it is at exactly this point, but this is only because I rounded the percentage to 2 decimal places. When I plot my actual graph I will plot the exact points, and write them down to 2 decimal places.
- I will take results from 0.2 mole solution to 0.5 mole solution instead of 0.0 to 0.5. It is difficult using the syringes to accurately measure out 0.5 of a mole but if I can do this accurately I will also take one reading at 3.5.
Finalised method for my experiment
Apparatus- to follow through this experiment I will use the following equipment:
- 6 cylinders of potato cut to roughly 6cm
- 6 test tubes
- 6 rubber bungs
- A small razor
- An electronic balance
- Two syringes- one for sucrose/one for water
- A beaker of 1 mole sucrose solution
- A beaker of distilled water
- A cork borer
- Paper towels
- A stop watch
- Test tube rack
Method- below is the method I will use to discover the sugar concentration in my chosen potato. You should repeat the experiment twice to ensure accuracy, at the same time as your first readings if you wish.
- Using a cork borer and a small razor cut cylinders of potato to a size of roughly 6cm. This is not extremely important because we will be measuring percentage increase and decrease but it is important to keep the surface area roughly the same.
- Measure out different concentrations of sucrose solution as shown below, using separate syringes for the distilled water and sucrose solution to prevent contamination.
Make sure you shake the sucrose solution and distilled water well so they are completely mixed. As you can see there is always 10 cm³ of solution.
- Place a bung in the top of each test tube to prevent evaporation of water, which would make the results useless.
- Using a paper towel, remove any surface water from each potato, which could increase the mass and weigh them on the weighing machine. Be careful to record the weight of each potato, and into which solution is has been placed.
- You should now have six labelled test tubes in a test tube rack. Using a stop watch leave the potatoes in solution for one hour.
- After one hour, remove the potatoes from their test tubes, remove surface water (for reasons stated above), and re-weigh. You should repeat the experiment twice to ensure accuracy. Record your results in a table like the one below. You may wish to record flaccidity or tergidity, by feeling the cylinders and how hard they are.
Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrinks and pulls away from the cell wall. These cells are said to be plasmolysed.
We are trying to find a molarity of solution that means there is no net movement of water.
Then plot a graph of percentage difference on the y-axis and molarity on the x-axis. Where the line crosses the x-axis will be the concentration of sugar in the potato.
Fair Test- to ensure a fair test I must do the following:
- Take all potato samples from the same potato as different varieties may have different solute concentration or different water intake potential.
- Use the same balance to take all readings.
- Take the test always in the same room, under the same conditions.
- Repeating the test to ensure accuracy, and to remove any outliers.
- Drawing a line of best fit on the graph.
Variables- what I will keep constant to ensure a fair test, what I will measure, and what will vary as the measurement changes:
Safety
This experiment isn’t particularly dangerous but I must be careful with the blade and cork borer, which are very sharp and could cause damage. It would be sensible to place the potato on a tile before cutting it.
I must keep my hands clean and dry when I handle the potatoes so I don’t pass on any water.
My Experiment
From my experiment I attained the below readings, having carried out the experiment two times. My readings were taken to 2.d.p and my averages have been rounded to 2.d.p. Because I had more time than I initially expected I left the potatoes in the solution for 1 and ¾ hours.
Conclusion
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From my graph I have discovered the sugar concentration of the potato I used was 0.285 which I discovered from reading off where y=0 and therefore there is no percentage increase or decrease.
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My graph shown above follows a general straight line as I predicted. It was difficult for me to predict actual values because my preliminary experiments were less accurate than my final experiment, but the graph follows the same trend I predicted. It appears that for some reason there is a slight curve as the line gets closer to the x-axis because the difference between each percentage change begins to get smaller. At first I thought that maybe the graph should have been a curve because of the time I left it for and the speed osmosis happened at as the potato got close to turgidity and flaccidity. However upon thinking about it I think this is just a slight anomaly in my results due to inaccuracies and so I have continued to plot a line of best fit as a straight line and not a curve. It makes sense that with a sugar molarity of 0.1 a certain amount of water would need to enter the cell and with 0.2 roughly half that amount would need to.
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I can see from the graph that sugar solutions of above molar 0.285 cause a decrease in mass and sugar solution below this point cause an increase in mass.
- When the molarity increases the percentage change decreases- they are inversely proportional to each other. By finding out the gradient and y-intercept I have found the equation of this straight line to be y= -50x+14.
Scientific knowledge and understanding
Following what I previously thought the potato cells, working to stay alive, took in, or gave out the water depending on the concentration of the tissue, and the concentration of the solution it is surrounded in.
I have found out the sugar concentration by placing slices of potato in different concentrations of sugar and finding one concentration where the potato doesn’t change in mass. Due to osmosis if there is a difference in concentration between the sucrose solution and the concentration of solutes in the potato there will be a change in mass due to the movement of water.
Osmosis is defined as:
‘A special type of diffusion where there is a net movement of water across a selectively permeable membrane from a relatively dilute solution to a relatively concentrated one.’
- If the potato is placed in a solution of high solute concentration water will leave the potato, its mass will decrease and it will become soft. (flaccid.) This is because there will be a certain solution inside the membrane and a more concentrated one outside so molecules of water will leave in an attempt to equalise the concentration.
- If the potato is placed in a solution of low solute concentration solution water will enter the potato, its mass will increase and it will become hard (turgid.) This is because there will be a certain solution inside the membrane and a more dilute one outside so molecules of water will enter in attempt to equalise the concentration.
From my scientific background and information I now know that when y (the percentage change) = 0% there will be no net movement of water and this must be the point where the sugar concentration is equal to the concentration of glucose inside the potato. Water will move in and out of the partially permeable membrane, but will not move more in on direction than another because the concentration doesn’t need equalising and there isn’t one solution that is more dilute than the other.
Evaluation
In general I think that my experiment produced results which are reliable and I would be happy to use as a guideline in further experiments, making my evidence valid. They appear quite accurate because they are a straight line and when y=0 my x value is 0.285 molar. The accepted sugar concentration of a potato is 0.24 molar and therefore for an experiment without access to very accurate measuring equipment I consider this a successful result. However, there are many things which I learnt that would need to be improved and I could work upon.
Looking at my graph there are no outstanding anomalous results, although all the results do no lie on the same straight line. As shown below there are almost two separate lines on my graph. However, I think this is just co-incidence and all of my results were not entirely accurate.
My investigation was fairly carried out in many ways, for example carrying out the experiment twice under the same conditions, however here are things I found that may have caused accuracy to decrease and were difficult to perform so therefore I would want to alter if I repeated the experiment:
- I could not ensure that the potato tubers were of the same surface area which may have meant the intake or out-take of water happened at different speeds. If I could use an accurate machine to produce them of identical size it would add to accuracy.
- I tried to make sure my experiment was fair by patting the potatoes dry, because excess water may have meant that the weighing was not accurate but it was difficult to make sure that that same amount of water was wiped off each potato.
- Carrying on from that I also found that having dried and weighed the potatoes I had to leave them while I dried the other potatoes to ensure that they all went in the sugar solutions at the same time when I started the stop clock. This meant that while the potatoes were sitting on the bench water was evaporating from them, which changed their mass and made my results less reliable. To improve this I may be able to use an individual stop clock for each potato tuber so they could be immediately placed in the solutions after beings weighed. Even having left them on the bench it was difficult to put them all in at the same time which would be helped by separate watches.
- At the beginning of my experiment I made the very big mistake of not completely covering the potatoes in solution which would have meant water could evaporate from the potatoes. Luckily more solution was made up and added so it shouldn’t have impeded my results too much.
- If I were to repeat the experiment I would also like to use a more accurate syringe to make it easier to measure decimal places and then I could take more results between 0.2 molar and 0.3 molar to investigate that area more closely which again would help increase accuracy. A more accurate balance may also help.
- It is good that I have repeated my results but I would like to have done so three or more times to again increase reliability.
Further work
- As I have mentioned I would like to extend the range particularly between 0.2 and 0.3 molar to obtain more results around the area of the potato’s sugar concentration.
- I would also like to repeat the above experiment with the same method but measuring the changes in the potato at smaller intervals. It would be difficult to decide these intervals, so I would use a preliminary experiment to help. I would like to find out how the rate of osmosis works and whether it begins slowly and then speeds up, begins quickly and slows down, or happens at a constant rate. I would also like to find out how long it takes for the sugar concentrations to be balanced out by measuring at small intervals and seeing where it stops changing. From the evidence which I obtain I would plot graphs of :
- Time taken to balance out sugar concentration against molarity.
- Time against percentage change, keeping the molarity constant and then comparing this with graphs of a different constant molarity.
- I would also like to repeat my preliminary work, my proper work and my further work on a variety of different species of potato and then maybe different species of vegetable. Not only would this enable me to compare the sugar concentrations of different vegetables but it would also allow me to see whether if I placed a variety of vegetables in a molarity directly proportional to their own molarity osmosis took place quicker in any particular vegetables. I could not simply place them in the same molarity to discover the rate of osmosis because I would expect those with molarities very different to that they are placed in to change concentration more quickly.