Potatoes are an excellent source of carbohydrate in the form of starch. Potatoes store food as starch, as it is insoluble, and doesn’t draw in water by osmosis. Because potatoes store starch, not polysaccharides, which are soluble in water, potatoes have a high water potential.
Plant cells, unlike animal cells, have a strong cell wall made of cellulose, in addition to a cell membrane, which means that when they take up or loose water by osmosis, they do not burst or shrivel up.
Plant Cells in dilute, equal and concentrated solutions:
Turgid Equilibrium Plasmolysed
Water Movement Water Movement Water Movement
Preliminary Investigation
Method
I filled 3 beakers with sucrose solution, of different concentrations. One of 30% sucrose, one with 15% sucrose, and one with 5% sucrose. I cut three strips of potato, all with a length of 7cm. I then left them in the separate beakers for 36 hours. I carefully dried them off using paper towels and recorded their changed length. I repeated this experiment once more.
Results
Graph
(Using Averages)
This graph gives a very rough presentation of the results because of the lack of substantial data. I need to complete more repeats of these experiments with a wider range of data to give more detailed and accurate results in my main experiment, but this was not necessary in the preliminary because the main focus was to find any faults with the method of the experiment, not to derive a solid conclusion from the results.
The preliminary has made clear to me that simply measuring the length of the potato slices is not accurate enough, as it does not take into account the amount in which the potato has expanded or contracted width-ways. This means that in my main experiment I will measure mass change instead of length change.
Main Investigation
Method
Key Variables:
My variable will be the concentration of the solutions. I will use equal amounts of solution to make it a fair test, but will vary the concentration of sucrose in each solution. I will do a replicate of each solution, for more accurate results.
Controlled Variables:
- I will use as similar potato size as possible, but this is not essential as I am measuring the mass change and not the actual mass in this experiment.
- I will use tweezers when taking the potatoes out of the solutions so as not to alter the mass.
- I will use a different potato for each replicate to give more reliability of results.
- I will measure the solution exactly with the use of accurate syringes.
- I will leave no excess water when weighing.
Safety
For safety, I will ensure that I wear safety goggles, and make sure I cut downwards on a flat surface to protect my hands. I must also keep paper towels on the desk, so I can keep the area dry and avoid spillages. I must make sure that the apparatus is clean and in good working order. I must also wear a labcoat.
Apparatus
- Beakers (100ml) x 6
- Potato (fresh)
- Cork Borer
- Weighing scales
- Sucrose Solution ( Different concentrations)
- Distilled water
- Tweezers
- Tissue Paper
- Syringes
- Cellophane
Diagram cellophane
Method
- Cut out enough potato slices for three experiments of 6 different concentrations, plus a control. These are all 3.5cm in length.
- Measure out the correct percentage of sucrose and water to give varying concentrations into 100ml beakers.
- Measure the mass of the potato slices and which beaker they have been placed in.
- Place all potato slices in their beakers.
- Place cellophane over the top of these beakers to ensure that outside influences do not affect the results of the potatoes.
- Leave for 48 hours.
- Remove the potatoes using tweezers and measure the masses. Record the results.
Results
Graph – See next page
Conclusion
My results, but mostly my lines of best fit show quite clearly, that as I increased the concentration of sucrose, the mass decreased. This was because the water moved out by osmosis, as the water potential inside of the potato cell was now higher. When I decreased the concentration, the water potential was greater outside of the potato cells. This caused water to move in, by osmosis. My prediction, therefore, was correct, in saying that the higher the concentration, the more mass my potato pieces would loose. According to my line of best fit, my potatoes had an average water potential of 16.3%. I can determine this by looking at where my line of best fit crosses the x axis of my graph, indicating when there would be no change in mass. There were quite a few anomalies, where the points were very far away from the line of best fit, which had an effect on the rest of the data.
Evaluation
Even though it was lucky that I was able to fit in three repeats within the time restrictions, more repeats would always be useful. I would produce more replicates, using different potatoes, to prove that by coincidence, my conclusion doesn’t apply to just the potato I used. A wider range of concentration samples and potato samples would also be useful in calculating the potato’s water-potential more accurately. I found quite a few anomalous results, and this means that my results could have been more accurate and closer to my line of best fit.
Also the potato chips could have partially dried in the time I had to place all the chips in the beakers with the solutions in. this would have altered the results. This could have been prevented if more assistance was used and or more time allowance was given for the experiment.
The potato I used may have contained more starch towards the centre or different areas would contain more starch, so I could be using chips with different starch content that would change the rate of osmosis.
For an extended investigation, I could test osmosis using various fruits instead of a potato. This is because I know that fruits have a lower water potential because they contain no starch, and also sugar molecules are soluble and therefore osmosis should occur faster, which will be interesting to see what extent osmosis differs in fruits then potatoes.
