To find out the concentration of salt inside potato cells by conducting an experiment that uses the principles of osmosis.
Aim: To find out the concentration of salt inside potato cells by conducting an experiment that uses the principles of osmosis.
Background Information: Osmosis is the movement of water molecules from a high concentration to a low concentration until equilibrium is reached. This only involves the movement of water and is therefore different to diffusion. A situation where osmosis is extensively used is in plants, where they use it to get water from the roots to the rest of the plant. This also ensures the efficiency of the plants hydrostatic skeleton. The principle for this type of transport, being only water based, is the use of a selectively permeable membrane. The membrane consists of phospholipids. These have to main components, a hydrophilic head and a hydrophobic tail. These molecules then arrange themselves into a bilayer. This bilayer has a hydrophobic interior, which forms the basic anatomy of the membrane. Water particles are small enough to fit through this bilayer, and so can freely pass through it. However, such particles as sugar or salt are too big to fir through the membrane and are thus kept out.
Prediction: I predict that if the salt solution is hypertonic, higher than the inside of the potato, the potato should lose weight. This is due to the potato having a smaller concentration of salt and therefore a higher concentration of water, causing it to loose water, to reach equilibrium. Thus the loss if water will in turn cause a loss of weight. If the concentration of the salt solution is hypotonic, lower than the potato, then the potato will gain weight due to the same principles. If the concentrations are the same, equilibrium is already reached and therefore there will be no weight gained or lost.
Preliminary readings: Prior to the conduction of my main experiment I carried out some preliminary readings to make sure the experiment will work, and any problems that might occur. I also used my preliminary readings to determine the range of my testings. As these were only preliminary readings, I left the potatoes in the solutions for only 15 minutes, this may affect it if the full process was not carried out, but it will give me a vague idea. Here are my results
Concentration of salt solution
(grams per litre)
Starting weight (grams)
Finishing weight (grams)
Difference between starting and finishing weight (grams)
2.96
3
+0.04
6
2.97
2.95
-0.02
From these readings I can see that the solution is between 1 and 6 grams per litre of water. I know this because when I tested with 1 gram per litre the result was a gain in weight, meaning there is less salt per litre in the water. In contrast, when I tested with 6 grams, there was a ...
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Concentration of salt solution
(grams per litre)
Starting weight (grams)
Finishing weight (grams)
Difference between starting and finishing weight (grams)
2.96
3
+0.04
6
2.97
2.95
-0.02
From these readings I can see that the solution is between 1 and 6 grams per litre of water. I know this because when I tested with 1 gram per litre the result was a gain in weight, meaning there is less salt per litre in the water. In contrast, when I tested with 6 grams, there was a weight loss, meaning there was more salt per litre in the water solution. Therefore, I only need to test concentrations between 1 and 6 grams.
Risk assessment: There are no major risks, with the exception that there may be a hazard from broken glass, if any of the glass equipment if dropped or knocked, this is the only risk I can see.
Equipment: The equipment I will use is, potato, glass test tubes, scales, distilled water, paper towels, salt, a scalpel, distilled water, spatula, measuring cylinder, a bore, and a sieve, also a lot of stationary equipment will be used, such as pens, labels, paper etc.
Method:
*First using the bore cut out a cylinder of potato
* Use the scalpel knife to cut the pieces down to size, 12 are needed
*Get 6 test tubes and label each one to clearly show which concentration each is.
*Weight out the salt needed on the scales for each concentration, which needs to be divided by 50, as we are only using 20 ml's of water, so 1 gram = 0.02 grams
* Add each weighed amount of salt to the corresponding test tube
* Weigh each piece of potato before putting it into the solution, so that a comparison can be made between starting and finishing weight.
* Leave experiment for 48 hours, to allow the process to complete.
* Empty test tube of water through a sieve and then collect the left over pieces of potato, remove and excess water with a paper towel.
* Weigh the new potato and make a record of the readings.
* The experiment if now complete, except for the analysis of results, each experiment should be finished with the cleaning away of equipment and any other mess created.
* Log all results into a table, and make comparison between the starting weight and finishing weight, then take an average weight for each concentration.
Fair Test: here are the key factors that I incorporated into my experiment to make it a fair test:
* All potato used was from the same potato ensuring the same salt concentration throughout.
* Using scales, very accurate measurements are taken to make sure that we can get an accurate result at the end of the experiment.
* The scales where further used to ensure the accurate measurement of salt concentration.
* The experiment was repeated so that an average could be taken, giving a more accurate result.
* Each experiment was left for the same amount of time.
*Although there may have been minor differences in the time the potato had in the salt solution, as osmosis is very slow, this would have minimal effect if any at all.
Concentrations Used:
(Gpl = grams per litre) there are 1000 ml in 1 litre, and we are only using 20 so we have to divide 1000 by 20 to find out how much to divide the solutions by. 1000/20 = 50
GPL = 0.02 2GPL = 0.04 3 GPL = 0.06 4 GPL = 0.08 5 GPL = 0.1 6GPL = 0.12
Analysis: From my results I can clearly see that the isotonic solution of salt water will be somewhere between 3 and 4 grams per lire because when the experiment was carried out with 3 GPL the potato gained weight, in comparison, when the experiment was carried out with 4 grams per litre, the potato lost weight. This means that somewhere between these to figures lies the concentration of salt in a potato cell.
When I analyse my prediction and my results comparatively, I can see that my prediction was right, to prove this further though, I will need to do an experiment showing this more accurately, maybe 3.5 GPL.
Evaluation: I believe that my experiment was fairly reliable, but there may have been one anomalie, this would have been for grams per litre. From my results I would guess that the isotonic solution would be much closer to 3 than 4 as, at 4 grams there was a -14.5% change (providing this was not an anmalous result), and at 3 only 3.3% so 3 is only 3.3% away from being isotonic whereas 4GPL is 14.5% away. If I then look comparitvely at my graph and my results table then I shows that the isotonic solution is approximatley 3.2 grams per litre. I think that I have a good detailed method and if the experiment was repeated, or read by some one else, they would be able to follow my steps exactly and obtain very similar results.
I would improve my investigation in the following ways:
* I would be more accurate with my testings
*I would have added in another testing per concentration, giving me 3 results and therefore a more accurate average
* Maybe leaving the experiment for a longer period of time, as osmosis is a very slow process.
I think that I could have extended this project by mainly being more accurate, but there are some factors I could have incorporated to make this a more accurate experiment, Firstly if I had bigger pieces of potato, and difference in size would be a smaller percentage of the overall mass. Also after we were about to test the potatoes on the scales, we handled them, and wiped them with a paper towel. This could have drawn out a very small amount of water, from the surface of the potato affecting our result. Also our skin sweats, and sweat contains salt, this could have affected the experiment both when taking results and when primarily setting up the experiment, cutting the potato. To counteract this I could have used some equipment that would not affect it in this way.
Conclusion:
From my graph I can conclude that the internal salt concentration in a piece of potato is approximately 3.2 grams per litre. I think though that the test for 4 grams per litre was anomlaous, and therefore voids this conclusion. If this result was maybe -3.5%, then the result would be 3.5 gramms per litre. However the reasons for this not being an anomalous result, are the other results taken at concentration above it, they are all fairly close together suggesting that this is right. Overall from my experiment, we can see how osmosis works, trying to obtain an equilibrium. This helps scientist a great deal, especially when tryin to find the concentration of salt in water, or something similar. My experiment has been beneficial as it has shown me how hard it is to be accurate and fair when doing an experiment, and has given me experience. In conclusion I think that my experiment was a success, and showed clearly the effects of osmosis.
