- 10 test tubes
- Tile
- 4 solutions, which are 1.25%, 2.5%, 5% and 10%
- Ruler
- Razor blade
- Calculator
- Test tube rack
- Measuring cylinder
- Distilled water
- Weighing scales
- Goggles
Working safely
Safety is an important aspect in every experiment, even if the experiment seems to be very harmless. And that is why we take this into consideration, no matter what.
We will be using a very sharp razor blade, which could injure someone if it’s not handled properly. And we will also be careful that the solutions don’t get into our bodies internally, just in case, because we are not fully aware of the damage it could do to us. But other than that, there weren’t any bigger matters to be cautious of. To ensure safety, the test tubes will be kept in test tube racks so that they cannot be knocked over and will be stored safely. The potato chips will be cut on a white tile with a blade so that no one or thing will be cut or damaged. Also while using the blade everyone will have to wear safety goggles.
Method
A range of sucrose sugar solutions will be prepared with concentrations distilled water, 1.25 %, 2.5 %, 5 % and 10 %. This will be done by adding varying amounts of distilled water to varying amounts of sucrose solution. Sections of potato will be cut using a razor blade on a tile and will be measured using a ruler. We cut the depth of the potato to 5cm by 1cm by 1cm. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur. The mass of each chip will be measured as well so that more results can be obtained. One potato chip will be placed in each test tube each time so that I can take an average for each tube. I will do two experiments for each solution. I will use 20 ml of each concentration of sugar solution and once in the test tubes they each will be labelled. The potato pieces will then be placed in the different test tubes and then left for one day for osmosis to fully take place and be stored in a dry place by the teacher. Then the potato pieces will be removed, the surface solution removed using paper towels and then they will be re-weighed. If I then have time afterwards I will repeat this experiment again as to obtain a second set of results. This will hopefully produce more accurate results from which I will be able to draw a more accurate conclusion.
Step by step Method:
1. I took two average sized ground potatoes and checked that they were both healthy and hard.
2. Using a razor blade I peeled the potatoes and cut each one into an even block approximately 5cm by 1cm on a white tile.
3. Using a razor blade and ruler I cut the potato into chips, which were 5 cm long. I had to be very careful whilst cutting the potato, as the blade is exceptionally sharp. I then had 15 chips.
4. Taking a test tube rack I placed 10 test tubes and then labelled them distilled water, 1.25%, 2.5 %, 5% and 10 %.
5. Using a measuring cylinder I measured out different amounts of sucrose solution (20ml) and distilled water, which I then poured into the test tubes in a percentage ratio giving me the various percentage concentrations.
6. I will then weigh every potato chip on an electronic balance and made sure they weighed the same.
7. I swiftly put 1 potato chip into each beaker and then waited for osmosis to occur. I then left it for a day.
8. Whilst waiting I set up a basic table for my results.
9. After one day I drained out the solutions in the sink and placed all the chips on the paper towel in the order I had put them in the test tubes as to not confuse myself as to which chip came from which solution.
10. I dried each chip with the paper towel and then recorded how each potato increased or decreased in size with a ruler.
11. I then recorded all my results on a basic table.
Diagram
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1cm
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Obtaining
I obtained these results by taking two average sized ground potatoes and checked that they were both healthy and hard. Using a razor blade I peeled the potatoes and cut each one into an even block approximately 5cm by 1cm by 1cm on a white tile. Then I used a razor blade and ruler to cut the potato into chips, which were 5 cm long. I was very careful whilst cutting the potato, as the blade is exceptionally sharp. I then had 15 chips. Taking a test tube rack I placed 10 test tubes and then labelled them distilled water, 1.25%, 2.5 %, 5% and 10 %. Using a measuring cylinder I measured out different amounts of sucrose solution (20ml) and distilled water, which I then poured into the test tubes in a percentage ratio giving me the various percentage concentrations. In my planning I said that I would weigh the potatoes but unfortunately they were not available for this investigation. I then swiftly put 1 potato chip into each beaker and then waited for osmosis to occur. I then left it for a day. The next day I drained out the solutions in the sink and placed all the chips on the paper towel in the order I had put them in the test tubes as to not confuse myself as to which chip came from which solution. I dried each chip carefully with a paper towel and then recorded how each potato increased in mass as to see if they increased or decreased in size with a ruler.
Whilst doing this I made sure I worked safely. I used a very sharp razor blade, and made sure I used it carefully as it could injure someone if it’s not handled properly. Also while I used the blade everyone wore safety goggles.
And I was also careful with the solutions that they don’t get into our bodies internally, just in case, because we are not fully aware of the damage it could do to us. I also stored them safely on the desk making sure know-one could knock into the solutions.
Analysing evidence and concluding
In this investigation we have found out that osmosis is a very importing factor in this experiment. Osmosis is defined as the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration. In a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium). Knowing that osmosis will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysised (when cytoplasm is drawn away from the cell wall) if water flows out of them, and thus change their volume, we want to test the hypothesis that: If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand. This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. With the potato cells, the cells would increase in length volume and mass because of the extra water. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.
The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.
However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.
In my table of results the trends that appear on my graphs and tables are quite similar. I recorded two sets of results both using the same equipment and using the same approach to find the results. When I recorded my second results the weather that day was much hotter than the day before which could have increased the rate of reaction of osmosis. The two result tables are very accurate as the differences between them are almost exact. Which shows that I can take measurements safely and accurately. I managed to make my table by simply taking away my average of the potato to the size of the potato before osmosis took place. For example my average for distilled water after 24 hour is 5.8cm, which I took away from its privies size, which is 5cm. (5.8-5= 0.8cm)
Using the graph, with lines of best fit, I can draw a conclusion from my experiment. Firstly I can see that the correlation of my graph has a high and strong negative correlation, meaning that as the concentration increased the change in millimetre for the potato decreases. This is because when the concentration is increased osmosis in a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. . When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).
The results show that, in accordance with my prediction, the cylinders will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane, such as a cell membrane. This concentration is the osmolar concentration (the total solute concentration) of the sap inside the cell.
It shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. When the concentration reaches above 0.8 M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed. From the graph an estimate to the concentration of the potato cell can be made as 2.7 M, as this is the point where the potato is not increasing or decreasing in mass, this is known as the isotonic point. This is where no osmosis is taking place; both the potato and the solution have an identical molar concentration. The next point, 5 M looses approximately 4.0 %. This shows that the water potential of the sugar solution in the beaker is weaker than that of the potato chip. The next, 10 M, looses approximately 8.0 % in mass. This shows that the salt solution has an even weaker water potential than 5 M and that osmosis took place. This is why the potato lost even more mass, and it shows that the water potential in the beaker is less than that of the potato chip. This pattern carries on through the graph, and even more mass is lost, as more water moves out of the potato into the solution. My results also match with my initial predictions.
Evaluating evidence
In my method I carried out each procedure with the intention to make it as fair and accurate as possible. Each pupil in my group had their own separate job in this investigation so there was a less chance of a mistake or an anomalous result to take place. There were not any anomalous results, but some results were not as close to the line as others. This may have been caused by human error, or out of my results could have been inaccurate, and changed the average drastically. Or perhaps the potato chip was not cut accurately, or that part of the potatoes, cells did not loose/gain mass well. My results did vary a lot for the same concentration. The fact that a certain part of the potato may not be the same as another was shown as the chips all of the same length were not the same weight, or even close. This is another reason to use cloned potatoes. My results were consistent, and there were no results that they were ignored. I could extend my enquiry by testing the percentage change in mass with morality using a different substance. By this I mean using a different vegetable, perhaps celery or cucumber. Then I could find out whether osmosis occurs with the same patterns and trends with any vegetable.
In my opinion the experiment went well. I gained a good set of results and altogether I am pleased with the investigation. The graph obtained from the results has a strong resemblance to the one in the prediction. This suggests that the experiment was fairly accurate. Although 2 repetitions of each concentration are sufficient, there wasn’t an anomalous result. A possible factor affecting my results is that although the cylinders were taken from the same potato there is no guaranteeing that the density remains the same throughout the potato. Another factor is that the potato from which the cylinders are taken could be abnormal - this could be prevented by amalgamating sets of results, for example of a whole class, where each experimenter used a different potato. I think that three sets of results was a good number.
The improvements that I would introduce to the investigation would to see how temperature would affect the rate of osmosis. Temperature could also be changed for example the samples could be placed in different water baths and brought up to different temperatures to see if temperature played its part in the osmosis of potatoes. 5 sets of 5 potatoes could be placed in water baths at 10oC, 20oC, 30oC, 50oC and 60oC. Then leave them for 24 hours making sure all the variables in the first experiment still apply however just using one molar solution. Then after 24 hours re-weigh the samples and record the result. I would expect that at high temperatures the potato samples would osmosise the most. This is because at high temperatures the solutions water molecules would move faster and therefore equal the concentration faster. A preliminary experiment could be set up beforehand to find out how long the experiment should be kept going because if the concentration of the potatoes equalises then the weight of the potatoes will be almost exactly the same.
Also we could use different types of potatoes like new potatoes, King Edwards etc could be used to see if the potato type actually matters when doing this investigation. Instead of just putting one potato in the test tube we could put three, as it should give you a better set of results and accurate graphs. By doing this we can see if less or more osmosis will take place, as it will be hard for the solutions to react with all the potatoes.
However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.10m, 1.15m, 1.20m, and so on. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate.
The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a mechanical device to cut the potato, as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.00g but to 0.0000g. Other investigations could include using different varieties of potato or different cell tissues e.g. carrot. As this would be interesting to find out if other types of vegetables react the same way.
Obviously it would have been good to do more repeats. Two tests were managed each time but if one had been wrong this could have dramatically changed the average time and therefore rate of reaction. In some cases we were finding it hard to finish the investigation in time so I would have preferred to have more time so we could had have more time preparing and less room for mistakes and under strict time it was hard to get perfect results.
Overall I think the investigation worked really well as my results and graphs are quite accurate as I made the investigation fair as possible by repeating the investigation twice and using the right equipment to make it safe. My results were accurate and completed twice to assure a higher quality result assuring us that our results were even more accurate. Our results were very similar so it proves that we carried out the investigation carefully so that we achieved good results.
The accuracy of the experiment was accurate to suit our purpose to make it more accurate several steps could have been taken.
1: Human error: Human error could have been reduced by taken more accurate measurements to a uniform result. This could be attempted by using a tool which could be set to a set length to cut the potato.
2: Instead of the range of the potato being 0.0 to 0.30 the range could be decreased to 0.20 to 0.30. This would produce more accurate results. Also increasing the number of results so instead of taking results from 7 different molarity levels 10 molarity levels could be taken. This would make the lines of best fit on the graphs more accurate, therefore the end results would be more accurate.
3: If the potatoes did not rest against the sides then they would all have the same amount of surface area. This is the same for the potato samples that float therefore exposing themselves to air and the samples that sink stopping osmosis occurring on the areas that are touching the bottom of the test tube.
Reliability: The results were reliable to take a reasonably accurate result. However the steps above (1,2 and 3) would produce an even more accurate result.
