In 1.0M solution. When osmosis takes place in the potatoes, which are in a solution of high concentration of sucrose, the water molecules in the potatoes will move by osmosis through the selectively permeable cell membrane into the solution of low concentration of water. The water molecules will move out of the cells of the potatoes into the solution. This happens because osmosis is the movement of water molecules from an area of high concentration to an area of low concentration. So therefore the water molecules in the potatoes will move through the selectively permeable cell membrane out of the potato into the solution, which is less concentrated in water than the cells. The water molecules have then moved from an area of high concentration to an area of low concentration. These potatoes will be soft and flimsy because they contain less water than they did before. These potatoes will weigh less than they did before. Their weight has decreased because they don’t have the same number of water molecules in their cells as before. The cells are plasmalysed. The cells cannot possibly loose any more weight (water) because the cells will die.
I predict that the results graph will look like this:
I predict that the cell sap concentration will be around 0.4M. The cell sap concentration is when the cell is neither turgid nor plasmalysed. When the concentration of water is the same in the cells and in the solution. In the graph you know if osmosis has taken place when the weights of the potatoes change in a roughly straight from the top corner at 0.0M to the bottom right corner of 1.0M.
A diagram of the process of Osmosis looks like this:
The particles of water will keep defusing until the particles are uniform.
The apparatus we are going to use in the investigation is:
- Measuring cylinder, 10cm3
- Test tubes, 18
- Test tube racks, 3
- Scalpel
- Cork borer
- Ruler
- Balance, 2 decimal places
- Potatoes
- Beaker of Sucrose solution (1.0M)
- Beaker of distilled water
- Sticky labels
To make the investigation as accurate as I can, I will use 18 test tubes so that I can do three repeats for each concentration of sucrose. This will ensure that I get the fairest results. Each repeat I do I will label with an A, B, or C to be able to identify exactly which piece of potato is which. I will use sticky labels to do label the different test tubes. I will use the three results for each concentration to find the average percentage change in weight of the potatoes. I will plot my results in a table like the following:
I carried out the investigation as follows:
Safety:
The hazards of the investigation are; people could trip over bags or stools. To prevent this happening put the stools under the benches and bags on the back of the room out of the way. Be sensible with equipment, e.g. knives. Any broken test tubes must be thrown away quickly to avoid people cutting themselves.
Procedure:
I collected 18 test tubes and placed each one in a test tube rack. On each test tube I placed a sticky label saying which concentration is in the test tube and which number repeat it is. I used a small measuring cylinder, which reads up to 10cm3. For the concentration of 0.0M I measured 10cm3 of distilled water. For 0.2M I measured 8cm3 of distilled water and 2cm3 of sucrose. For 0.4M I measured 6cm3 of distilled water and 4cm3 of sucrose and so on for each molarity. The readings I took from the measuring cylinder were from the bottom of the meniscus because water clings to the sides of the measuring cylinder. Before I place the pieces of potato in the test tubes with the solution I will make sure that I have filled each tube so that some of the potatoes have not been in the solution for longer than others. I will then after all test tubes have been filled with the right amount place the pieces of potato in the tubes, after these have been weighed.
I will cut 18 cylindrical pieces from as few potatoes as possible with a cork borer, this makes sure they are equally round with the same circumference. Each piece will be the same length and size. The surface area of each should be the same. This is important for a fair test. Then after I cut the pieces I will weigh each piece individually and record its weight, then I will place them all in the test tubes at the same time. I will discard any pieces of potatoes that are not the right shape, incomplete or too small.
When I finish setting up the investigation I will weigh each piece of potato and note its current weight to 2 decimal places. Then after weighing them I will place all 3 test tube racks at in the same place so that they all are exposed to the same temperature. I will leave the investigation in the same place for 25 hours before I collect the results. I will not leave the potatoes longer so that the cells are not too badly damaged to be handled when I collect my results.
When I collect my results I will take out each of the potatoes and place them in order on paper towels so that any excess solution can be removed. The potatoes will be dried for an equal amount of time. I will place them in order so that I know which piece came from which solution and I can easily and accurately record my results. I will place each piece individually on a two decimal place balance and note the weight I read from the balance in the table on the previous page. I will use the information that I have collected to plot a graph similar to one on page 2. I will use the graph and table to write a conclusion and evaluation of the investigation.
Method My method was exactly how I described it in my procedure of my plan of the investigation. I did however first use a balance of only one decimal place, but then changed over to one that is more accurate with two decimal places.
Results weight and change in weight of potatoes, at the start and end of the investigation.
Conclusion
To determine the range of concentrations I used, I did a pilot experiment before everything, by placing two pieces of potato in a) 10cm distilled water and b) 10cm 1.0M sucrose solution. I then decided that it would be appropriate to use concentrations from 0.0M to 1.0M.
From my results table and graph I found that the cell sap concentration of the potatoes was 0.15M of sucrose in the solution. Therefore the tissue of the potatoes could not loose nor gain water at 0.15M of sucrose, so the concentration of water was even in the test tube and in the tissue. I think that these results are not perfectly accurate, as I would have liked them to be. My results do not match my hypothesis because the cell sap concentration was lower than I predicted. I predicted the cell sap concentration would be about 0.4M. I think that what went wrong was that at the beginning of the investigation I measured the concentrations wrong and inaccurately. Or the potatoes that I used, because all the cylinders didn’t come from the same potato, had different properties. I wanted to find out if that is the case. To find that out I redid the molarities of 0.0 and 0.2. I repeated the molarities twice before I wrote my results for those molarities.
My results could have been wrong because, the measurements of solutions/potato pieces could have been wrong. Also the different pies of potato did not all weight the same, in some cases there were quite large differences in weight. The different potatoes I used to get the cylinders of potatoes could have had different properties so their cell sap could of varied.
The results I obtained were as follows:
This time I found that the cell sap concentration is not at 0.15M but more than 0.2M. Therefore my investigation went wrong at some point. I think that it went wrong when I was measuring the molarities of the distilled water and sucrose solution. I think that the cell sap concentration would be 0.3M if I did more concentrations.
Evaluation
If I were going to repeat the experiment I would alter the following:
- I would use a burette instead of a measuring cylinder to measure the volume of liquid. It is more accurate than a measuring cylinder.
- I would use calipers to measure the length of the potatoes. Calipers are more accurate than rulers.
- I would make sure that the pieces of potatoes all weighed within a 0.05g ratio. So my results are as fair as possible.
- I would make the pieces of potatoes smaller than I used so that I can get all the pieces from the same potato. I used different potatoes to get all the pieces. Different potatoes may have different properties.
- I would leave the test tubes with the potatoes in a water bath so they remain at a constant temperature.
- When I dry the potatoes I should make sure that I dry them all the same amount of time. I should do this because the tissues I used could have absorbed more water from the potatoes.
- To make my results more accurate I could do more concentrations than I did. Instead of 0.0, 0.2, 0.4, 0.6, etc I should use 0.0, 0.1, 0.2, 0.3, 0.4, etc up to 1M.
- I could possibly use another vegetable.
During my investigation nothing went wrong. There was nothing unexpected that happened. However my results were not what I expected. The results that I obtained, from the experiment to determine if my investigation was right, showed that my results from my primary investigation were inaccurate. The secondary results showed that my cell sap concentration was closer to what I predicted at the beginning of the investigation. I think that I got the results in my primary investigation because of inaccurate measuring of the liquids.
From my graph I can see that there is an odd result at the molarity of 0.6M. The graph follows the pattern that I predicted in my hypothesis but the results do not follow the pattern that I predicted. The results that I obtained had a cell sap concentration at a very different molarity than I predicted. In my