My Predictions
As the concentration of the glucose solution increases, the potato slices will lose increasing amounts of water.
This is because of the negative water potential values of the potato and the solutions. The potato tissue has a certain negative value. If the glucose solution has a greater negative value than the potato tissue, and is therefore of a higher concentration, the potato tissue will lose water by osmosis until the concentrations are equal or until the potato has lost all its water. Therefore the water will diffuse by osmosis out of the potato from an area of high concentration down the concentration gradient. This will result in a loss of mass for the potato.
If however the glucose solution has a lesser negative value than that of the potato tissue then the potato tissue will take in water through osmosis until the two solutions are of equal concentration. Therefore the water will diffuse by osmosis into the potato from an area of high concentration down the concentration gradient. This will result in a gain of mass for the potato.
Diagrams of Apparatus Used:
Method
In this experiment we first of all cut discs of potato by using a cork borer and then we used a scalpel to cut slices of the cylinder. 20 of these discs were cut and then divided into 5 different groups. We separately weighed each of these groups on filter paper so before weighing them together we found the weights of each piece of filter paper then subtracted that from the weight of the filter paper and 4 disks of potato together to find the weight of the potato disks. We then placed a different group of potato discs into different test tubes. Each of these test tubes contained 15 millilitres of glucose solution of different concentrations. These concentrations increased by 0.25 moles/dm3 each time, starting at 0.00 M (distilled water). We put bungs in the tops of the test tubes and left them for 24 hours. When we came back we removed the potato disks from the test tubes being very careful not to mix the groups. We then blotted the potato disks dry and weighed them in the same way. We recorded the weight of the groups before and after they were put into the solutions and we then worked out the change in mass as a percentage of their original masses.
My Preliminary Results
From my preliminary results I can see that as the concentration of the glucose solution increased, the potato cells lost a greater amount of mass each time.
Alterations to be made:
Although the results of the preliminary experiment are pleasing and generally follow my predictions, I will need to make some alterations in order to make it more accurate for my final experiment. The first alteration I will make is to increase the concentration of the glucose solutions by 0.20 moles/dm3 each time instead of 0.25moles /dm3 as I did in my preliminary experiment. I will also increase the number of potato disks in each test tube. This will hopefully cancel out any mistakes or anomalies that may have occurred in my preliminary experiment. These changes will provide more accurate results and therefore it will also give a more accurate graph.
MY FINAL EXPERIMENT
Method:
For our final experiment we decided to use the same method and quantities of solutions as for our Preliminary experiment, and made all of the alterations listed above. We hoped that this would give us a far more accurate set of results.
Diagram of Apparatus;
1 2 3 4 5 6
Results
From my experiment I have obtained these results.
These are the weights of the different groups of 12 potato disks before putting them into the solutions.
These are the weights of the potato disks after 24 hours in the glucose solutions.
The percentage changes in mass were as follows:
Analysis
The first observation I made was after the potato cylinders had been placed in their solutions for a few hours. The potato disks in the 0.00M, 0.20M and 0.04M glucose solutions were floating and the potatoes in the 0.60M, 0.80M and 1.00M solutions were at the bottom of the test tube, this lead me to drawing my next conclusion.From my experiment it is clear that as the concentration of the glucose solution increases, the mass lost by the potato increases. The loss in mass can only be reliably attributed to a loss of water from the cells of the potato because no other substances are able to pass through the potato’s partially permeable membrane. The potato cells floated if they had lost mass, and were lower down in the test tube the more mass they had gained.
In my explanation I have used the examples of potato cell walls for partially permeable membranes, and glucose solutions and the contents of the potato cells provided the two substances on either side in my experiment. But of course this occurs with any partially permeable membrane and solution.
The graph shows that the potato in the 0.0M and 0.20M solutions is hyper-osmotic; this means that there is higher water potential in the distilled water and 0.25M solution than in the potato, which has a high concentration of solute. Therefore this is why the water diffuses by osmosis down the concentration gradient from high concentration to low concentration, resulting in the potato gaining mass.
The opposite occurs in the solutions where the molarity is higher. In the 0.40M, 0.0.60M, 0.80M and 1.00M solutions, the potato is hypo-osmotic because there is a higher concentration of water inside the potato than in the solutions, therefore the water diffused out of the potatoes by osmosis, down the concentration gradient and into the solutions outside; this resulted in the loss of mass.
Although Osmosis is the passage of water through a partially permeable membrane fro a low concentration to a high concentration, water is not consciously passing through the membrane in order to equalise the concentrations either side of it. In actual fact there is no unequal water concentration on either side that makes the molecules pass through the membrane. This is because the membrane does not affect the water molecules at all – they simple pass straight through the pores in it and the pressure in the pores is entirely balanced. However on the side of the membrane that holds the glucose solution, there are glucose molecules which cannot pass through the partially permeable membrane. This is represented in this diagram of a partially permeable membrane, water molecules and glucose molecules.
These glucose molecules are what give the solutions its negative value. The unequal pressure that these molecules exert on the membrane by random collision with the pores in it causes the partially permeable membrane to expand. This means that the cells swell therefore encompassing more water. The cells stay swollen because the glucose molecules are still colliding with the pores, unable to get through, but they are doing this less often and so the cells cease to expand.
From this explanation I have given I can say that my predictions were correct about the results of my experiment but that they did not cover fully the detail that that I have since discovered.
Evaluation
Although I am pleased with the results of my experiment I would make one or two changes next time I did it.
My first change would be to increase the number of solutions I carry out my experiment with. Instead of increasing the concentration by 0.2 moles each time I would increase it by 0.1 moles. By doing this I would make my results a great deal more accurate than they were this time.
Another change I would make would be to repeat my experiment twice more in order to dispose of any anomalies that may have occurred. Although I did not repeat my experiment this time, by using twelve disks of potato in each test tube there are fewer anomalies than there may have been; because if one of the potato disks acted abnormally then it would be almost cancelled out by the ones that behaved normally.
There was one anomaly in my experiment. This occurred at 0.4 moles/dm3; in test tube 4. This could have been caused by a few different sources of error. Although the effect of these will have been toned down by the other slices there is still an effect from them.
Particles can gain energy by heat transfer. Therefore if the temperature of one or both of the concentrations increases then the particles will gain more energy and move a lot faster. They will thus collide more frequently will the partially permeable membrane and so the speed at which osmosis takes place. It is possible that one of the test tubes or solutions may have been at a higher temperature than the others. This would have caused the particles to move at a greater rate and therefore the rate of osmosis would also increase slightly. This could be counteracted by sitting the test tubes in a warm water bath during the experiment in order to keep a steady temperature. However, this would not be a very practical way to keep the temperature constant, as the warm water would not stay at the same temperature for 24 hours, it would have to be kept warm with a heater of some sorts. This could not be relied upon to keep the water at a regular temperature; it would actually keep increasing the temperature.
Concentration has a great effect on plant cells; if the concentration of the glucose solution is higher than the concentration inside the potato cells, then water will pass out of the potato cells and into solution by osmosis. If the glucose solution were less concentrated than the cell contents, then water would pass into the cell through osmosis. However, if the concentration inside and outside the cell are exactly equal, then water will pass out of and into the cell at the same rate through random collision with the partially permeable membrane. This means that the cell will be unaffected by being placed in a glucose solution. I do not think that it is very likely that this is the cause of the anomaly in my results. This is because we did not mix the solutions that we used. They were prepared especially for our investigation. Despite this, it is possible that human error is the cause for one of the solutions being either slightly too strong or weak than it should have been. This would be a perfectly reasonable origin of the anomaly in my experiment.
- Surface area of partially permeable membrane:
The surface area of the potato is both very hard to measure accurately and very difficult to keep at a constant level. The greater the surface area of the potato is, the greater the area of partially permeable membrane there is available for osmosis to take place. Accordingly, the greater the surface area of the potato, the greater the amount of osmosis which can take place. It is not unlikely that this is the cause of the anomaly; because the surface area of the potato disks, as I have already stated, is very hard to keep at a constant level. It could well be the case that one or two of the potato disks in the test tube with 0.40 moles/dm3 glucose solution had a slightly greater surface area due to the angle at which they were cut.
Apart from these causes, there may be others that are not factors which directly affect the rate of osmosis.
Firstly, when I dried off the excess water on the potato cylinders after the experiment and before I weighed them, I used a paper towel. This might have either taken some water out of the potato or it might have left some excess water on the potato. This part of the experiment is difficult to come up with an accurate and fair method, as other ways would also lead to some slight mistakes.
Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result.
In my preliminary experiment I used four potato disks in each test tube and varied the glucose concentrations by 0.25 moles each time. Although this preliminary experiment was useful to collate the data I later obtained in my final experiment, they were surplus results as the number of disks I used in my final experiment ensured that any anomalies hardly affected my results at all.
Finally, I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, and then I would be able to see the cells in greater detail and draw some more observational results.
From studying the possible causes for the anomaly in my experiment I have concluded that the most likely cause is human error in cutting the potato disks, or a change in temperature for the test tube in question.