To investigate the effect of varying concentration of glucose solutions on the osmotic activity between the solution and potato slices.
INVESTIGATING OSMOSIS IN PLANT CELLS
AIM:
To investigate the effect of varying concentration of glucose solutions on the osmotic activity between the solution and potato slices.
SCIENTIFIC THEORY:
Osmosis is defined as the net movement of water molecules from a region of high concentration to a region of low concentration of water, through a partially permeable membrane, which only lets smaller molecules through. This can also be described as movement down the concentration gradient. The molecules continue to diffuse through the membrane until both sides reach a state of equilibrium. This is also known as the isotonic point when the molecules are equally distributed on both sides of the membrane so that no one area has a higher or lower concentration than the other.
Plant cells have a strong cell wall and a partially permeable cell membrane around them. When cells are surrounded by a solution more dilute than their own, (hypotonic solution-with more water than solute molecules), the cell swells up and becomes "turgid". This is because the water molecules diffuse from the solution to the plant to equal out the concentration levels. The strong cell wall prevents them from bursting.
Figure 1- A turgid plant cells in more dilute surroundings (from "Biological
Sciences Review")
When they are surrounded by a solution more concentrated than their own, (hypertonic- less water molecules), they shrink and become "plasmolysed". This is because the water molecules are highly concentrated in the plant, so they diffuse out into the solution to create a state of equilibrium.
Figure 2- A plasmolysed plant cell in more concentrated solution (from
"Biological Sciences Review")
For this particular investigation potato slices will play the role of the plant cell. I will place the potato slices into six different glucose sugar concentrated solutions and record the percentage increase or decrease in the mass of the potatoes.
PRELIMINARY EXPERIMENT:
Preliminary work was carried out to help clarify the method of the experiment and make changes. It also helped to me decide the range of observations to be made so that the results obtained are as accurate as they could possibly be.
Two unknown glucose sugar solutions were given, Solution A and Solution B. 3 Potato slices were kept for 24 hours in each solution. The results were as follows:
Table 1- Results of preliminary experiment
Mass before (g)
Mass after (g)
% change
SOLUTION A
2.1
2.4
4.3
2.1
2.5
9
SOLUTION B
2.1
.5
-28.6
2.1
.6
-23.8
As there was a percentage increase in the mass of the potatoes slices in Solution A, I know that Solution A had a high concentration of water. Solution B had a low concentration of water, as there was a percentage decrease in the mass of the potatoes.
WHAT I LEARNED FROM MY PRELIMINARY
Two potato slices were put into each solution. The percentage change of the potatoes in the same solution differed slightly. This was due to an inaccuracy in the method. This means that it was not as accurate as it should have been. As a result of the preliminary the following changes were made to my method:
> Measure and weigh the potato slices more accurately. Do not lean on the worktop when taking readings from the mass balance as this sometimes affects the reading.
> Put three potato slices into each solution rather than two so the results are more accurate.
> Use separate measuring cylinders for the different solutions to prevent the solutions from being mixed and therefore reducing the accuracy of the results.
> Measure the volume of the solution as accurately as possible by making sure that the bottom of the meniscus is parallel and as close to ...
This is a preview of the whole essay
> Measure and weigh the potato slices more accurately. Do not lean on the worktop when taking readings from the mass balance as this sometimes affects the reading.
> Put three potato slices into each solution rather than two so the results are more accurate.
> Use separate measuring cylinders for the different solutions to prevent the solutions from being mixed and therefore reducing the accuracy of the results.
> Measure the volume of the solution as accurately as possible by making sure that the bottom of the meniscus is parallel and as close to the measurement mark on the measuring cylinder as possible.
PREDICTION:
I predict that the higher the concentration and molarity of the glucose sugar solution, the more the potato slice will shrink and lose water, and become "flaccid". As a result the mass of the potato will decrease because water will move out of the potato by osmosis into the solution. When the potato slices are kept in the lower concentrated glucose sugar solutions they will swell up and become turgid. The mass of the potato will increase as a result. This is because the water molecules will move by osmosis from the area of high water concentration (the solution) to the area of low water concentration (the potato)
APPARATUS NEEDED:
6 petri dishes and lids
A No. 5 Cork Borer (1cm diameter)
3 potatoes
2 measuring cylinder
Stirring Rod
A cutting board
A scalpel
A ruler
An electronic mass balance (to 2 d.p.)
6 different glucose concentrated solutions (0M, 0,2M, 0.4M, 0.6M, 0.8M, 1.0M, 1.2M)
METHOD:
) Using the potato tuber, scalpel, ruler and mass balance, 18 slices of potato will be cut out, six from each potato. This must be done as accurately as possible as a change in the surface area of the potato may act as a variable and affect the rate of osmosis.
2) Measure 40 cm3 of each 6 solutions. Pour each of the measured solutions into a separate petri dish. The different solutions will be 0M, 0,2M, 0.4M, 0.6M, 0.8M, 1.0M & 1.2M.
3) Three potato slices will be placed in each petri dish so that an average result can be taken at the end. The petri dishes will then be labelled depending on the molarity of the solution in them. The petri dishes will be left for 24 hours (1 day)
4) After 24 hours the potato slices will be removed and weighed. Before they are weighed, they should be dried carefully so that only the potato and not the solution is being weighed. The new masses of each slice will be recorded as accurately as possible.
FAIR TEST: So that it is a fair test certain aspects of the test will carefully have to be kept the same whilst the one key variable, the concentration of the solution, changes. Other factors in the experiment need to be carefully controlled so that the experiment is fair and so there are no other variables. The volume of the solution, the size (surface area) and mass of the potato are non-variables. The temperature of the surroundings should also be attempted to keep the same as much as possible as a dramatic increase could affect the experiment. All readings must be taken as accurately as possible so that the results achieved are reliable.
SAFETY: It is very important and the following precautions must be carried out to prevent any accidents:
* Working area should be dry and clean
* Move bags and coats away
* Tie hair back and wear a safety overall to protect clothing
* Be extra careful when using the scalpel and potato tuber as they are very sharp and one can easily be cut.
* Wipe up any spillages.
There was no need for safety glasses as no corrosive solutions were used the experiment.
SOURCES OF INFORMATION:
) Mr Rossi- My biology teacher
2) "CGP Biology Revision Guide for GCSE Double Science"
3) P. 14- p.16 "Biological Sciences Review-January 1995"
4) "A Science Dictionary for GCSE" by Ron Joyner
5) Website: " www.ukonline.co.uk/website/spinneret/life/osmdia.htm"
6) Preliminary Results from classmate Yasmine Li
ANALYSING EVIDENCE AND DRAWING CONCLUSIONS
From Tables 2 and 3 and Figure 1, it can be seen that as predicted the potato slices that were put in a low glucose concentrated solution gained mass and the potato slices that were put in a high glucose concentrated solution lost mass. Table 4 shows the visual observations made at the time. They correspond with the quantative data as the loss and gain of mass is visible as the potato slices placed in a low glucose concentration visibly swelled and the slices placed in a higher concentration of glucose solution could be seen to shrink and became extremely thin and wrinkly.
The graph in Figure 1 shows the straight line of best fit for the percentage change in mass of the potato chips over the course of 24 hours. It can be seen that all of the readings and mean results are very close to the straight line of best fit, clearly showing that my results are fairly accurate and reliable.
It can also be observed from Figure 1 and Table 3 that the potato slices placed in a o.4M concentrated solution and below have a percentage increase in their mass. This means that the 0M, 0.2M and the 0.4M solutions were hypotonic as they were more dilute (contained more water molecules) than the plant cells that they surrounded and so as a result there was a net movement of H20 molecules from the solution to the plant cells to equal out the concentration levels. As there was now more water molecules in the plant cells, they swelled up and became turgid resulting in the whole potato slice increasing in size. The results in table 4 correspond with this finding, as they appear to have visually increased in size and swelled.
Figure1 and Table 3 also show that the potato slices placed in a 0.6M concentrated solution and above have a percentage decrease in their mass. This means that the 0.6M, 0.8M, 1.0M and 1.2M solutions were hypertonic as they were more glucose concentrated and so had less water molecules than the plant ells that they surrounded. As a result there was a net movement of H20 molecules from the plant cells to the surrounding solution to equal out the concentration levels. As there was now less water molecules in the plant cells they shrunk and became plasmolysed resulting in the whole potato slice decreasing in size. The results in table 4 correspond with this finding as the potato slices appear to have visually shrunk in size and become shrivelled up and wrinkly.
My graph and my results therefore clearly match my initial predictions that in a low glucose concentrated solution the potato slices will increase is mass and size as the cells become turgid and that in a high glucose concentrated solution the potato will lose mass and shrink as it loses water and its cells become plasmolysed.
The graph also shows the x-axis and line of best fit intercept to be 0.37M. It is at this point where the potato is not increasing or decreasing in mass. This is an extimate of the isotonic point where there is no net movement of water from the glucose solution into the potato from the potato into the solution. This is where no osmosis is taking place and both the potato and the solution have an identical molar concentration.
CONCLUSION
As the concentration of the solution increases and it is less dilute, the mass and size of the potato decreases as a result of osmosis. This is because there is a net movement of water molecules from its higher concentration in the potato cells to the lower concentration of water molecules in solution surrounding it to reach a state of equilibrium and balance out the concentration level.
The solution becomes more dilute and its concentration increases the mass and size of the plant cell will increase due to osmosis. This is because there is a net movement of water molecules from its higher concentration in the solution to the lower concentration of water molecules in the plant cells it surrounds so that the concentration levels of water and glucose are equal.
EVALUATION
Overall In my opinion I think the experiment was very successful. I think that the experiment was carried was at a reasonable standard of accuracy as I obtained a large quantity of accurate data from which I was able to create informative graphs and draw precise conclusions. Also most of my results followed a similar pattern.
The graph in figure 1 showed a visual pattern and nearly all of the points were close to the line of best fit. The data in Tables 2 and 3 were close to the mean calculated and the information in Table 4 followed a consecutive pattern.
Some results were not as close to the line of best fit as others. They were:
> Anomalous 1- at o.8M
> Anomalous 2- at 1.2M
I think that the anomalous results were mainly due to human error or a few inaccuracies in my method. They were:
* The potatoes were measured on a scale which had an accuracy of 2.d.p. If a more accurate measurement is needed a scale to 4d.p. could have been used. (e.g. not 0.00g but 0.0000g.)
* The measuring of the solutions was not as accurately as it should have been. A burette could have been used as a more accurate way to measure out the solutions. It would ensure that I had an accurate amount of fluid in each test tube.
* When the potato slices were removed from the petri dishes and dried, I may have dried some more thoroughly than others and so some would have had more excess water, which would add to the mass.
* Some solution spilt out of the petri dish which was filled completely to the top. If repeating the experiment the volume used could be reduced to fit more appropriately in the petri dish.
In general however I think that my results were fairly precise as my method to carry out this experiment was fairly accurate. As a result it was a reasonably fair test. Some of the main positive aspects of my procedure that were the cause of the accurate results were:
* The lids on the petri dishes were kept on during the 24 hours to prevent water evaporation, as this would have increased the molarity of the solution. A loss of H20 molecules through evaporation would have increased the glucose concentration as the same number of glucose molecules would have been dissolved in less water.
* The experiment lasted for 24 hours and so this was more than enough time for the sufficient osmosis to occur.
* A wide range of different concentrated glucose solutions was used and so a wide range of data was collected
* Three potato slices were used for each concentration to calculate a mean and so a wide range of data was collected.
* The No 5 Cork borer used to cut the potato slices had a diameter of 1cm and so all the had the same surface area making it a fair a accurate test.
All of these factors contributed to a fairly reliable set of results.
However there was limited time and I needed to test a wide range of concentrations to improve the validity of the data being collected. As a result two groups set up the experiment together. One group measured 0M, 0.4M, 0.8M and 1.2M solutions and the other measured the 0.2, 0.6M and 1M solutions. However all the final measurements were taken by the first group. As a result the results could have been inaccurate as both groups had different cutting and measuring methods for the potato slices and the solutions. Also different equipment was used e.g mass balance and potato tuber although they were to the same accuracy. However as the first group took all the final results only one drying method was used.
To further investigate osmosis in plant cells:
> I could measure the mass change every 1 hour for 4-6 hours to work out when all the percentage change took place and not leave the slices in the solution any longer than they should be. However this is rather difficult in a school situation.
> I could possibly use more concentrations so that my results would be more varied. I.e. 0.10M, 0.30M, 0.50M, 0.70M e.t.c. Many concentrations around 0.37M, the estimated isotonic point, should be tested on to find out a more accurate level at which there is no net movement of water or glucose to equal out the unbalance.
> I could cut all the potato slices from one potato so that the variation between individual potatoes does not interfere in the experiment and undermine the experiment's accuracy