To explain this theory I will show how a cells activity would change when put in a high concentration of sucrose solution and then in a low concentration of sucrose solution using the theory of osmosis and water potential.
Based on the background information that I have provided above, I can predict that when the potato tuber cells will be placed in a solution that has a higher water potential than that of the cell; water will move in the cell via osmosis down its concentration gradient across the partially permeable membrane. The plant cell itself has a negative solute potential and therefore negative water potential. Due to this the potato tuber cell will absorb water until it becomes fully turgid. This means that the protoplast starts to push against the cell wall thus bringing about an increase in the pressure potential of the cell. As a result the water potential of the cell also increases until an equilibrium is reached whereas in the water potential of the cell equals the water potential of the solution that it is absorbed in. In fully turgid cell pressure potential is equal to solute potential.
Consequently it is evident that the potato core will gain mass when placed in a solution that has higher water potential than that of the potato tuber cell. This is because the water will move in via osmosis.
Conversely, when the potato tuber cells will be placed in a solution that has lower water potential than that of the cell; water will leave the cell across the partially permeable membrane, via osmosis. Accordingly, the protoplast starts to shrink until it exerts no pressure at the cell wall at all. I can predict that this the point at which it can be assumed that the concentration of sucrose solution will be same as the water potential of the potato cell. This is because at this point the pressure potential is zero as a result of which the water potential is equal to the solute potential. This point at which plasmolysis is about to occur is referred to as incipient plasmolysis.
When the potato cells will be placed in more concentrated solutions, more and more water will leave the cells via osmosis. As a result, the protoplast will continue to shrink and begin to pull away from the cell wall. This process is called plasmolysis. Hence the cell solution becomes more concentrated until equilibrium is reached whereas in the water potential outside the cell is equal to the water potential outside the cell.
As a result, the potato core will lose some of its mass when placed in a more concentrated sucrose solution, as the water will leave the cell.
Based on all the information that I have provided above, I can predict that the core, which has the least change of mass after it has been removed from the solution, will have approximately the same water potential as that of the solution that it has been placed in. As I will be plotting a graph of my results with percentage change in mass against the molarity of the sucrose solutions, I can predict that at the point where my graph will cut the x axis will be the point at which it can be assumed that the solute potential is equal to the water potential of the potato tuber cell. I will the convert the value of the concentration of the sucrose solution into the solute potential in kPa. Based on my preliminary experiment I can suggest that the value of the concentration of sucrose solution at which incipient plasmolysis will occur will be roughly around 0.3-0.4 moles. This is because incipient plasmolysis is 50% plasmolysis and this occurred between the concentrations of 0.3 and 0.4 in my preliminary experiment. This is what I expect my graph to look like, crossing between these ranges.
In this experiment there are certain variables that should be kept constant and some that should be changed. Below I have shown in the table the variables that should be changed and the reason for the change.
Before completing the actual experiment I did a preliminary experiment similar to what my actual experiment be like but instead I chose to use an onion. In this experiment the aim was to observe the process of plasmolysis and to record the plasmolysed state in the epidermal cells of the onion. If a plant cell is in contact with a hyper tonic solution, that is a solution that has a higher solute concentration than the cell contents, water leaves the cell by osmosis via the cell membrane. Water is lost first from the cytoplasm and then the sap vacuole through the tonoplast. The protoplast the living part of the cell (cytoplasm and the nucleus) shrinks and eventually pulls away form the cell wall – this process is called plasmolysis, and the cell is said to be plasmolysed.
In the experiment I used 6 different concentrations of sucrose solution varying from 0.1 molar to 1.0 molar shown in the table below:
I removed a strip of epidermis from the inner surface of one of the fleshy storage leaves of the onion bulb. First slitting it with a scalpel, and tearing back the single layer of cells with forceps can remove the epidermis. I cut up this epidermis into seven 5 x 5 mm (approximately) pieces. I put each of these in the different concentrations and waited for about roughly 20 minutes. After this I removed each piece and transferred it to a slide and added a drop of iodine. Carefully I covered it with a coverslip and examined the cells with a microscope. I counted the number of cells I could see and the amount that had been plasmolysed. Obviously there were too many to count so I made approximate result, which are shown below in the graph:
As you can see as the concentration of sucrose solution increases the percentage of onion cells plasmolysed increases. This what I expected because the more concentrated the solution that the cells are placed in the more the lower the water potential is. So the water in the cell moves out by osmosis and the protoplast shrink away from the cell. The external solution passes the cell wall and fills up the cell, and then the cell had reach equilibrium.
Although I had some decent results, the experiment was not really fair because most of the variables were approximated; such as when counting the amount of cells and those plasmolysed, it is not possible to count them perfectly, so I have decided to weigh the potato before and after they have been put in the solution.
Also from the experiment I learnt that it was difficult to cut the pieces of onionskin into equal pieces because it was too small. Also it is impossible to count the number of total plasmolysed cell because there are too many. I made a rough estimate but this is not good enough for the actual experiment so I have chosen to weight he potato and than measure the change in weight, which will be more accurate.
This will lead to more reliable and accurate results. We observed that as the added solution became more concentrated the cells became more plasmolysed. In the higher concentration of solvent the number of molecules increased in each group and therefore the groups moved more slowly.
From this experiment I realized that I need to use a range of concentrations going up equally so I have chosen the range of 0.2 - 1.0 M going up each 0.2 M. This will give consistent results, which will be concise to make an evaluation upon.
The experiment was done once but it was not fair to do it only once it should be done at least twice of which the average should be done to ensure reliable and more accurate results.
Safety is a key issue that must be considered prior of proceeding with the experiment. If these certain safety rules are not considered they could consequence in inaccurate results due to being unprepared, you have a good chance of wasting time and making a mistake. As well as this you are being hazardous to the rest of the class as well yourself while performing the experiment. Below are the rules that you should comprehend:
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Before proceeding with the experiment
Keep your lab bench organized and free of books and other clutter
Know how to use the eyewash, fire blanket and first aid kit
Pay close attention to any infoòmation give at this time
Ask questions if you are not sure of anything
- Long hair and bulky clothing are dangerous in the lab
There is a chance of catching fire, as well as being drawn through chemicals
Wear appropriate clothing
Tie long hair back
- Rings, watches, and jewelry are dangerous in the lab
Corrosive or irritating liquids may get underneath a ring or watch and produce irritation
Dangling jewelry may catch on a piece of labware and cause an accident
Should be worn to avoid any chemicals getting in the eyes, inform teacher if this does occur IMMEDIATELY!!
Glassware should be handled with care avoid leaving it near the edge of the table.
Notify the teacher if any glass is broken, it should be disposed of in and appropriate container
Sharp instrument such as the scalpel should not be used in any other activity apart from when needed
Often labs involve the use of chemicals. Please be careful to keep chemicals off your skin and out of your mouth and eyes. If you get any chemical on your skin or in your eyes, INFORM your teacher immediately!
Below is a table of the equipment that I have chosen to use for the experiment, I have commented why I have chosen that particular piece:
I will use a simple technique to determine the water potential of potato cells. Solutions of varying osmotic potential (using sucrose molarity) from pure water to 1.0 molar of sucrose are used with potato cores. Seven solutions are used, each increasing in molarity from 0.2 molar to 1.0 molar, one of them being distilled water. The potato samples are weighed so they are all equal before they are placed in the solutions and after 20 minutes in each solution. The percent weight change is plotted on a graph and the point at which no weight change occurs is determined. The solution in which the potato cells do not change weight has the same water potential as the potato cell contents. This is the point on the graph where the line intersects 0 (zero).
- Make sure the work surface is clear from books and clutter
- Measure 15ml of 0.2 molar concentration solution in a measuring cylinder and poor it into one of the test tubes
- Repeat this with the rest of the different concentrations of sucrose solution including distilled water
- Cut up the potato into 6 equal masses using the cork borer of 1.7 grams
- Weigh them and measure the length before putting it into the each of the test tubes. Put a bung on each test tube so the reaction is not affected by the surrounding air
- After 24 hours take out the potato cores and place them on a paper towel for about a minute so excess solution is removed.
- Weigh each piece of potato and measure the length record it in the table
Also the whole experiment has to be done twice to ensure a fairer set of results, which can be found by finding the average of the two results and then plotted into a graph.
Here is a diagram of how I will set up my equipment:
Once I have completed the experiment I will record the data in a table like below:
After calculating the percentage change in weight, this can be used to work out how the water potential increases and decreases depending on the molarity of the solution.
Below are the results of my experiment which I have organized into this table and worked out the average of both experiments:
From this table it is not possible to see how the result look visually and whether they are reliable or not and if they are close to the trend line or not so I have decided to plot my results in a graph like below: I have plotted concentration of sucrose solution against the percentage change in mass.
From the graph it is evident that as you increase the concentration of sucrose solution the percentage change in decreases moving from positive to negative, this supports my hypothesis that I made in my prediction. The results look fairly reliable up to an extent as they are close to the trend line and there are no anomalous results as shown on the graph. This shows that I had carried out the experiment fairly and accurately, although they are not all exactly fitted on the line of best fit, this small difference may be due to many reasons, as it affected all the different concentration
From this graph it is possible to work out the point at which it has cut the x-axis accurately using the equation of the line. At this point incipient plasmolysis occurs, the pressure potential is zero. From the use of excel I have drawn a trend line which is more accurate then the one that I have done previously by hand (-74.429 being the gradient and 27.381 being the y-intercept following the rules of mathematic where y = mx +c). If it is not accurate it is not possible to see where the line cuts the x – axis. Below I have worked out this point:
The equation of the line is Y = -74.429x + 27.381
At this point y = 0
∴ 0 = -74.429x + 27.381
-27.381 = -74.429x
x = -27.381/-74.429
= 0.367880799
This is the molarity of sucrose solution at which point incipient plasmolysis occurs. This is a very accurate number because it is up to 9 decimal places, which is not necessary but is very accurate. On my and drawn graph you can clearly see that this same point is at 0. 38, which is not accurately because the line of best fit has been drawn approximately. As we found out from the experiment even small changes in concentration can have an effect in the percentage change in mass.
Here is a table of solute potentials of sucrose solutions (at 20°C). This will help me work out the water potential
Some of the information that I used for my coursework plan was taken from the following references:
- http://www.purchon.com/biology/osmosis.htm information used on 22nd January 2004
- Cambridge Advanced Sciences Biology 1, Mary Jones etal, 2000
- Bidwell, R.G.S. 1974. Plant Physiology. MacMillan Publ. Co. New York.643pp
- Weier, T. E., C.R. Stocking and M.G. Barbour.1974. Botany: An Introduction to Plant Biology. 5th ed. John Wiley & Sons.
- http://web.ukonline.co.uk/webwise/spinneret/life/osmdia.htm
- Biology for Advanced Level – By Glenn and Susan Toole
- Plant Biology – By Peter H. Raven
- Www.npcspud.com/research-Suttle.html
- Step.sdsc.edu/personal/vanderschaegen/handouts/labs/waterpot.html
- Arbl.cvmbs.colostate.edu/hbooks/molecules/aquaporins.html
- Www.biologie.uni-hamburg.de/b-online/e04/kartofst.htm
- Gecko.gc.maricopa.edu/~lsola/bio108.htm
- Ccollege.hccs.cc.tx.us: 16080/instru/Biology/AllStudyPages/Cells/cells.htm
- Www.sri.bbsrc.ac.uk/SCIGRPS/SG5/Modelling.htm
