Simpkins. J, William, J.I, Advanced Biology, Collins Educational, 1990
A diagram to show a plant cell in full turgidity and total plasmolysis.
Phillips. W.P, Chilton. T. J, A-level Biology, Oxford University Press, 1993.
A diagram to show turgid and plasmolysed cells.
As osmosis is the diffusion of water molecules, and as diffusion is the random movement of particles from areas of high concentration to low concentration, it might be expected that any factors that speed up or slow down the movement of these particles will affect the rate of osmosis.
I will be able to calculate the water potential by finding where the line of percentage change for mass and length crosses the axis. This will then give me a value for a concentration where the percentage change is zero. On a second graph which demonstrates the relationship between molarity and solute potential of sucrose solution I can plot the values gained from mass and length and thus read off its solute potential. It is predicted that the isotonic solutions, for both length and mass to remain unchanged, to be of the same molarity.
Variables:
Fair Test
All variables, apart from the independent variable, must be kept constant in order to allow for a fair test. These variables include:
- The temperature. This is because by increasing temperatures one is increasing the kinetic energy of the molecules and as a result the diffusion rate will also increase.
- The length and diameter of the swede chips in order to allow for uniformity.
- The volume of solutions used, in order to allow for consistency.
- The same apparatus used, in order to allow for consistency, especially the top pan balance as errors often occur from balance to balance so it is vital that the same one is used through out the experiment.
Preliminary results:
The results appear generally to be as expected, where the mass and length increase up until 0.6M for both. However, I feel that there are a few factors that could be changed in order to improve the experiment:
- Where I used cling film to cover each boiling tube to stop evaporation I would use bungs. This would be easier and ensure that there was definitely an airtight seal
- I used swede chips of length 5cm. I believe that this is too long as not many chips can be extracted from each piece of swede. I think a more efficient but just as effective length would be 3cm.
- After the swede had been left overnight in a concentrated sucrose solution and was re-weighed it was very damp and had a lot of excess moisture on it. This would also have been weighed so its mass would have probably been greater than it actually was. To overcome this problem I intend to dab the swede with a paper towel before re-weighing to remove this excess moisture. It will then be weighed, re-dabbed and then weighed again. This process shall continue until a reoccurring weight is reached.
Equipment:
- Swede- To be placed into boiling tubes to investigate its weight and length change.
- 2 measuring cylinders- To measure out the volume of distilled water and sucrose solution.
- Cork borer- to extract swede which will have the same diameter
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Ceramic tile- To cut the swede safely into 3cm long pieces
- Ruler- To measure the swede before and after being immersed in solution
- Top pan balance- To weigh the swede before and after being immersed in solution
- 18 boiling tubes- To hold one swede chip in each tube with different concentrations
- 2 boiling tube racks- To hold the boiling tubes
- Distilled water- To mix solutions to desired concentration
- 1 molar sucrose solution- To mix sucrose solutions to the desired concentration
- Knife- To cut the swede chips to a length of 3cm
- Pipette- To accurately measure out the solutions
- Funnel- To pour the solution into the boiling tube without spilling any solution.
- Bungs- To place on every boiling tube to stop evaporation.
Method:
- Using a cork borer extract several tubes of swede making sure the same cork borer is used so that the swede tubes are all the same width in diameter.
- Cut the tubes of swede into 3cm long pieces using a ruler for accuracy and cut safely with a knife on a ceramic tile. 3 chips will be needed per concentration; so 18 chips shall be needed in total. This will allow room for anomalous results so an average of the three results can be taken to be analysed.
- Pat the swede chips with a paper towel to remove any excess moisture so that the mass reading is true and accurate. Weigh its mass on a top pan balance, and record these masses exactly and ensure the same top pan balance is used as they can often be inaccurate and using the same one should reduce this factor dramatically.
- To compose the concentrations of sucrose solution: 0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1M use the table below:
Table to show volumes of distilled water and sucrose solution to make different concentrations:
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Place the 20cm3 of solution into a boiling tube and place one swede chip in the same boiling tube and put it to rest in a test tube rack and place a bung on the boiling tube to stop any excess moisture entering or leaving the tube.
- Use three chips of swede per concentration of sucrose solution in separate boiling tubes.
- Leave the boiling tubes overnight
- The next day remove the sucrose solution from each boiling tube and place each chip onto paper towels and dab each piece of swede to remove any excess moisture.
- Weigh each chip on the top pan balance and then re-dab with the paper towels and reweigh, keep doing this until the weight is fairly constant and record it.
- Directly after weighing the chips place it on the ceramic tile and using the ruler measure the length of the chip accurately and record it.
- Once this has been completed for all 18 chips place all the apparatus away properly and dispose of the swede chips.
Once the results are in tabular form the percentage change can be calculated and a graph drawn to show this percentage change against concentration for mass and length. Where each line of best fit crosses the axis at 0% a concentration value can be taken. Next, from a graph which show the relation of solute potentials and concentration the concentrations gathered for mass and length can be plotted and a solute potential can be gained. Theoretically these values should be the same.
Diagram:
Observations:
There are two factors to observe during this experiment: Change in length (cm) and change in mass (g). This will be carried out by weighing the mass before and after the swede chips have been submerged in the sucrose solution and measuring the length again before and after the swede chip has been submerged in different concentrations of sucrose solutions.
Reliability:
In order to conduct the experiment in as a reliable manner as possible, thereby diminishing the chance of anomalous results occurring, 3 sets of results shall be collected for each concentration so 18 chips shall be used altogether. This will then allow me to take an average percentage change of the three chips making my results far more reliable, hopefully helping to eliminate anomalous results and make it easier to spot them.
Safety:
- Always wear safety goggles to protect your eyes from harmful chemicals
- Tie long hair back
- Take extra care when cutting the swede chips with the knife to avoid cutting yourself
- If liquid gets into the eyes flood the eye with gently running tap water for 10 minutes, and if irritating seek medical attention.
- As with any scientific experiment, care should be taken to ensure that gangways are kept clear to avoid people tripping over bags and other obstructions, and that work areas are kept clean and organised to reduce error and spillage.
- Safety to you and other colleges is of the up most importance and must be upheld at all times.
Results:
The results will firstly be constructed into a table that will show its mass and length change and then its percentage change, which can be calculated by:
Percentage change = Mass / length change x 100
Original mass / length
From the percentage change a graph can be plotted to show the percentage change in relation to the concentration of sucrose solution for both the mass and length. Where the line formed by joining the plotted point crosses the axis is its concentration of sucrose solution where an equilibrium is reached. This value can then be looked up on a graph to show the solute potential for swede.
Tables to show the results obtained for the length and mass of swede chips before and after being left in a range of concentrations:
Relationship between molarity and solute potential of sucrose solution (at 20°c):
Green, Stout, Taylor, Biological Science 2 (Cambridge 1988) p478
Tables to show the percentage change of the mass and length of swede chips in varying concentrations:
Analysis:
From the graph plotted to show the relationship between concentration and the percentage change of mass and length it is clear that there are many anomalous results present and it seems that only 4 results out of the 6 concentrations of mass and length change would show a pattern that was expected where the percentage change goes from a positive value then gradually slopes to a negative value. However, when all 6 concentrations are considered and a curved line is drawn a fairly straight downward slope does not occur but a line that goes up and down is formed. This leads the results to be very inconclusive, as so many anomalous results have arisen. The anomalous results for the percentage change in mass were for the concentrations, 0.4M and 1.0M. The 0.4M solution shows a percentage increase on average of 3.27% up on 0.2M, as the slope then shows a dramatic decrease to 0.6M we can assume that the 0.4 result is anomalous. Breaking down the average and looking at individual results for 0.4M would suggest that the second result obtained was anomalous as it brings a percentage change of 21.35% whereas the other two results are 8.19% and 6.72%, this would therefore dramatically increase the average percentage increase. If this anomalous result was not considered then the average of the two results would be 7.45%, decreasing the average nearly by half and if this result was potted it would not fluctuate and a slope would be formed. The result for 1.0M is also abnormally high and although it is still negative at -1.37% it is a large jump from -8.34%. Again breaking down the results the 3rd result is very high and a positive value, which is very unusual. Eliminating this result will leave an average of –3.88%. This may seem still a high value looking at the previous concentration, 0.8M, it has a very low negative number of -15.88% and if this is not considered the above would increase to –4.57 which seems more acceptable. Now looking closely at the percentage change for length the results for 0.2M and 0.4M appear to be anomalous. When considering the results individually for 0.2M it is hard to know which result is anomalous as the results stretch from –3.33 to 0 then to 3.33. The results for 0.4M seem to be far to negative as two are -6.66 and one at –3.33, these two results cause the line to decrease but then increase and then decrease again. If the results for these two concentrations were not plotted then a continuous sloping line would form which would also cross the axis at a higher concentration close to where the mass does thus making the value for its solute potential more negative and could help prove my prediction that the solute potential for mass and length should equal each other. With the results obtained the solute potential for mass is –1750kPa and for length -1450kPa, these results have a very large difference of 300kPa and I think that the solute potential for length should be more towards –1750kPa. Although it was not ideal to have so many anomalous results and these should have been retested at the time I can see a general pattern, which fits my prediction loosely. With more testing this conclusion could be stronger.
Conclusion:
Referring back to my prediction and when I can conclude that generally the more concentrated the solution after a certain molarity the greater the net movement of water out of the swede cells is making the cell shrink and its mass and length decrease. At a low concentration of sucrose solution i.e. Pure water at 0M and at 0.2M the percentage change is of a positive value whereby the net movement of water by osmosis is moving into the cell thus making the swede increase in mass and length, proving that the net movement of water will move into the cell making it turgid until an equilibrium is reached and the net flow of water into and out of the cell is equal. After this equilibrium when the concentration of sucrose solution is increased the net movement of water is moving out of the cell making it plasmolysed and caused its mass and length to decrease. I cannot, however prove my prediction that the solute potential of mass and length should be the same as from my results there was a large difference, but the general pattern the graphs formed fitted with my prediction.
Evaluation:
A lot of anomalous results have occurred from this experiment, mainly for 0.4M, which was anomalous for both mass and length. This could be due to a number of reasons that are discussed below, but it seems most likely that the concentration may not have been correct. The concentration 0.2M was also anomalous for length and 1.0M for the mass. These two anomalies are largely out of pattern and so do not fit the line of best fit, these can also be explained by many reasons which should account for them but on the whole a poor experiment was carried out.
I believe that the results for the mass of the swede chips were most reliable. I have based this conclusion on the fact that the weighing of mass is a lot more accurate than for the length as mass is calculated to two decimal places electronically and length can only be measured to the nearest millimetre and this is done by the human eye so there is a lot more room for error. Therefore I would take the value of the solute potential for swede as –1750 as this was the value for mass.
After analysing my results it is apparent that there are many problems with my experiment however I do believe that a lot of the errors may have been due to human error, e.g. inaccuracy when cutting the length of the swede chip and when re-measuring it. There are many other factors though that could have affected the success of the experiment. One problem was that the same piece of swede was not used for all the chips, this could have great consequences on the rate of osmosis as different pieces of swede could contain various amounts of moisture and solutes already in it which would affect the net flow of osmosis. Next time it is vital that a big enough piece of swede is used so that all 18 chips are cut from the same swede. Also, when the sucrose solutions were being mixed they were mixed individually in a small measuring cylinder for one boiling tube at a time. Inaccuracy in measuring the volumes of water and sucrose solution could cause some solutions to be more concentrated than another. To resolve this, one 60cm3 volume of sucrose solution will be made up and then 20 cm3 placed in three boiling tubes which should make each of the three boiling tubes have exactly the same concentration of solution. Also as we were simply measuring the solution in measuring cylinders this could also lead to inaccuracies of the human eye. I believe using a burette would be a lot easier to extract exact quantities. I still think that more moisture could be removed from the swede chip before and after the experiment is carried out to make sure a true reading of the water inside the cell is read rather than a greater weight as moisture is on the chip, however, it was difficult to tell when the chips had been dried enough so as to not over dry them and thus water would move out of the cells by osmosis due to the water potential outside being lower than inside the cells. A major problem as expected was the inaccuracy of the top pan balance, especially when re-weighing the chips, as the same top pan balance was not available which could mean that I was unable to get a true reading for the change in mass. Also if the chips were weighed inaccurately at any point along the experiment it would alter the final results and percentage change. However, this is hard to be controlled as human error is a major fact in this but it is difficult to restrict this problem. Finally I cannot be sure whether or not the temperature was constant at all times as the experiment took place overnight. Therefore I think it would be better if the experiment took part over 4-6 hours and the temperature checked every 30 minutes and adjusted if necessary using hot or cold water in a water bath then placing the boiling in that water bath. This should then allow enough time for osmosis to take place and an equilibrium reached but is also in a more realistic time for the temperature to be monitored. I also think that more concentrations should be experiment on around the solute potential, i.e. between 0.5M and 0.6M. This could make the outcome of solute potential a lot more accurate. After reviewing my results it is clear that as so many anomalous results have occurred these anomalies should have been redone which would have hopefully eliminated these and given me stronger proof on which to base my conclusion. To extend this investigation further I would use a greater variety of concentrations e.g. 1.4M to extend the graph further and prove whether my prediction is true as the concentration increases even further.
Bibliography:
- Barber. M, Boyle. M, Cassidy. M, Senior. K, Biology, Collins Educational, 1998
- Green. N.P.O, Stout. G.W, Taylor D.J, Biological Science 1&2, Cambridge University Press, 1996
- Simpkins. J, Williams. J.I, Advanced Biology, Collins Educational, 1990
- Roberts. M, Reiss. M, Monger. G, Biology Principles and Processes, Nelson, 1993
- Jones. M, Fosbery. R, Taylor. D, Biology 1, Cambridge University Press, 2000