The results do not appear to have produced any anomalous values thereby suggesting that the method used to carry out the experiment was sound. Valid results are also likely to have been produced due to the number of concentrations used. This amount of concentrations enabled the trend of water potential to be recognised clearly thereby allowing any possible anomalous results to be easily identified. A further positive aspect of this study was that the equipment required was easily set up and was in no need of any specialist skills and thus the results were obtained with no evident difficulty.
Due to the above factors it could be argued that it would be appropriate to use the results obtained form the preliminary study as a basis for the prediction for the point of incipient plasmolysis of the potato tissue and the method used as a foundation for the experiment due to be undertaken. However, there are possible flaws evident in the data collected. For instance, the preliminary study involved the process of analysing the cells within the onion tissue for signs of plasmolysis thereby relying purely on the judgement of those carrying out the experiment. There was also a choice involved as those carrying out the experiment were able to decide which cells they wished to analyse. As a result of these above factors, it would be correct to establish that it would be unlikely for any two of the same experiment to produce the identical results. This is because different individuals are likely to possess different opinions regarding the state of the cells within the tissue (i.e. whether they are turgid, incipiently plasmolysed or plasmolysed.) Furthermore, any misjudgement could result in the results obtained being invalid. Vision may have also been impaired due to factors such as the tendency of the onion tissue to ‘fold up’ once removed from the solutions forming a double layer of cells. This would have an effect o the accuracy of the results because under magnification, both layers appeared intermingled thereby reducing the definition of the imagery of the individual cells. A further evident flaw in the method for the preliminary study was the lack of attention paid to the stringent timing of waiting 20 minutes before the cells were analysed. Allowing the cells to be immersed for the full time period before analysis would have allowed the sucrose solution to have had its full effect upon the onion cells, hence giving a more accurate reading of the number of plasmolysed cells. The method possessed further flaws, especially with regards to the solutions involved. First and foremost, the solutions used were not made up by those carrying out the experiment themselves but were instead given to them. As a result, there may have been uncertainty regarding their validity. Also, there was no specific volume of solution in which the onion tissues were immersed as no measurements were taken. This may have had an impact on the level of osmosis and thus, the reliability of the results obtained. In addition, although the number of solutions used was appropriate for the nature of the experiment, the range of concentrations made could have been further improved. This would have been possible by more concentrations being made up close to the predicted strength of concentration that would cause the cells to be incipiently plasmolysed. This would enable a more precise value for the concentration at which incipient plasmolysis occurs to be achieved. Lastly, the preliminary study aimed to investigate the water potential of onion tissue whereas the proposed experiment involves the analysis of potato tissue. There is therefore uncertainty regarding the fact of whether or not the potato tissue will behave in the same manner as the onion tissue and mainly, whether the cells within the tissue will be incipiently plasmolysed at a similar concentration to that of the onion tissue. It is on this basis that it may be seen to be inappropriate to use the results obtained from the preliminary study as a foundation for the prediction of the experiment due to be undertaken.
Secondary data:
Figure 3 shows the results obtained form an experiment carried out by a US School, the aim of the experiment being to investigate the water potential of potato tissue when immersed in varying concentrations of sucrose solution by analysing the percentage change in mass after immersion. When percentage change is equal to 0, the water potential of both the external solution and the potato tissue are the same, thereby enabling the point of incipient plasmolysis to be identified. Figure 7 is a graph drawn form the results obtained. It is evident from the graph that the point at which there is no percentage change in the mass of the potato tissue and thus the point at which the cells within the tissue are incipiently plasmolysed is at a concentration of 0.46 M. This is similar to the concentration at the point of incipient plasmolysis of the onion tissue in the preliminary study thereby providing backing for the validity and use of these results as a foundation for the prediction of the proposed experiment.
It could be argued that the results obtained form the US School are valid for use in predicting the outcome of the experiment due to be undertaken as it was too based on investigating the water potential of potato tissues, rather than the onion tissue central to the preliminary study. Despite this, there ate many aspects of the results obtained that have caused uncertainty to arise regarding their validity. First and foremost, the reliability of the data can be questioned due to the fact that there are a number of errors existent in the results obtained. For instance, the mass difference for concentrations 0.00 M, 0.20 M and 0.80 M have been miscalculated thereby causing the results to be incorrect. Also, the percentage change in mass for the concentration 0.80 M is an anomalous value therefore causing further impacts on the validity of the results. The reason for such major flaws occurring are likely to be sue to the fact that the study was undertaken by US students who did not possess the required expertise and specialist skills to carry out such an investigation accurately, especially with regards to calculating the mass difference between initial and final mass of the tissue, before and after immersion. Also, the range of concentrations used as part of the experiment were not designed to allow the concentration at which incipient plasmolysis occurs to be identified accurately. This was due to the fact that the concentrations were evenly spread from 0.00 M to 1.00 M, rather than a number of concentrations close to the predicted concentration being used. As a result, an accurate reading from the actual experiment was unable to be obtained and thus, the graph detailing the results and a predicted trend had to be used as a means of identifying the point of incipient plasmolysis.
Figure 4 shows the results obtained from an experiment carried out by the Havering Sixth Form Biology lab technician, Marilyn. She aimed to investigate the water potential of potato tissue when immersed in varying concentrations of sucrose solution by calculating the percentage change in length of the potato tissue after immersion. The point at which there would be no change in length of the potato tissue due to equal values of water potential between the external solution and within the cell would be the point at which the cells within the tissue are incipiently plasmolysed. Marilyn’s results show that the concentration at which the incipient plasmolysis occurs to be 0.40 M. A t this concentration, the protoplast within the potato tissue will no longer exert a force upon the cell wall, (the cell will no longer be in a state of turgidity as in the concentrations 0.00 M to 0.39 M) and the water potential of the external solution will be equal to the solute potential and will thus achieve the value of -1120 kPa. For concentrations 0.41 M to 1.00 M, the protoplast will exert less and less of a force against the cell wall and will begin to shrink away thereby leading to a state of plasmolysis.
Of all the studies mentioned it could be argued that the results obtained from Marilyn’s experiment are the most valid due to a number of factors. Firstly, the range of concentrations used by Marilyn (a number of concentrations close to the predicted point at which incipient plasmolysis was likely to have occurred) allowed the results obtained to be of an accurate nature. The results were also likely to have been valid due to Marilyn’s expertise and skills in the area due to her professional background. Furthermore, it is likely to have be regarded as appropriate to use Marilyn’s results as a basis for the prediction due to be made as her experiment was also central to the water potential of potato tissue specifically.
Dissimilar to the investigation carried out by the students at the US School, Marilyn aimed to investigate the water potential of potato tissue through the means of measuring the change in length of the tissue after immersion, rather than measuring the change in mass. Although using length is likely to be less accurate (especially due to the fact that the electronic balance used to measure mass is far more precise (0.001 g accurate) than the vernier calliper (0.1mm accurate) used to measure length), it is likely to be a more reliable means of measuring the water potential as there is less room for error. As a result, this method shall be adopted as a means for investigating the water potential for the experiment due to be undertaken. Marilyn did not however use the same initial length for each of the potato tissues before immersion in to the sucrose solutions. Although this did not have any effect o the validity of the results, it would have been easier to have the same initial length when comparing different readings.
The preliminary study and the secondary data have enabled an understanding to be developed of the best possible means of carrying out the proposed experiment. The following summarises the methods that shell be adopted as a result of these previous studies:
- The experiment shall aim to investigate the water potential of potato tissue and thus the point of incipient plasmolysis through the means of measuring the change in length of the tissue after immersion, (no change in length shall indicate the point of incipient plasmolysis). Length shall be the dependent variable as it is a more reliable means of investigating water potential (in relation to mass) as there is less room for error.
- The concentrations shall be made up by those carrying out the experiment themselves in order to ensure validity.
- The tissues shall be immersed in a precise amount of solution, which shall be the same for all concentrations. This shall enable the data obtained to be more reliable.
- A similar number of solutions shall be made up to those used in the aforementioned studies. This is due to the fact that this amount will allow the trend of water potential of the potato tissue to be recognised clearly and will thus identify if any results are anomalous.
- In order to allow each concentration to have its full effect on the cells within the tissue, the tissue shall be left immersed for a full 24 hours. As a result, the results obtained are likely to be far more valid.
- A range of concentrations shall be made up for the potato tissue to be immersed in, in which there is a number of concentrations close to the concentration at which incipient plasmolysis is likely to occur and less concentrations further away from this value. This shall enable the most accurate value for the concentration at which incipient plasmolysis occurs to be obtained.
Prediction:
The proposed experiment shall aim to investigate the water potential of potato tissue when immersed in various sucrose solutions of different concentrations in order to identify at which concentration the cells within the tissue are incipiently plasmolysed thereby identifying the normal sucrose concentration of the potato cells. Using the findings from the preliminary study and the secondary data, the following prediction regarding the outcome of the experiment can be drawn:
It is predicted that the concentration at which the cells within the tissue are incipiently plasmolysed is at 0.40 M. This concentration has been predicted mainly due to the influence of the experiment carried out by Marilyn. For all solutions less concentrated than this value (0.00 M to 0.39 M), the solution shall have a higher water potential than the tissue and thus the net movement of water shall be from the external solution. This movement of water through osmotic means shall cause the protoplast of the potato tissue to exert a strong force against the cellulose cell wall thereby causing the tissue to become turgid. As a result the potato tissue shall show an increase in length after immersion due to their state of turgidity for this range of concentrations. The concentration at which solute potential is at its lowest (the least concentrated solution) will thus have the greatest water potential, as evident from the equation (Ψ = Ψs + Ψp). At the concentration of 0.40 M, it is predicted that the water potential of both the external solution and the potato tissue will be equal. Therefore, on this basis, the normal sucrose concentration of the potato tissue must be 0.40 M. At this concentration the protoplast within each individual potato cell will no longer exert a force upon the cell wall. Pressure potential will therefore be equal to 0 and thus the water potential of both the external solution and the cell solution shall be equal to the solute potential. The water potential of both solutions of concentration 0.40 M at the point of incipient plasmolysis can therefore be established as -1120 kPa (refer to Figure 5). As a result, there will be no change in length of the tissue after immersion. For all concentrations more concentrated than this value (0.41 M to 1.00 M), there will be a net movement of water out of the potato tissue through osmotic means. This is due to the fact that as solutions become more concentrated, the water potential of such solutions shall decrease. As the potato tissue shall have higher water potential when immersed in such concentrations, water shall leave the tissue, causing the volume of the cells to decrease (the cell shall shrink) and the protoplast to pull further and further away from the cell wall. In this state, the cell is said to be plasmolysed and the tissue is described as a flaccid due to its reduced shape. As a result, the tissue shall show a decrease in length after immersion for this range of concentrations.
Apparatus:
Method:
- Label 9 sample tubes (using the glass marker) with the different sucrose concentrations that shall be made as part of the experiment. The labels should read: 0.00 M, 0.10 M, 0.25 M. 0.30 M, 0.40 M, 0.45 M, 0.50 M, 0.80 M and 1.00 M. Then place these tubes in the sample tube rack.
- 20 cm³ of each of the 9 different sucrose concentrations must now be prepared. This involves the following procedures for each concentration:
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0.00 M: For a sucrose concentration of 0.00 M, 20cm³ of distilled water must be measured using the pipette and inserted into the first sample tube (labelled 0.00 M) using the pipette filler.
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0.10 M: For a sucrose concentration of 0.10 M, 18 cm³ of distilled water must be measured using the pipette and inserted into the sample tube labelled 0.10 M. 2cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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0.25 M: For a sucrose concentration of 0.25 M, 15 cm³ of distilled water must be measured using the pipette and inserted into the sample tube with the appropriate label. 5cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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0.30 M: For a sucrose concentration of 0.30 M, 14 cm³ of distilled water must be measured using the pipette and inserted into the sample tube labelled 0.30 M. 6cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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0.40 M: For a sucrose concentration of 0.40 M, 12 cm³ of distilled water must be measured using the pipette and inserted into the sample tube with the appropriate label. 8cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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0.45 M: For a sucrose concentration of 0.40 M, 11 cm³ of distilled water must be measured using the pipette and inserted into the sample tube labelled 0.45 M. 9cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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0.50 M: For a sucrose concentration of 0.50 M, 10cm³ of distilled water must be measured using the pipette and inserted into the sample tube with the appropriate label. 10 cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the same sample tube through the same means.
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0.80 M: For a sucrose concentration of 0.10 M, 4 cm³ of distilled water must be measured using the pipette and inserted into the sample tube labelled 0.80 M. 16cm³ of 1.00 M sucrose solution must also be measured using the pipette and inserted into the sample tube through the same means.
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1.00 M: For a sucrose concentration of 1.00 M, 20cm³ of 1.00M sucrose solution must be measured using the pipette and inserted into the sample tube (labelled 1.00 M) using the pipette filler.
- Next, prepare the potato tissue by first removing the impermeable outer skin through the means of the potato peeler. Once removed, use the cork borer in order to produce 9 potato cylinders. The glass rod should be used as a means of pushing these cylinders out of the cork borer.
- Each of these potato cylinders should be measured using the vernier calliper and thus cut to the initial length of 3 cm using the scalpel on the cutting tile. Each of these 9 potato cylinders should now be identical in length.
- Each of the 9 individual potato cylinders should now be immersed in a different sample tube (and thus a different sucrose solution) through the use of tweezers. The lids should then be placed on each of the 9 sample tubes in order to rid the possibility of evaporation occurring which would have an adverse effect on the sucrose concentrations of each of the solutions. The potato tissues should be left immersed for a full 24 hours.
- After this time period, the potato cylinders should be removed from the sample tubes, again through the means of tweezers. The vernier callipers should now be used to measure the length of the potato cylinders after immersion. These values should be recorded in a results table in order to allow comparisons to be made to the initial length before immersion and ultimately, the point of incipient plasmolysis to be identified.
- Repeat the experiment a further two times and record all results. These repeats shall eradicate the possibility of invalid results being obtained and shall also allow an average reading to be calculated.
Variables:
A variable is any factor that is likely to change within the experiment, there are three variables which shall be looked at specifically, they are: independent variables (those factors that shall be changed); dependent variables (those factors that result due to the changes made to the independent variable and thus they are the aspect which is being measured) and lastly, the controlled variables (they are those factors that must be kept constant in order to ensure that reliable results are obtained).The following details each of the three variables that are existent in the proposed experiment:
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Independent variable: The aim of the experiment is to investigate the water potential of potato tissue when immersed in varying concentrations of sucrose solution and therefore find the concentration at which the cells within the tissue are incipiently plasmolysed. In order for this to be made possible, different concentrations shall have to be tested. Therefore the factor that shall be changed within the experiment will be the concentration of sucrose in each solution and thus the water potential of each of the different solutions.
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Dependent variable: The dependent variable is the factors that result due to the independent variable, which is the varying sucrose concentration. The dependent variable must therefore be the change in length of the potato tissue caused due to the varying concentrations of sucrose, and thus the varying water potentials of the external solution. This length is the factor that shall be measured in order to make comparisons between the states of the potato tissue in the different concentrations and ultimately, to identify the point of incipient plasmolysis.
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Controlled variables: The controlled variables are those factors that must be kept constant for all concentrations in order for the experiment to produce reliable results. They include factors such as the amount of time the potato tissues are immersed (that is 24 hours for all concentrations); the volume of each of the different solutions (which is 20cm³) and also the type of potato used. This is because the normal sucrose concentration may differ for different varieties of potatoes and thus the point of incipient plasmolysis will vary also. A further variable which must be kept constant is the temperature in which the sample tubes are kept. This is because temperature can have an effect on the level of osmosis that occurs.
Risk Assessment:
When carrying out an experiment of this nature, it is important to acknowledge the need for safe working. There are many aspects within the investigation that are hazardous and could thus, cause possible harm to those carrying out the experiment. Focal to the experiment is the use and production of various solutions. There is therefore a likelihood of spillages occurring. Such spillages could be hazardous as they could cause falls and other such accidents. In order to prevent such incidents from occurring, those involved in carrying out the experiment must therefore ensure that any spillages are cleaned up immediately. It may also be appropriate to warn others if a spillage occurs so that they are aware of the potential risk. Furthermore, another potential hazard lies in the great use of glass ware within this experiment. A number of apparatus required in order to carry out this investigation appears to be made of glass and therefore of a delicate nature, such equipment include pipettes and the sample tubes. It is thus possible that breakages may take place. Breakages are likely to cause great harm if caution is not taken. Such measures that are required to be enforced if breakages were to occur include using a dustpan to clear all fragments of broken glass and disposing of such material through an appropriate means and also, alerting those in the surrounding area. The final potential hazard existent in the carrying out of the proposed experiment is the use of sharp objects such as the scalpel and cork borer when cutting the potato cylinders. Recklessness when handling such objects is more than likely to result in accidents and bodily harm. In order to avoid such events, those carrying out the experiment must ensure that they take great care and handle the objects by the correct means, i.e. cutting the potato cylinders away from themselves correctly on the cutting tile.
Bibliography:
- Jonathan Wilson: Biology Tutor, Havering Sixth Form College
- Essential AS Biology for OCR: Glenn and Susan Toole
(ISBN 978-0-7487-8511-7): Pages 60-63
- Cambridge Advanced Sciences Biology 1: Endorsed by OCR
(ISBN 0-521-78719-X): Pages 55-58
- Secondary data: Results obtained form US School and
Marilyn, Biology Lab Technician, Havering Sixth Form College