INVESTIGATION Potato Osmosis INTRODUCTION Osmosis is a process that occurs at a cellular level that entails the spontaneous net movement of water through a semi-permeable membrane from a region of low solute concentration to an area of high solute concentration in order to equalize the level of water in each region. Involved in this process are hypotonic, hypertonic and isotonic solutions. A hypotonic solution is one with a lower osmotic pressure, indicating that the net movement of water moves into the said solution whereas a hypertonic solution is one with a higher osmotic pressure, thus the net movement of water will be leaving the hypertonic solution. Lastly, an isotonic solution entails no net movement of water across a semi-permeable membrane as the two substances involved display osmotic equilibrium. AIM To observe the effect of solutions different levels of concentration on potatoes, considering the process of osmosis METHOD (see Potato Osmosis exercise document) Table of -H 2 O Dilutions Solution / ml ±1 Distilled H 2 O/ ml ±1 Concentration/M 100 0 1.00 75 25 0.75 50 50 0.50 25 75 0.25 0 100 Official Timeframe of Experiment Insertion Time: 29 November, 2007 8:54 am Extraction Time: 30 November, 2007 9:40 am Legend for Potato Differentiation Potato Mark Potato 1 Vertical line Potato 2 Horizontal Line Potato 3 Notch (Fig. 1) Diagram demonstrating the correct preparation of a potato cylindrical strips marked according to the legend DATA COLLECTION
Raw Data Table Mass and Lengths of Potato Strips Pre- & Post-Experiment Concentration/M Potato Samples Initial Mass/ g±0.01 Mass after Solution / g±0.01 Initial Length /mm±1 1.00 Potato 1 1.93 1.42 40 35 Potato 2 1.88 1.31 40 34 Potato 3 2.02 1.56 40 38 0.75 Potato 1 1.98 1.42 40 36 Potato 2 2.09 1.46 40 37 Potato 3 2.01 1.36 40 37 0.50 Potato 1 2.02 1.38 40 38 Potato 2 2.00 1.47 40 36 Potato 3 2.00 1.45 40 38 0.25 Potato 1 2.01 1.98 40 40 Potato 2 1.98 1.86 40 39 Potato 3 2.01 1.98 40 41 Potato 1 2.04 2.48 40 44 Potato 2 1.98 2.46 40 45 Potato 3 1.98 2.17 40 44 Length after Solution/ mm±1 Observations Pre-Insertion - Generally rigid in structure although slightly bendy - Pale yellow in colour - Moist - All strips appear the same/similar in structure and size at this point Observations Post-Extraction - Strips immersed in 1.0M Solution are very soggy, soft and appear shrunken - Strips immersed in 100% H 2 O are very rigid, swollen, turgid and appear larger/longer - they are slightly bent and cannot be straightened due to their rigidity - Strips become progressively soggier as the solutions they are immersed in are higher in concentration of (Fig. 2) Potato strips from the same potato arranged in descending order of concentration to demonstrate the differences in structure post-extraction
DATA PROCESSING Calculating Percentage Change *Required in order to calculate percentage change in mass and in length Processed Data Table Average Percentage Change in Mass and Length of Potato Strips Concentration/M 1.0 0.75 0.50 0.25 % Change in Mass/% change/% % Change in Length/% -26.42-12.50-30.32-26.51-15.00-22.77-5.00-28.28-1 -30.14-30.25-7.50-32.14-7.50-31.68-5.00-26.50-28.56-1 -27.50-5.00-1.49-6.06-3.02-2.50-1.49 2.50 21.57 1 24.24 18.47 12.50 9.60 1 change/% -10.83-8.33-6.67 10.83 Processed Data Table Difference between % Change in Mass and Length and Change % Change Concentration/M in Mass 1.0-26.42 Change - Mass % Change Difference in Length 0.09-12.50 Chang - Length Difference -1.67-30.32-26.51-3.81-15.00-10.83-4.17-22.77 3.74-5.00 5.83 0.75-28.28 1.97-1 -1.67-30.14-30.25-8.33 0.11-7.50 0.83-32.14-1.89-7.50 0.83 0.50-31.68-3.12-5.00 1.67-26.5-28.56-6.67 2.06-1 -3.33-27.5 1.06-5.00 1.67 0.25-1.49 1.53 0-6.06-3.02-3.04-2.50-2.5-1.49 1.53 2.50 2.5 21.57 3.1 1-0.83 24.24 18.47 10.83 5.77 12.50 1.67 9.60-8.87 1-0.83
Average Percentage Change/% Potato Osmosis Average Percentage Change in Mass and Length of Potato Strips at each Concentration 30 Average Percentage Change in Mass and Length of Potato Strips at each Concentration 20 10 0-10 0 0.25 0.5 0.75 1 1.25 R² = 0.989-20 -30 R² = 0.978-40 Concentration/M % change in mass % change in length CONCLUSION & EVALUATION As can be seen from the above graph and observations, we can ascertain that as concentration in the solution decreases, the mass and the length exemplified after an approximate 24-hour period increases. Thus, we can state that there appears to be a negative correlation between concentration and the mass and length of the potato strips, clearly evident in the above graph which shows an exponential decrease in both mass and length. This can also be initially seen in the post-extraction observations where it is evident that the potato strips immersed in lower concentration were far more turgid than those immersed in 100% solution which were flacid and fragile (see strip-comparison in Fig. 2). This occurrence can be explained through the process of osmosis. As mentioned in the introduction, a hypertonic solution is one with higher osmotic pressure meaning that the net movement of water leaves the solution. This would explain the physical changes the increase in mass and length as well as the increase in turgidity - in the potato strips immersed in 100% H 2 O solutions or low -concentration solutions. Since the solution it is submerged in is higher in concentration in water molecules, or hypertonic, the water molecules will diffuse into the area of lower H 2 O-concentration (the potato strip) in order to achieve equilibrium. Alternatively, the decrease in mass and length in the potato strips submerged in highly concentrated solutions can be explained by its immersion in a hypotonic solution. Hypertonic solutions, as mentioned
before, are described as those with lower osmotic pressure, indicating that the net movement of water moves into the solution. Therefore, as solution is less concentrated in H 2 O molecules than the potato strips, the decrease in mass and length and loss of turgidity results from the net movement of water leaving the potato strips, which is higher in osmotic pressure, and diffusing into the solution. Nevertheless, there are several possible sources of error that could have greatly or negligibly affected the outcome of the experiment. First, we must note the varying external factors resulting from an uncontrolled environment the biology classroom. Primarily, these would include varying temperatures and humidity which could potentially affect the rate of osmosis as increased temperature results in increased diffusion while increased humidity results in an increased number of water molecules. Secondly, we must note the human errors involved, for example, miscalculations in experimental preparations. These would include the miscalculation of solutions leading to an inaccurate concentration of as well as the possibility of impurities in the concoction in the first place while imprecise cutting of the potato strips could ve affected the surface area and thus the rate of osmosis. This leads us to the errors resulting from variances in the substances used. As already discussed previously, differences in surface area of each potato strip caused by imprecise cutting as well as the marks (lines and notches) imprinted would ve affected the rate of osmosis while the concentration gradient between each potato strip is likely to differ as well. This stems from the differences in water content of each potato, as, for example, a potato with high water concentration in highly concentrated solution would have a faster rate of erosion. Further affecting factors could include barriers to diffusion such as the size of pores which would also determine the rate of osmosis. All the mentioned errors above hold the possibility of skewing the data. Subsequently, such errors could have an effect on the reliability of the results. The level of accuracy which has been used throughout this investigation would come into question as a combination of these errors would not permit such precision. Values of percentage change have been taken at two decimal places corresponding with the actual values of mass and length, however, this could be seen as far too precise. A better option would have been to take percentage change as whole numbers or at one decimal place. Nevertheless, we attempted to reduce the potential errors through several measures. With surface area, a cork borer was used in order to uniform the size of the potato strips while the varying concentration gradients were controlled through the completion of several trials (three trials with three potatoes) in order to limit error. Furthermore, to control the effects of the external environment, foil was secured over the beaker containing the submerged potato strips. However, if we refer to the graph, we can see the minimum and maximum spread for each data-point is generally close-set while the R 2 value, which calculates the spread of the datapoints from the line of best fit, are both relatively high both around 0.9. This demonstrable trend indicates a limiting of the amount of error, and thus fairly reliable results despite possible errors. Overall, the results ultimately seem reliable although it might ve been even more reliable by reducing the level of precision (decimal places) when recording it. Ultimately, potential improvements will stem from attempting to reduce the amount of error in this investigation, particularly involving controlling the external environment and the miscalculations. To control the external affecting factors, the solution containing the potato strips can be kept overnight instead in a controlled environment with consistent temperatures and humidity. Limiting the human error would be difficult and time-consuming as this would involve
highly-precise instruments or even more focus dedication from the experimenter during preparation. Finally, nothing can be done to uniform the response of the materials used, thus the completion of even more trials limits the potential error and allows the formation of generalizations. Despite the improvements proposed, those relating to limiting human error and completing more trials may prove to be futile as they are not only time-consuming, but the demonstrable trends resulting from this experiment indicate that no further improvements are necessary to reach the desired conclusion. Having established that there is no real need to pursue drastic improvements for the initial experiment, we can now proceed to discuss possible extensions to the investigation. While we already know the results of osmosis on a potato, we may now wish to better understand it. This can be done by recording the progress of the potato s transformation either (a) over a period of time (perhaps 24 hours) or (b) until it has reached the point of equilibrium. The mapping of this progress would involve the periodic removal of the samples in order to measure its mass and length, after which it can be compiled into a graph to chart the transformation under osmosis. Alternatively, we could compare the progress of a potato to another type of vegetable or fruit in order to ascertain water content of each. Lastly, the submerged potato strips may be subjected to different kinds of environment, particularly, varying humidity and temperature, without the protection of a foil cap. This would reveal how much of an impact environmental factors would have on the osmotic process and how would the effects manifest. In relation to the question of the sailor, this could represent the life-span one would expect when trapped in certain climates.