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An investigation into osmosis in potato tissue.

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Introduction

AS LEVEL BIOLOGY/ HUMAN BIOLOGY PRACTICAL WORK AN INVESTIGATION INTO OSMOSIS IN POTATO TISSUE 1. PLAN AIM The aim of this investigation is to place pieces of potato tissue into salt solutions of different strengths and to measure changes in the potato tissue. INTRODUCTION Osmosis is the net movement of water molecules from a region where they are of a high concentration (the solution has a high water potential) to a region where they are of a low concentration (low water potential) across a partially permeable membrane. The diagram below shows this process, where the water potential on the right is lower than that on the left as the molecules of water on the right are in lower concentration. Over time, the net movement of water molecules will be from left to right until such time as the water potentials on both sides are in equilibrium. The significance of the process described above for plant cells is that if plant tissue is immersed in a solution of a higher water potential than itself, the net movement of water molecules will be into the cells, thus causing the cells to swell up. Due to the fact that plant cells are surrounded by a strong cellulose cell-wall, the membrane will not burst but the tissue will become increasingly turgid until a point is reached where the pressure potential exerted by the cell wall on the membrane prevents further uptake of water by the cell. At this point the cell is said to be fully turgid. In whole plants it is the turgidity of their cells that helps hold them upright and spreads the leaves out to the sun so that they can photosynthesise efficiently. If plant tissue is placed in a solution of lower water potential than its own, the process described above will occur in reverse; the net movement of water molecules will be into the surrounding solution and the cell will become increasingly flaccid as pressure exerted by the membrane on the cell wall decreases. ...read more.

Middle

Once all the initial masses had been recorded, the chips were placed in the solutions and the time was noted. After one hour, the chips were removed from the solutions, dried by flicking them twice as before and the final masses were recorded. Any observed changes in a group of chips' turgidity were also noted. RISK ASSESSMENT During this experiment it will be necessary to use several potentially dangerous pieces of equipment: a scalpel to cut the potato samples to the correct length and glass measuring cylinders and bottles containing the various solutions. In order to overcome these hazards, specific precautions need to be taken: * All glassware should be kept in a place where it cannot be knocked over and broken, preferably at the side of the laboratory once the solutions have been measured. * The scalpel should be used with care and again put in a safe place once it is no longer required for the experiment. * Any spillages should be cleared up immediately. * There should be a clearly marked first aid kit to hand in the laboratory. 3. RESULTS RESULTS OF PRELIMINARY EXPERIMENT Table 1 - The change in length of potato chips soaked in water and 10% salt solutions Solution used Initial length of chip Length after immersion In solution. 10% NaCl (Water potential = -8000 KPA) 3cm 2.3cm Distilled Water (Water potential = 0 KPa) 3cm 3.8cm RESULTS OF FULL EXPERIMENT Table 2 - Initial Mass of Potato Chips Water Potential (KPa) Initial Mass of Potato Chips (g) 1 2 3 4 5 Mean (3sf) 0.00 1.02 1.04 1.02 1.04 1.01 1.03 -200 1.04 1.01 1.02 1.02 1.00 1.02 -400 0.97 1.00 0.98 1.00 0.99 0.99 -600 0.95 0.97 1.00 0.96 0.97 0.97 -800 0.96 1.00 1.00 0.98 0.97 0.98 -1000 1.02 0.99 1.00 1.00 0.99 1.00 Table 3 - Final Mass of Potato Chips Water Potential (KPa) ...read more.

Conclusion

Although the discrepancies caused by this were worked around by dipping the chips in the solutions and flicking them dry before the experiment as well as after it, this flicking dry of the chips itself introduces another source of error. This is that it is difficult to ensure that the same amount of liquid is shaken off each time, and where only small differences in mass are being dealt with, the proportional importance of the mass of a drop of water is significant. To improve the accuracy of this, more care could be taken to ensure that all the chips are shaken with the same degree of force each time, or alternative methods of drying could be used. One possibility would be to place the chip on a dry piece of paper towel after immersion and roll it back and forth twice. Another potential source of error lay in the fact that some water from the chips remained on the balance after they had been weighed. The method of taring the balance was used several times to reduce the significance of this. However, as this was not carried out between each weighing, there was a potential error in the reading of the balance of up to 0.0049g. Once again, as very small masses are being dealt with here, a small difference such as this is potentially very significant. To reduce the chance of such factors influencing the results, the balance should be dried scrupulously between each weighing. Once the method of the experiment has been modified to adopt these changes, the investigation could then be carried out with many repeats and using more water potentials as discussed above. However, whereas in the experiment carried out here, it was relatively easy to compare the potatoes used for the samples and see that they were of the same breed, approximate age and state of turgidity, when conducting the extended experiment, it would be very important to ensure that these factors are closely monitored to avoid bringing another potential source of error into the results. ...read more.

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