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permeability of beetroot membranes

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An investigation into how the concentration of a solution affects the rate of osmosis in a potato Definitions of important terms: Osmosis: Osmosis is the movement of water through a partially permeable membrane from a low concentration into a high concentration. This continues until both solutions are of equal concentration. Osmosis is a natural occurrence that can be simulated with an artificial membrane such as a visking tube. The rate of osmosis can be determined by the difference in water potential between the two substances on either side of the partially permeable membrane. Water potential (symbol??): Water molecules possess kinetic energy, which means that they are continually moving around when in a gas or liquid state. The higher the concentration of water molecules in a system, the greater the total kinetic energy of water molecules in that system and also the higher its water potential. Water potential is a measure of the energy available in an aqueous solution to cause the migration of water molecules across a semi-permeable cell membrane during osmosis. Water moves from areas of high (less negative) to areas of low (more negative) potential. Pure water is given the value zero. Aim of My Experiment: I am going to study whether or not the amount of water that may be taken into a potato cell by osmosis is affected by the concentration of a solution on one side of the partially permeable (cell) membrane. For this experiment I am going to use slices of potato as my partially permeable membranes and glucose solutions. MY PRELIMINARY EXPERIMENT For this investigation it was necessary to carry out a preliminary experiment in order to both gain a rough idea of what my result might be for the final experiment and to refine the method and techniques used in this experiment. This first experiment was carried out in class and the method used is a great deal less accurate than I hope to make my final experiment. ...read more.


Concentration of glucose solution (M) Weight of filter paper (g) Weight of paper + potato disks (g) Weight of potato disks (g) Original weight of potato disks (g) Change in weight (g) 1.00 M 0.58 g 2.89 g 2.31 g 3.07 g - 0.76 g 0.80 M 0.71 g 3.49 g 2.78 g 3.35 g - 0.57 g 0.60 M 0.66 g 3.45 g 2.79 g 3.04 g - 0.25 g 0.40 M 0.59 g 2.74 g 2.15 g 2.91 g - 0.04 g 0.20 M 0.63 g 3.83 g 3.20 g 3.18 g +0.02 g 0.00 M 0.70 g 4.46 g 3.76 g 3.28 g +0.48 g The percentage changes in mass were as follows: Concentration of glucose solution (M) Change in mass in percentage (%) 1.00 M - 24.76 % 0.80 M - 17.19 % 0.60 M - 8.34 % 0.40 M - 1.99 % 0.20 M + 6.29 % 0.00 M + 14.63 % Analysis The first observation I made was after the potato cylinders had been placed in their solutions for a few hours. The potato disks in the 0.00M, 0.20M and 0.04M glucose solutions were floating and the potatoes in the 0.60M, 0.80M and 1.00M solutions were at the bottom of the test tube, this lead me to drawing my next conclusion.From my experiment it is clear that as the concentration of the glucose solution increases, the mass lost by the potato increases. The loss in mass can only be reliably attributed to a loss of water from the cells of the potato because no other substances are able to pass through the potato's partially permeable membrane. The potato cells floated if they had lost mass, and were lower down in the test tube the more mass they had gained. In my explanation I have used the examples of potato cell walls for partially permeable membranes, and glucose solutions and the contents of the potato cells provided the two substances on either side in my experiment. ...read more.


It could well be the case that one or two of the potato disks in the test tube with 0.40 moles/dm3 glucose solution had a slightly greater surface area due to the angle at which they were cut. Apart from these causes, there may be others that are not factors which directly affect the rate of osmosis. Firstly, when I dried off the excess water on the potato cylinders after the experiment and before I weighed them, I used a paper towel. This might have either taken some water out of the potato or it might have left some excess water on the potato. This part of the experiment is difficult to come up with an accurate and fair method, as other ways would also lead to some slight mistakes. Another way of improving the results would have been to leave the experiment running longer, this would have enabled me to find the saturation point (when the potato can no longer take in any more water) and dehydration point (when the potato cannot lose any more water) and therefore get a more accurate result. In my preliminary experiment I used four potato disks in each test tube and varied the glucose concentrations by 0.25 moles each time. Although this preliminary experiment was useful to collate the data I later obtained in my final experiment, they were surplus results as the number of disks I used in my final experiment ensured that any anomalies hardly affected my results at all. Finally, I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, and then I would be able to see the cells in greater detail and draw some more observational results. From studying the possible causes for the anomaly in my experiment I have concluded that the most likely cause is human error in cutting the potato disks, or a change in temperature for the test tube in question. ?? ?? ?? ?? Georgia Barnett ...read more.

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