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Isotonic Point of Potatoes

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Aim: To determine the isotonic point of a potato. Research Question: To determine the isotonic point of a potato by placing it in different concentrations of salt (NaCl) solution and measuring the change in mass after a fixed time period. Background Information Osmosis is defined as the movement of water molecules from a region of low solute concentration, to a region of high solute concentration, through a semi permeable membrane, which is one that allows only certain small particles to pass through. A solution with a high water concentration, or low solute concentration is also called a hypotonic solution, and one with a low water concentration is known as a hypertonic solution. When a cell is placed inside a hypertonic solution, water molecules diffuse out of the cell (which has a higher water concentration, or is hypotonic) through the cell membrane, due to the high solute concentration outside the cell. This process is called exosmosis, and it makes the cell flaccid, as it loses water. The very theory can be applied to plants, which causes wilting. When a cell is placed in a hypotonic solution, water molecules diffuse from the solution into the cell, as the solvent concentration of the cell is lower. This causes the cell to become turgid, and it is called endosmosis. Source - http://biosimplicity.webs.com/ A solution which has the same concentration as that of the cell has no concentration gradient as both the cell and its environment have the same water potential. Thus the net movement of water is zero and the system is in equilibrium. Water molecules flow in and out at an equal rate by osmosis, causing the cell size to stay the same. ...read more.


thin * They had become very soft and completely flaccid * They could bend very easily and became very delicate, thus had to be held carefully * The colour had become very pale, and the cylinders appeared slightly translucent. * The cylinders seemed to be thinner from the centre. Table showing change in mass and percentage change in mass of the potato for each trial of the different concentrations Percentage change = (change in mass/mass before) * 100 Concentration of NaCl solution (M) Trial Mass before (?0.01g) Mass after (?0.01g) Change in mass(�0.02g) Percentage change in mass (%) 0.1 1 1.53 1.80 0.27 17.6 2 1.46 1.77 0.31 21.2 3 1.53 1.81 0.28 18.3 4 1.58 1.90 0.32 20.3 5 1.56 1.87 0.31 19.9 0.2 1 1.53 1.71 0.18 11.8 2 1.53 1.72 0.19 12.4 3 1.51 1.67 0.16 10.6 4 1.41 1.62 0.21 14.9 5 1.42 1.57 0.15 10.6 0.3 1 1.16 1.22 0.06 5.2 2 1.15 1.20 0.05 4.3 3 1.14 1.19 0.05 4.4 4 1.09 1.18 0.09 8.3 5 1.05 1.12 0.07 6.7 0.4 1 0.89 0.84 -0.05 -5.6 2 0.90 0.85 -0.05 -5.5 3 0.82 0.79 -0.03 -3.7 4 0.91 0.88 -0.03 -3.3 5 0.86 0.83 -0.03 -3.5 0.5 1 1.17 0.93 -0.24 -20.5 2 1.13 0.92 -0.21 -18.6 3 1.11 0.95 -0.16 -14.4 4 1.16 0.94 -0.22 -19.0 5 1.12 0.92 -0.20 -17.9 Calculation of average percentage change in mass of the potato Formula used: - Where x is the percentage change in mass; n is the number of trials and is the mean Concentration (M) Average or mean % change in mass 0.1 17.6+21.2+18.3+20.3+19.9/5 = 19.5 0.2 11.8+12.4+10.6+14.9+10.6/5 = 12.1 0.3 5.2+4.3+4.4+8.3+6.7/5 = 5.78 0.4 -5.6-5.5-3.7-3.3-3.5/5 = -4.32 0.5 -20.5-18.6-14.4-19.0-17.9/5 = -18.1 Table showing calculations for standard deviation: Concentration Trial % change in mass Deviation (d) ...read more.


* The temperature or pressure were simply monitored and not regulated, hence a change in temperature or pressure would result in a change in the rate of diffusion, which would in turn affect the final mass. * The surface area of the potato cylinders may not have been equal for all trials as the cork borer may have been pierced in the potato at different angles. * If the potato pieces were not completely dried before weighing then the mass of the residual water on the surface of the pieces (which had not diffused into the potato) would be taken into account. Consequently this would increase the mass of the potato, thus resulting in errors in the results. * Two different potatoes were used as the first potato was not enough to obtain 25 potato cylinders. As a result the isotonic points of both potatoes may have been slightly different. Improvements to the experiment: * The air conditioner and fans should have been switched off while taking readings of mass * Instead to keeping the beakers out, they could have been kept inside a fridge. This would prevent evaporation of water. The beakers could also have been covered with cling film to prevent evaporation of the water. This would ensure that the concentration of the solutions remained constant, and thus the rate of diffusion would not get affected. * In order to make the experiment more accurate, concentrations between 0.1M, 0.2M, 0.3M and 0.4M could have been taken, such as 0.15m, 0.25m etc. This would make it easier to find the exact isotonic point of the potato, and thus the experiment would have been much more accurate. * One potato should have been used in order to maintain a fair test ...read more.

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Marked by teacher Luke Smithen 01/01/1970

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