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# Investigate and find the water potential of baking potatoes and sweet potatoes in (N/mm2) using various strength solutions of sucrose (mol).

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Introduction

16th March Biology Coursework Aim To investigate and find the water potential of baking potatoes and sweet potatoes in (N/mm2) using various strength solutions of sucrose (mol) Hypothesis I believe that the baking potatoes will have a less negative water potential. In the experiment they will not increase in mass at a lower of sucrose molarity than the sweet potatoes. Baking potatoes contain more water than sweet potatoes. This is clearly obvious, cut a baking potato into cubes and it releases a lot of water, more so than sweet potatoes. Water potential determines the rate of osmosis. Water movement form a high concentration to a low one is the means for testing the aim. Distilled water and other strength solutions will flow into the sweet potato more readily than into the baking potato. This is because baking potatoes have more water, and thus a less negative water potential. Sweet potatoes have less water content and will therefore have a more negative water potential. Both potatoes will have a lower water potential then the distilled water and other sugar solutions. As water diffuses into their cells they will increase in mass. The weight difference will be converted into a percentage so that the water potential can be determined. When plotted on a graph, percentage change of mass against solution molarity, the line of best fit will cross the x-axis. At this point the potatoes will not have increase in mass, their mechanical pressure will offset the solute potential. At this point there would be no diffusion gradient and osmosis would cease. The potatoes would have the same water potential as their surroundings. Using tables I can determine what the water potential is in kPa or N/mm2 by means of comparison using the graph obtained by this experiment. (See appendices i) Obviously the sweet potato and the baking potato would have different water potentials. Their lines of best fit would cross at different points. ...read more.

Middle

The thermometer must be out of sunlight because that could give a biased reading. Rinse out all components (not electronic like the stop clock or the thermometer) three times to remove impurities that could affect the accuracy of the solutions. Rinsing the equipment three times dilutes impurities by a greater factor than just doing it once. Using a borer cut out a whole cylinder of potato out of the baking potato. Using a white tile as a backboard, cut 108 slices of potato, taking more cylinders out of the potato then needed, of roughly the same width using a scalpel. Take care with the scalpel as it is much sharper than a normal knife. The white tile will protect the surface you are working on. The 108 slices will be sufficient for the entire experiment. 36 slices will be required for the different solutions, six to each test tube. The remainder will allow for two extra repeats. The potato slices must be the same width and diameter to have the approximately same surface area, this is a primary factor over differences in mass as they are generated into percentages later on. The surface area will affect how much water can enter the total volume of the potato slice. Label six test tubes with the values given below. Mix together the quantities of distilled water and 1.0 mol sucrose solution as directed below. Label Add Distilled Water Volume (ml) Add 1.0 Mol Sucrose Volume (ml) 0.0 10 0 0.2 8 2 0.4 6 4 0.6 4 6 0.8 2 8 1.0 0 10 You must cover then shake each test tube to mix up the sucrose and water, this will give an even water potential. The values of the mixtures give the molarity of the solution as said on the label. These molarities give a wide range of possible readings, sufficient enough to give a proper reading. ...read more.

Conclusion

This could also change the position of the line of best fit. Anomalous results could be caused by laziness or fatigue, or because the equipment was not washed well enough with distilled water before use. Also, if you do not keep wiping the scales and resetting it then the weights might be recorded wrongly. When making solutions it is important to check volumes in the pipette by looking at the level of the meniscus on a level plane and not at an angle, this could give differing volumes and inaccurate concentrations. The limitations of the experiment were that the slices were not kept for sufficiently long periods of time in their solutions, and this prevented the maximum effect of osmosis from being carried out and thus a more representative percentage mass change. In subsequent investigations, the time may be extended to seven and a half minutes to allow for extra diffusion of water. More accurate readings for mass change would give a more accurate value for osmotic potential. You could also find the optimum temperature to submerge the slices in, because the efficiency of the protein based water channels is dependent on their hydrogen bonds which are easily affected by temperature changes. A preliminary experiment could find this out by using a sucrose solution of 0.4 mol and trying different temperatures to see which one put on the most weight, which would therefore be the optimum temperature as the protein channels are working their best. Finding the best temperature would give more true values for mass change as the cells would be working at their best. This in turn would give a better osmotic potential value that was more accuracy. Appendices (i) Molarity (mol) Osmotic Potential (N/mm2) 0.05 -0.13 0.10 -0.26 0.15 -0.41 0.20 -0.54 0.25 -0.68 0.30 -0.86 0.35 -0.97 0.40 -1.12 0.45 -1.28 0.50 -1.45 0.55 -1.62 0.60 -1.80 0.65 -1.98 0.70 -2.18 0.75 -2.37 0.80 -2.58 0.85 -2.79 0.90 -3.00 0.95 -3.25 1.00 -3. ...read more.

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