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# To determine the water potential of potato tuber tissue

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Introduction

To determine the water potential of potato tuber tissue Introduction Osmosis is the net movement of water down a gradient of water potential through a partially permeable membrane until the water potential gradient is constant. When a tissue looses water, it looses mass, and so we can take a measure of how much osmosis has taken place by weighing the tissue before and after it spends time is solution. If there is no mass change then the potato tissue will have the same water potential as the external solution, and so the two water potentials are in equilibrium, and there is a flat gradient of water potential. The formula for the water potential of a plant tissue is written: as the water potential equals the solute potential added to the pressure potential (from Biology 1 by OCR): ? = (+)?p + (-)?s The units for these values are recorded in KPa. Osmosis occurs until there is no net movement of water, because the water potential is equal on both sides of a partially permeable membrane. Preliminary Work I conducted a pilot study before I planed my actual experiment to gauge what sort of length potato to use and to see if my values for sucrose molarity were adequate. For my GCSE coursework I conducted an experiment to see the effect of varying the surface area of potato chip in sucrose solution. I used a 1M sucrose solution and the potato chips lost about on average about 0.6g, for chip volumes varying from 2cm3 to 8cm3, left in solution for 45mins. This is quite substantial so I decided to make my dilutions from 1M and diluting it sufficiently. I found from previous experiments that the water potential for a potato tuber would be about 0.3M, so I chose a range from 0.3M up to 0.5M, using distilled water as a control. From various Internet sources (www.essaybank.co.uk), I found that the water potential can differ allot from different potatoes, and so I could not quote any source for this. ...read more.

Middle

I have volumes above 10cm3, which is what the pipettes go up to, and so is the next most accurate thing to use. It is much easier to use than a larger pipette, and quicker. Accuracy to 1cm3 is fine for large volumes Beaker To hold the sucrose solutions and water. It is large enough to hold the volumes of water and sucrose that I need, so I don't have to constantly refill from stock solutions. Ruler To measure and then cut the potato chips to 3cm length. All the potatoes need to be the same length, and so an average ruler is the easiest thing to use to measure the potato chip lengths. Cutting Board To cut and core the potato on. The corer and blade are sharp and it would be unsafe to use them into a hand. The lab benches may be dirty, and so we must cut onto something clean. Dilutions Table for different solute potential solutions. Concentration/M Volume Of H2O/cm3 Volume of 1M Sucrose/cm3 Final Volume/cm3 0 17 3 20 0.2 16 4 20 0.25 15 5 20 0.3 14 6 20 0.35 13 7 20 0.4 12 8 20 The range of my independent variable is based around the water potential of the potato tuber tissue. It would be difficult to make the solutions accurate to more than 0.05M, and so I chose two readings above and below the expected 0.3M, and used distilled water as a control. I shall repeat my readings two times each, so that I can see if there is a pattern, and to see if there are any anomalous readings. For example, if two readings are the same and another is wildly different. If I encounter a result, which is far away from two others in a solute potential range, then I will not include it when I average my results. To be wildly different I would expect it to be 0.1g to 0.15g away from the other two readings. ...read more.

Conclusion

The graduating cylinders only measured to the nearest 1cm3, which is not very accurate, but should not have affected my results too much, as I was using large volumes. However, I could have used pipettes, it would have been much more accurate, to the nearest 0.05cm3, this would have made my measurements much more accurate. I did two repeats for each value of solute potential for my results, and I believe this to be sufficient, as my results were all consistent between readings within a molarity range. The change in the % mass change within a range is a maximum difference of 1.3% (see results table). The maximum difference in the -970KPa range is only 0.1%, which is very small, and so seems reliable. Vertically, the results change at a seemingly constant rate, which can be seen from my graph, comparing results with my best fit line, which is what I predicted, and this supports my prediction well. All of the errors in my experiment would have affected my results, however, I believe that the blotting error was the main error overall. It needed to be controlled, because it could significantly affect the mass of my chips if there was excess liquid, or if too much had been taken out while blotting. I tried to have consistent blotting, and so any error due to too much or too little blotting is likely to be minimal given that the error is quite likely to be consistent. The improvements I can put to my method, especially to the blotting error would benefit the experiment. With blotting at exactly the same extent the excess water will not contribute to the mass change and so the results will more truly reflect my independent variable. There uncertainties certainly affect my results, but the question is, how valid can a conclusion drawn from them be? The errors will obviously affect my results, and make them harder to draw conclusions from. However, the results agree with the preliminary work I conducted and are consistent with each other, and I think I can be quite firm in my conclusions. Michael Hutton-Ashkenny ...read more.

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