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2Determining Water Potential of Potatoes Students

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  NO.06 DETERMINING WATER POTENTIAL OFPOTATO TUBER CELLS: THE WEIGHINGMETHOD If a plant cell is in equilibrium with an external solution of such a concentration thatthere is not net loss or gain of water, the water potential of the external solution will be equal to the water potential of the cell.Use of this fact can be made in estimating the water potential of a plant tissue.Samples of tissue are immersed in a range of external solutions of different strengths.The solution that induces neither an increase nor a decrease in the volume or mass of the tissue has the same water potential as that of the cells in the tissue.The cells to be investigated in this experiment are those of the potato tuber. Changesin mass will be used as an indication of whether the cells are taking up or losing water. Procedure !abel six specimen tubes distilled water, #., #.$, #.%, #.&, #.' mol dm (% . )laceapproximatel* one third of a tube of distilled water in the first, and an equalvolume of each of a series of sucrose solutions of different strengths and+molarities in the remainder. -ach tube should be firml* stoppered.$Using a cork borer and a raor blade, prepare six solid c*linders of potato.-ach c*linder should be approximatel* #mm in diameter and $mm long.Slice up each c*linder into six discs of approximatel* equal thickness. )laceeach group of discs on a separate piece of filter paper.%/eigh each group of discs. +In each case weigh them on the piece of filter  paper alone, and subtract the one from the other to get the mass of the discs.0ecord the mass of each group.&)ut one group of discs into each of the labelled tubes and as *ou add eachgroup, record its mass. Stopper the latter firml* and leave for not less than $&hours.'1fter about $& hours remove the discs from each tube. 0emove an* surplusfluid from them quickl* and gentl* with filter paper, using the standardised procedure for all of them. Then re(weigh them. 0ecord the new mass of eachgroup of discs.23raph *ou r results b* plotting the percentage change in mass +change in massmultiplied b* ## divided b* srcinal mass against the molarit* of the sucrosesolutions. The latter, being the independent variable, should be on thehoriontal axis4 the former on the vertical axis.5Calculate the water potential of the potato cells as follows. 6ind the point on*our graph corresponding to a percentage mass change of ero. The molarit*of sucrose corresponding to this ero mass change can now be read from thehoriontal axis. 6rom table &. find the water potential of a sucrose solution of that molarit*. That is the water potential of *our sample of potato cells.-xpress *our result in a k)a. $&%''$#5'.78C)age  of $  For consideration Criticise this method of finding the water potential of plant cells. 9ow might it be improved:$/hat was the reason for dividing each c*linder into six discs, and wh* was itnecessar* to standardise the procedure for dr*ing the discs:%/ith what kind of plant tissue might it be possible to use a change in volumerather than mass for estimating the water potential:&9ow does the value of the water potential differ from the osmotic potential of the solution in the vacuole:'In constructing *our graph did *ou ;oin up the points with straight lines or ;usta smooth curve: <ustif* whichever technique *ou used. Molarity (mol dm)Osmotic potential kPa #.#'(%##.#($2##.'(&##.$#('&##.$'(2=##.%#(=2##.%'(>5##.&#($##.&'($=##.'#(&'##.''(2$##.2#(=###.2'(>=##.5#($=##.5'($%5##.=#($'=##.='($5>##.>#(%####.>'(%$'#.##(%'##Table&. 0elationship between molarit* and osmotic potential of sucrose solutions $&%''$#5'.78C)age $ of $
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