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The aim of this experiment is to determine the water potential of celeriac cells by investigating the osmotic effect of different concentrations of sucrose on the cells.

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Introduction

Biology Assessed Practical - Rosalind Brock Spring 2003 The water potential of Celeriac Aim The aim of this experiment is to determine the water potential of celeriac cells by investigating the osmotic effect of different concentrations of sucrose on the cells. Background Theory Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of high water potential to low water potential. Water molecules always move from areas of high concentration, to areas of low concentration. In an ordinary solution or mixture, the particles of both substances diffuse so that there is a uniform spread of particles throughout the mixture. However, if there is a partially permeable membrane, only the water molecules are small enough to pass through it and the larger solute molecules remain on one side of the membrane. The water molecules will move from where there are more to where there are less of them. Since diffusion occurs at random, the water molecules will in fact be passing both ways, but the net change will be from high concentration to low concentration. The tendency of a solution to lose water in this way is called its "water potential". So, water moves from areas of high water potential to areas of low water potential. The symbol for water potential is the Greek letter psi - Q, and it is measured in kilopascals (kPa). Water potential has two components, pressure potential (Q[p]) and solute potential (Q[s]). The overall Q of a cell is the sum of the two components. The Q of pure water is set at zero by convention, so the Q of any cell or solution is =< 0. Since osmosis occurs down a concentration gradient, solutes will lower the water potential of a solution. The amount the water potential is lowered by the solutes is the solute potential, so Q[s] always has a negative value. ...read more.

Middle

This is a limiting factor of the mass loss. Equally a cell cannot go on gaining mass for ever, since at some point the turgor pressure will balance out the solute potential and the cell will have Q = 0, (i.e., fully turgid) and not gain any more mass, so there is also a limiting factor for the mass gain. Therefore, I would expect a graph of the shape shown below. [image008.jpg] Main Experiment Planning a fair test Independent variable: Sucrose concentration Dependent variable: Percentage change in mass Controlled Variables: starting length of celeriac tubes, starting thickness of celeriac tubes, exposed surface area of celeriac, type and age of celeriac, volume of sucrose solution, time tubes are left in solution, amount of drying on absorbent paper. � All the tubes will be cut to 7cm long in order to control starting length, � All the tubes will be cut with a size 3 borer to control starting thickness, � Using the same length and thickness will control surface area, and the tubes will be cut perpendicular to their length to give an equal "end" area on each tube. � All 24 tubes will be cut from one large celeriac if possible � 10cm^3 of sucrose solution will be used in all cases. � All the tubes will be left in their solutions for half an hour. � The tubes will all be rolled once on the absorbent paper to dry off just the drips of solution. Apparatus Size three borer - completely accurate if the same one is used for all 15 tubes Ruler - accurate to 1mm Scalpel Cutting mat Top pan balance - accurate to 0.01g Test tubes, Cling film, 10cm^3 syringes - accurate to 0.1cm^3 Forceps Absorbent paper Risk assessment Method 1. The top was cut off a large celeriac in order to give a flat surface. 2. The celeriac was placed flat side down on a cutting mat. ...read more.

Conclusion

The best way to minimise the effect of this error would probably have been further repeats, since it is difficult to control the density of the celeriac. The volume of sucrose solution could have been better controlled by using more precise measuring equipment, a Pasteur pipette and filler for instance. The time that the tubes were left in the solution could be more precisely measured if the test tubes were done one at a time, or at least staggered over regular intervals, so that the potato tubes could be put in and taken out at the precise moment. However, since equilibrium should have been established by the time of removing the celeriac tubes, the effect of slight differences in the time the tubes were in sucrose should have been minimal. The amount of drying was also quite hard to control. Since the higher concentrations are more viscous, less solution tended to come off the celeriac tubes, but if gentle and uniform pressure were applied to the tubes as they were dried, perhaps by doing it mechanically, then this would be fairer. It would be beneficial to do further repeats to find a more accurate average for each result. This experiment could be extended to find out whether a limiting factor was affecting the loss of mass. Using concentrations of up to 1.5M would also enable the point of no change in mass to be seen more clearly. If the dry masses of several 7cm celeriac tubes were taken and averaged, then the total mass of water could be found by comparing this figure to the mass of the tubes before they were dried. Then the percentage of the tube's mass that is water could be found. The experiment could be continued with higher and higher molarities, until the graph had completely reached a plateau. The percentage at which the tubes stopped losing mass could be compared to the percentage of water in the tubes, and if the figures were about the same, then this would prove the hypothesis that a limiting factor was total plasmolysis of the celeriac cells. ...read more.

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