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An Investigation to find the water potential of a plant cell.

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Introduction

Janine Regan February 2003 Preston College 46651 An Investigation to find the water potential of a plant cell Aim and Introduction The initial aim of the experiment was to determine the water potential of a plant cell. (Measured in kPa, ?). With the knowledge ofosmosis it was possible to discover the water potential of the cell. Osmosis states that water molecules move from a region of a higher water potential/concentration to a region of a lower water potential/concentration through a semi permeable membrane, until equilibrium. As emphasised, "until equilibrium", was the key part to the investigation, as it became the aim of the experiment. It then made sense that if a plant cell, let's say a potato sample was placed in a known created solution, such as in this case a sucrose solution and in due course no change in mass or volume in either the potato or solution occurs, we would then call this the isotonic solution. It would show that because no change had occurred then the solution outside the potato cell was at equilibrium, or had the same water potential as in the cell, as no movement occurred across the membrane of the cell. So this was the aim, to find the figure that matched the water potential inside the potato cell. For future reference it is necessary to point out the possible variables that could have affected the final results in any way: * Temperature * Surface area * Volume * Concentration Apparatus * Petri dish, with a lid ?7 * Scalpel ?1 * Tweezers ?1 * 10ml pipette ?2 * 1M sucrose (excess) ...read more.

Middle

The discs were placed in their groups in the weighing boat and weighed just as before with their change in mass recorded alongside their 'start mass'. Each time a group was weighed and replaced for another set of discs the boat was dried to prevent any excess fluid being added to the next weighing. 16) All the results were annotated into a table and the change in mass was calculated. Due to the different 'start' and 'finish' masses they needed to be standardised. This was done by calculating the percentage change, rather than just leaving the initial mass change. 17) The percentages were then plotted onto a graph and the point of which no change occurred was pointed out. To make it a bit more obvious where no change occurred the start and end mass was plotted, and where they intersected was the point of equilibrium. This percentage solution was then taken from the graph and compared to a given table for conversion, from percentage concentration solution to water potential. This was the estimated water potential of the plant cell. Results Concentration of sucrose solution (%) Mass of potato at start (g) Mass after 1 hour (g) Change in mass (g) Percentage change (%) 0.0 1.84 1.94 +0.10 +5.4 0.1 1.57 1.69 +0.12 +7.6 0.2 1.50 1.59 +0.09 +6.0 0.4 1.78 1.75 -0.03 -1.7 0.6 1.53 1.35 -0.18 -11.8 0.8 1.66 1.37 -0.29 -17.5 1.0 1.47 1.23 -0.24 -16.3 Discussion Provided are two types of graphs, presenting the data collected. ...read more.

Conclusion

Not just for the readings but for using the same borer, which kept the width and therefore surface area the same for each cylinder. As explained earlier in the investigation, surface area is a variable of osmosis, which in this situation was not required and needed to be kept under control. Likewise, the length of the cylinders were precisely measured with a ruler and cut with a scalpel, 24mm long, and then 4mm. The discs were all the same length for a purpose, to keep the surface area of the samples the same. Prior to weighing the saturated potato lumps they were picked from the petri dishes and placed on a paper towel, reason being the excess fluid needed to be drained/soaked so it didn't interfere with the final readings. From both the results and the way the investigation was carried out it seemed If this experiment were to be carried out once again I would consider placing the potato and solutions into a more controlled environment, for example control the temperature. Another suitable suggestion would have been to leave the specimens much longer, and therefore more detailed and accurate results. On that note, a further experiment would be to create sucrose solutions with the concentrations to two decimal places, between the found concentrations 0.2 and 0.4 (the area of equilibrium), such as 0.31, 0.32, 0.33, 0.34, 0.36, 0.37, 0.38, 0.39 and 0.40M sucrose solutions. With these values the graph would show a more precise isotonic figure and in turn a more accurate figure for the water potential of a potato cell. ...read more.

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