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An Investigation to Determine the Water Potential of a potato tuber

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Introduction

An Investigation to Determine the Water Potential of a potato tuber Background This experiment involves the process of osmosis. Osmosis is the process by which water moves across a membrane is known as osmosis and is described as the diffusion of water through a selectively permeable membrane The direction and amount of movement of water is dependant on the water potential. The water potential is based on the number and concentration of water molecules a solution has. For example, distilled water has the highest water potential, as it is the purest state of water. A highly concentrated solute solution has a low water potential. The highest water potential value (which distilled water has) is 0. Any dissolved solute-solution has a negative water potential value. Water potential is measure in the units Pascal. Water moves from a region of high water potential to a region of low water potential. Therefore, it moves down the water potential gradient. In this experiment we will be trying to determine the water potential of a potato tuber. Osmosis in this respect works on a cellular level. The cells that we are looking at are large, thin-walled, and usually have a large central vacuole. They are often partially separated from each other. In areas not exposed to light, as in a potato, food storage in the form of starch grains is the main function (Where light is present, e.g. in a leaf, photosynthesis is the main function). The amount of water present in these cells results in them having certain water potentials. If we place these tissues in different concentration sucrose solution we can see the if water has gone in or out of the cells. If the environment is hypotonic (having a lower concentration of solute than the cell) the net movement of water was into the cell, it would become turgid, the cytoplasm and vacuole pushed up against the cell wall, making the cell stiffer and wider and also result in the whole cell to increase in mass. ...read more.

Middle

* The time that was spent in solution is also an important factor. The time that is spent in the solution also affects how much water has had time to enter or leave the cell which means the cell has not had time to reach the isotonic point. This factor will be taken care of by allowing the potato pieces to stay in the solutions for a few hours, which is more than enough time for them to reach their isotonic point. Any difference in time that the potato strips spend in the solutions will be held to a minimum by placing and removing them within a small space in time. * We also understand that the potatoes are not identical and have not received the same environmental factors when being grown. Since we are doing repeats in this experiment, there is not enough potato to make all of the strips so more than one potato has to be used. This factor cannot be helped except for me to choose the healthiest potatoes and makes sure they are of the same type of potato. Apparatus 1x Scalpel - used to cut the potato strips to equal length 1x Tile - used to protect the table when cutting the potato strips 1x Glass Rod - used to stir the solutions when mixing water and sucrose solution 1x 30cm Ruler - used for the approximate measure of the potato strips 1x Cork Borer - used to accurately cut out the potato strips from the potato. 2x large potatoes - used as a source of plant tissue 200ml of distilled water - used to make the solutions 8x 250ml beakers - used to contain the water and sucrose solution and the other different molar solutions 200ml of 1M sucrose solution - used to make the solutions 1x marker pen - used to mark the beakers so that the contents can be identified 2x 50ml measuring cylinders - accurately measure the amounts of liquids to make the different ...read more.

Conclusion

all gave that same answers. My conclusion of finding the water potential of the potato cell could have been helped if I had a greater range of molarities with smaller increments which would show more accurately mass changes and also allow me to plot a better graph. The amount of repeats was sufficient and I believe that any more repeats would just give the same answers. Also the potato pieces itself was not definitely from the same potato and was not exactly the same size, although I did try to cut them to 2cm each, this could have effected the amount of water gained or lost. Anomalies include that in the yellow repeat in the 0.2 molar solution. This reading is not consistent as it indicates that that potato strip has not gained or lost any mass, which is inconsistent with the other repeats which show a range of 2.58% - 3.40% mass changes. This anomaly could have been caused by the strip not being weighed correctly at the beginning of the experiment or at the end of the experiment. You'll notice that the graph that shows all the results from each experiment do not all give the same mass change. Human error may have caused this. When the potato strips were removed from the test tubes and dried I may well have dried some potato pieces more thoroughly than others may and so some would have more excess water, which would add to the mass. I think the conclusion reached is quite accurate. Although there was an anomalous result the rest lay on a smooth curve which intercepted the x-axis at a credible point. The exact value of this point may not be exactly accurate as it is calculated from a freehand curve of best fit. The best thing to do if I was to carry on this experiment would be to make a 0.24 molar solution of sucrose and place potato strips in it. If I was right then the potato strips should neither gain mass nor lose any mass. ...read more.

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