An investiagtion to establish the isotonic point of potato cell sap

Topics: Concentration, Sucrose, Measurement Pages: 5 (1085 words) Published: February 24, 2004
I have been asked to investigate the effects of osmosis on potato cell sap. I will test potato cylinders in different strengths of sucrose solution and record any results in a table. I will then analyse my findings and work out the isotonic point of the potato cell sap.

Prediction

I have looked at my scientific knowledge from the background theory I carried out and I will base my prediction upon this information. In this experiment, I expect the different concentrations to show fairly noticeable effects on the lengths and masses of the potatoes, with the water diffusing into the potatoes at a high rate in the sucrose solution and therefore making their size bigger than at the start. As the distilled water solutions become stronger, I expect the rate of osmosis to increase (and, as a result, make the potatoes themselves comparatively bigger at the end). Also, the potatoes will be quite turgid at the start of the experiment, there will be already some water inside, and because of this I expect, in the stronger solutions, for there to be a decrease in the mass and length of the potato, due to a reversal in the effect of osmosis.

Apparatus

· 6 test tubes

· White tile

· 1 molar sugar solution

· Electronic balance

· Stopwatch

· Measuring cylinders

· Ruler

· 12 pieces of potato (0.8g each)

· 2 beakers

· Distilled water

Method

We cut 12 pieces of potato using the cork borer and knife, and then we weighed each piece to 0.8g using electronic scales. The lengths of each potato cylinder varied so we measured them using a ruler and wrote the different lengths down. We placed the potato cylinders on a paper towel in order to dry any excess residue from them, and keep it a fair test. Then we placed them into test tubes that contained different sugar solutions. We put 2 pieces of potato in each of the six test tubes. The concentrations of sugar solutions were as follows,

distilled water/sucrose solution: - 0/10,1/9, 3/7, 7/3, 9/1, 10/0

After 1 hour, we took the potato cylinders out of the test tubes and recorded their lengths and mass in a table. We used the table to work out the change in mass and used a calculator to determine the percentage change in mass, which we plotted in a graph.

Fair Test

To ensure the experiment was a fair test, we took many precautions to keep the results accurate. It was important to get all of the measurements, weights and volumes as precise as possible. We used an electronic balance for weighing the potato cylinders, a ruler for measuring the lengths and a measuring cylinder for the solutions. We also made sure that the potato cuttings were placed in the test tubes at the same time and that they were completely submerged in the solution. Another step to ensure a fair test was cleaning all of the apparatus thoroughly before the experiment. A stopwatch was used to time how long the potato cylinders stayed in the test tubes.

To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. In this experiment, the concentration of the sugar solution will be the variable. If any of the variables were not kept constant it would mean the test would not be fair; for instance if one of the potato cylinders was 1 cm longer, then the surface area would also be greater than the other cuttings and there would be more space for osmosis to occur. Doing all the tests at room temperature will keep it a fair test.

Results

We made a table to record our results and work out the percentage change in mass. We used two potato cylinders in each of the solutions so that we could average the two results and use this to work out the change in mass. We used a graph to plot the percentage change in mass for each of the different sucrose solutions; it was also used to show any anomalous results gained through error or accident.

Lengthbefore(cm)Mass(g)Sucrose solution (cm")Distilled water(cm")End mass(g)Length...
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