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To Investigate the Rate of Osmosis In Potatoes and Find Out the Sucrose Concentration of the Potato.

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Introduction

INTRODUCTION Aim: To investigate the rate of osmosis in potatoes and find out the sucrose concentration of the potato. Osmosis is the movement of water molecules from a solution of high water potential to a solution of low water potential across a partially permeable membrane. Water potential is the ability at a solution to lose water. A concentrated solution has a low water potential and a dilute solution has a high water potential. The diagram below illustrates the concept of osmosis. The diagram above shows a concentrated sucrose solution and a dilute sugar solution separated by a partially permeable membrane. This membrane allows small molecules, such as water, to pass through it but it does not allow larger molecules like sucrose through it. The water moves from the right to the left so that the sucrose concentration on the left is decreased. The concentrated solution becomes more dilute due to the extra water molecules entering it. In plant cells the cell changes when the concentration around it changes. A plant cell can become turgid or flaccid. When the concentration of water is higher outside the cell than inside the cell, water will move by osmosis down its concentration gradient, through the cell membrane and into the cell. As water goes in the vacuole and cytoplasm will swell. As the cell has cell wall, which is fully permeable, it will not burst. The cell's volume and mass will increase. The cell will expand to its maximum size and it is then said to be in a turgid state. When the concentration of water outside the cell is lower than inside the cell, the opposite happens. ...read more.

Middle

When the solution is 0.35 it is higher than the potato so water diffuses out of the potato by osmosis and the concentration of the solution is reduced. As the cells lose water they become flaccid. There is no water movement when the concentration is the same of the potato. From the preliminary I predict that the concentration of the cell sap is 0.3 M. However this may not be exactly correct but only approximate as a wide range of concentrations was used in the preliminary. The diagram below demonstrates this: 0.25 M 0.3 M 0.35 MAIN EXPERIMENT Aim: To find out the concentration of the cell sap in a potato by investigating the movement of water within a potato. Apparatus: - Potato x 1 - Pair of borers - Knife - White tile - Test tubes x 15 - Test tube rack x 3 - 1 molar sucrose solution - Water - Syringe (10ml) x 2 - Forceps - Filter paper - Electronic weighing scales - Sticky labels x 15 Method: 1. Using the knife the potato was cut into three pieces. 2. 15 cylinders were cut from the potato using the pair of borers. (3 cylinders per piece of potato) 3. The 15 potato cylinders were all cut to a length of 4cm using the knife. This meant that all 15 had the same surface area. 4. All 15 potato cylinders were weighed and their masses were noted. 5. The concentrations of 0.25, 0.275, 0.3, 0.325 and 0.35 molar were written on to the sticky labels. Each concentration had three labels and these were stuck on to the 15 test tubes. ...read more.

Conclusion

As I had done three tests for each concentration the results were more accurate as averages of the three tests were used to plot the graph. The third test for when the concentration was 0.275 M was anomalous as it was much lower than the other values (tests 1 and 2). Also, the average % mass change for when the concentration was 0.325 M was quite high and it was the furthest point from the line of best fit. These anomalous results were most probably caused by human error i.e. inaccurate measurement. Improvements 1. Instead of using potato cylinders I would use smaller discs. The cylinders would be cut into equally sized smaller discs, which would increase the potato's surface area. This would mean that the mass change of the potato would be more substantial and the results would how a clearer trend. 2. The variables would be kept constant in order to keep it a fairer test. This means that the concentrations, amount of solution, size of discs, time would all be measured more accurately. The temperature and pressure would also be monitored. 3. The potatoes would be dried using the filter paper evenly so that the results would not be largely affected. Extending the investigation The investigation could be extending in a number of ways b8ut still based around osmosis. In my investigation I investigated how the concentration of sucrose solution affected the movement of water into and out of the potato. Instead of the concentration of a sucrose solution, other variables such as temperature, pressure and humidity could be varied to see how they affected the rate of osmosis. Different vegetables could be used to see how they compare to how the potato was affected. Various root vegetables could be used such as carrots, onions, potatoes. ...read more.

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