Investigating the Effect of Concentration on Osmosis

Investigating the Effect of Concentration on Osmosis
Preliminary Investigation The aim of the preliminary experiment was to find a suitable range to collect our results. To find a suitable range we had to carry out the experiment to see if there was a substantial change in the size and weight of the potato chip after it had been placed in a 1 molar glucose and another chip in distilled water. At the beginning of the experiment we had to cut out potato chips of equal length and similar shape. We obtained the potato chip by using a cork borer; we made sure we cut off the edge with the skin on it, as this would affect the results at the end. Using a scalpel we sliced the cylindrical piece of potato into equal lengths, measured them and then weighed them. These are classified as our initial results. Since we had a boiling tube of glucose and distilled water, these were measured to the same volume earlier using a measuring cylinder. These two tubes were then placed into a water bath that was heated using a Bunsen burner to a temperature between 25-40ËšC.
The potato chips were then placed into each tube and left in there for 30minutes. After these 30 minutes the chips were weighed and then measured to see if there was any change. We used 50ml of glucose and distilled water and the temperature of the water bath was at 30ËšC. Results Solution Initial Length of Chip (cm) Final Length of Chip (cm) 1 Molar Glucose Solution 2.8 2.6 Distilled Water 2.8 3.0 Conclusion From the results we can see that the chips had changed in length; we can now use the same measurements in the main experiment as in the preliminary experiment. These measurements include the length, diameter of chip and temperature etc. When the chip was in distilled water the size had increased and in the one molar glucose solution we can see that the chip size had decreased, from this we can see that the concentration of glucose within the cell sap is between these two solutions. Main Experiment ? To design an experiment to determine the concentration of the cell sap of potato. Aim We are trying to find out the concentration of glucose in a potato chip. Prediction I predict that the concentration of the cell sap (glucose) within the chip will be between a 1 molar glucose solution and distilled water. I predict this as when the glucose solution is more concentrated than the glucose within the cell sap the potato chip will become smaller and therefore when the glucose solution outside of the chip is more dilute than the glucose within the chip, the chip will become longer. This is as when the glucose solution surrounding the chip is more concentrated than the glucose within the chip water from inside the vacuole will leave the cell, as the potato cell wants to gain an equilibrium with the surrounding glucose solution. Since the water has left the vacuole, the vacuole is then smaller and the cells become flaccid. When the surrounding glucose solution is weaker than the glucose within the cell sap, water from the surrounding solution will move into to the cell and this extra water will make the cell bigger and heavier. As the vacuole has become larger it pushes against the cell membrane making the cell turgid. The movement of water from a more dilute solution to a more concentrated solution through a partially permeable membrane is called osmosis. We will know the concentration of the glucose inside the cell sap, as there will be no movement of water in or out of the cell. Apparatus ===== Measuring Cylinders ? To measure the volumes of solutions Water Bath ? To get tubes of solution to correct temperature Test Tubes ? To place potato chips in with solution Ruler ? To measure the length of potato chips Cork Borer ? To get a potato chip from a potato Scalpel ? To cut off any skin off of the potato Diagram Method == - Set up apparatus as shown in the diagram above - Get a potato chip using the cork borer (0.7cm) - Make sure to that the potato has no dents and looks about cylindrical. - Cut off any skin off of the potato - Start a water bath to the temperature 30Â?C - While water bath is going, measure and weigh the chip and note them down in a table This is known as the initial length. The chip should be 3.0cm long. - Place chip in a test tube of 20ml of solution - Place this test tube in the water bath and let it stay in for 30mins. Ensure the temperature of the water bath stays at 30Â?C - After 30mins take chip out, measure and note down length in table - Repeat using the same type of solution 3 times and take an average so results will be correct. This eliminates a bad set of results due to an anomaly. - Repeat using different solutions. Variables ===== Independent ? Concentration of glucose solution Dependent ? Length of chips To make the test fair there were many factors that we had to keep the same and these were: Surface area of chip A chip with a larger surface area (e.g. A long flat chip or a chip with dents) will affect the results, as it will make osmosis occur quicker as more cells are in contact with the glucose solution. Volume of solution If the volume of the glucose solution did not submerge the chip it would mean that some potato cells were not in contact with the glucose solution so no osmosis would occur in these parts. Temperature If the temperature got higher it would affect our results as the higher the temperature the more energy the particles have. If the water molecules had more energy it would mean that they would diffuse quicker. This is known as the kinetic theory. If the temperature got higher than about 40Â?C it would kill the chip, as the temperature is too high. Time The time the chip is left in the solution matters as if it was left in for longer then osmosis will have fully taken place. If the chip had just been left in for half a minute there would not be sufficient time for osmosis to occur. Skin of potato If this skin had been left on it would mean that no osmosis would occur were the skin had been left on, as it is a hard protective outer layer. Type of potato Different types of potato have different types of membrane so water may diffuse out of one potato slower than another. Results The following section will contain the results obtained during the experiment. Length of Potato Chip in centimetres Type of Solution Start Result 1 Result 2 Result 3 Average Percentage Change Distilled Water 3 3.4 3.3 3 3.23 7.67 0.2M Glucose Soln. 3 3.0 3.1 3.1 3.07 2.33 0.4M Glucose Soln. 3 3.1 2.9 2.9 2.97 -1 0.6M Glucose Soln. 3 2.8 3.0 2.9 2.9 -3.33 0.8M Glucose Soln. 3 2.8 2.8 3.0 2.87 -4.33 1.0M Glucose Soln. 3 2.7 2.7 2.9 2.77 -7.67 Analysis ==== The results above show us that a potato chip will change its size when placed in different concentrations of glucose due to diffusion. The results obtained followed my prediction as the chip did shrink when the concentration of sugar in the beaker was between 0.4M and 1.0M and the chip grew in size when the concentration of sugar in the beaker was at 0 ? 0.2M. The basic principle of my results is correct, as the law of osmosis states water moves from a high water concentration to a low water concentration through a semi permeable membrane. My prediction stated that the concentration of the cell sap (glucose) within the chip will be between a 1 molar glucose solution and distilled water. I predict this as when the glucose solution is more concentrated than the glucose within the cell sap the potato chip will become smaller and therefore when the glucose solution outside of the chip is more dilute than the glucose within the chip, the chip will become longer. This is as when the glucose solution surrounding the chip is more concentrated than the glucose within the chip water from inside the vacuole will leave the cell, as the potato cell wants to gain an equilibrium with the surrounding glucose solution. Since the water has left the vacuole, the vacuole is then smaller and the cells become flaccid. When the surrounding glucose solution is weaker than the glucose within the cell sap, water from the surrounding solution will move into to the cell and this extra water will make the cell bigger and heavier. As the vacuole has become larger it pushes against the cell membrane making the cell turgid. The movement of water from a more dilute solution to a more concentrated solution through a partially permeable membrane is called osmosis. We will know the concentration of the glucose inside the cell sap, as there will be no movement of water in or out of the cell. For instance when the potato was placed in distilled water and 0.2M sucrose solution the glucose solution is weaker than the glucose within the cell sap, water from the surrounding solution will move into to the vacuole of the cell and this extra water will make the cell bigger and heavier. As the vacuole has become larger it pushes against the cell membrane making the cell turgid. We can see that this makes the chip longer. At 0.4M, 0.6M, 0.8M and 1.0M solutions, water from inside the vacuole will leave the cell, as the potato cell wants to gain an equilibrium with the surrounding glucose solution as the solution outside is more concentrated than the glucose solution inside. Since the water has left the vacuole, the vacuole is then smaller and the cells become flaccid and the length has decreased. The table above shows us that the glucose concentration within a potato cell is between 0.2M and 0.4M. 0.2M is where the cell increases in length the least and 0.4M is where the cell only shrinks a tiny bit. From the graph we can see that at 0.32M is the concentration of glucose within a cell as the line of best fit crosses the x-axis at this point meaning there is no change in size for the chip. No change in size means that the chip is in equilibrium with the solution around it so this is the concentration of glucose in the potato. Evaluation -???- In general my results followed the pattern stated within the analysis but having plotted the graph there is one anomaly. When the concentration of the sugar was at 0.8M we can see that I received a decrease of only 4.33%, which is too small a decrease. The decrease at this point should have been around 5.5%. This anomaly could have occurred due to a series of factors, such as: - The potato chips had not been dried properly - The measuring cylinder could have been dirty and mixed with other sugar concentrations - The test tubes could have been wet so the concentration of the sugar solution would have been altered - The sugar solution used may have been made up to the wrong concentrations as there was no way of measuring the exact concentration of the 0.8M solution - When carrying out the experiment the potato chip could have been taken out too quickly so osmosis did not have as long to occur as the other results With only 1 anomaly and all points falling into a smooth curve, the results have been accurate and each test carried out in a similar way. To make this experiment fair I only changed one variable throughout the whole experiment in this case was the concentration of solution; the potato sizes were kept relatively similar, each potato cylinder was cut from the same potato and the same sized containers were used. I made sure I used the measuring cylinder so I could as accurately as possible measure the volumes of sugar solution correctly and temperature of the water bath kept at 30Â?C. Using a Bunsen burner was a problem as it only heats the bottom of the beaker and when the solutions had a higher sugar concentration the chip would float so this was a slight problem and often had to be pushed down to keep it submerged so this means that the potato may not have been at 30Â?C. This would therefore affect osmosis as mentioned earlier to improve this a thermos flask or something similar could have been used to keep at a constant temperature at all times and not having to stick a Bunsen burner beneath the beaker when the temperature dropped. To measure the volumes of the sugar solution to a more accurate degree a syringe or pipette could have been used. A ruler was used to measure the lengths of the potato before and after the experiment. At times this was inaccurate as the potato chip may have grown or shrunk in a strange manner leaving an edge diagonal so depending on which side you measured it from the results may have changed. The ruler only gave results to the nearest millimetre. This was a slight problem, as during the results section 1mm will swing the results the wrong way as shown in the 0.8M results as if the 3rd result was at 2.9cm and not 3.0cm it would have not been anomalous. However in the lab there is no equipment that could improve this problem at the moment. The sugar solutions used may have been tampered with and at times we could not tell at exactly what concentration they were at except from the large sticker on the bottle so this may have affected the results. I feel that the experiment has been reliable as each test was repeated 3 times but if there was more time more tests could have been done giving a better set of results. The sugar solutions had been to an adequate range as I was able to find out roughly were the concentration in a potato lied. The basic principle of my results is correct, as the law of osmosis states water moves from a high water concentration to a low water concentration through a semi-permeable membrane as there were non that decreased in size in a trend and then suddenly increased in size and then decreased again. To measure osmosis to a greater degree we could have also found out the weights of the potato as osmosis also affects the weight of the chip as when a chip is placed in a more dilute solution than the one inside its cells it will also get heavier as well as longer as water will move into the vacuole. An alternative way to look at osmosis is to view the cells under a microscope and to see whether there is a difference in the vacuole size after we have placed the cell in a glucose solution so we can see the physical effects of osmosis. Instead of using potato cells we could use onion cells, as it is easier to see the cells in an onion than in a potato. Conclusion -???- After the experiment I have seen that concentration of glucose does affect osmosis within a potato cell. The concentration of glucose within a cell is 0.32M as at this point there is no net movement of water between the cell and the solution surrounding it. Anything lower than 0.32M will cause the potato cells to get larger (turgid) due to diffusion and anything higher than 0.32M will cause the cells to become smaller (flaccid)

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