GCSE Biology Coursework Osmosis : - The Potato Experiment

Transcription

1 GCSE Biology Coursework Osmosis : - The Potato Experiment Background Information Osmosis can be defined as the movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration. The semi-permeable membrane allows small particles through it but does not allow large particles such as sodium chloride. Osmosis will continue until a state of equilibrium is reached i.e. there is no area with a higher or lower concentration than another area. To land plants water and osmosis are vital as they play leading roles in the structural support of a plant. Lack of water will lead to a plant wilting (becoming flaccid) and possibly dieing. Osmotic pressure. If a plant was placed in a waterlogged area where the external solute to the cell (being less concentrated (or hypertonic) to the cell vacuole contents) the cell will not continue to take in water via osmosis for ever. The cell wall made of cellulose acts as a firm barrier to any more expansion. Once the cell is full of water it is said to be turgid. This means that the inward force is equal to that of the outward force. The inward pressure is called turgor pressure and the outward force is called osmotic pressure. The opposite of being turgid is being flaccid. This occurs if the plant is placed in solution that is hypertonic to its contents. So basically when the solution outside the cell is more concentrated. The cell loses the water content via the process of osmosis. The cytoplasm will eventually cease to exert any force on the cell whatsoever and so the cell becomes flaccid. In the experiment the pieces will either become flaccid stay the same or become turgid depending on the concentration of the solute inside and outside the cells. Aim The aim of the investigation is to investigate the movement of water in and out of a sample of potato by osmosis. Safety Planning

2 Then experiment involves sodium chloride. This is harmful if it splashes into your eyes and so you must wear safety spectacles at all times during the experiment. A razor blade is required in the experiment so you must take care as it could cause a serious wound. You must carry the knife facing downwards. You must also always use it on a tile to avoid damaging the bench. A cork borer is required. Do not use this with your hand on the underside of the potato as it may slip surprisingly well though the potato and stab. You must like the razor use this piece of apparatus on the tile to avoid damage to yourself and the bench. Also before beginning the experiment so as not to contaminate the experiment one must wash their hands. The Experiment The experiment will prove the theory of osmosis and will also tell me how much solute there is inside the potato. I will cut equal sections from a potato weigh them and place them in different concentrations of sodium chloride. After 24 hours I will take them out and weigh them. The difference in weight will prove the existence of the process of osmosis. The point of equilibrium is the point where the line on the graph crosses the axis. This is also the point where the piece of potato doesn t shrink or increase in mass or length. A Fair Test for the results to be accurate and reliable the experiment must be a fair test. A fair test is when all the non-variables in the experiment are kept the same. This means the factors like surface-area and volume of solution are all kept the same. for it to be a fair test the following should and will be accounted for: o The pieces of potato must be the same width length because they must have the same surface area exposed to the solutions. This means that if one cylinder is exposed to more solution than another cylinder then the one with the larger surface-area has a larger contact with the solution therefore the probability that osmosis occurs could be more and also to the extent at which osmosis occurs could be affected. This would then affect my results invalidating them.

3 To do this they must be measured down to a mm and the same cork borer must be used throughout the experiment. The weights must also be as accurate as possible and therefore the scales must be extremely sensitive. When the balance is being used before each potato cylinder is placed on the scales the scales must be set 0. There must be equal amounts of solution in each test tube. This again is to with how much solution the cylinders are exposed to. As this could again affect the rates of osmosis. All the pieces must come from the same potato. If I used a different potato the second potato may have a higher or lower solute concentration. All the pieces must remain in the solution for the same amount of time. If one pieces of potato stayed in the solution longer than a different one then it would have had more time for osmosis to occur. This could lead to inaccuracies in the results therefore invalidating them. I will have to keep the potatoes and solutions at the same temperature (room temperature in this case)throughout the experiment. Equipment A Potato A Cork Borer A Razor Blade A Tile Weighing Scales A Ruler A marker Pen Salt Solution Distilled Water Test Tube Rack Beakers The subject of the experiment. Used to cut the cylinders from the potato. To cut the cylinders down to the correct size. A base for cutting and boring on so as not to damage the bench. To weigh the potato cylinders. To measure the cylinders. To Mark the test tubes. for the solution. To be mixed with hacl to make the correct molarity solutions. To Keep the test tubes in. for measuring the solutions. Plan 1. Bore 12 tubes of potato out of the same potato(being careful not to cross them) 2. Cut the ends of the pieces (as there is an impermeable skin layer which if not removed could prevent osmosis and therefore make the experiment and unfair test) and cut them to the correct length. 3. Weigh each and record the weight in a results table. 4. using some sort of marking system mark one pair of each of the potato sticks as there will be two per test tube.

4 5. Mark each of the test tubes so you know which test tube holds what concentration of solution. 6. Make the sodium chloride solutions by adding varying amounts of sodium chloride to varying amounts of distilled water. The mounts are shown in the table below. Moles Distilled water /ml Sodium Chloride (NaCl) solution /ml Place 2 pieces of potato in each test tube. One unmarked and one marked. Make sure you know where each potato stick has been placed so you can identify its original weight after the experiment. 8. Leave the potatoes in the solution for 24 hours. 9. After the 24 hours drain the solution form each test tube weigh the cylinder they should be ribbed gently on dry tissue paper to remove any excess solution that wasn t actually absorbed by osmosis. This wont affect the water taken in by osmosis as this has been taken into the cells. If this were not done then the readings would be false and lead to inaccurate results and conclusions. 10. Draw a graph. The point at which the line crosses the x-axis is the salt concentration in the potato. What will happen? I think that the potatoes in test tube 1 will increase in size by a couple of mm and will increase in mass by about 0.25 to0.5g. I thi the same will happen for test tube number 2 and maybe test tube 3. However at some point either test tube 3 or 4 there will be a decrease in size and in mass by a few mm and about 0.5 to a gram. My basic theory is that the pieces will decrease in size and mass with higher concentrations of the solution and will increase and become turgid on the 2 or 3 lowest concentrations. Previous Work/Research I did a preliminary experiment of which several faults were recognisable for example I didn t dry all the cylinders equally on the tissue paper. The next time I took more care in obtaining the results. I realised the mistake after I looked at the graph for the experiment. It wasn t a curve but a fluctuating line. My thought was that

5 maybe I dried of the excess water form the some and not the others. Also the cuts I made in the potatoes to identify them were difficult to see when the pieces of potato shrunk and so identification was very difficult. On the second experiment I used a marker pen and larger cuts. Information sources I used were the school biology text book C.G.P. GCSE double science higher level biology and Encarta 96. These all helped with my understanding of osmosis and where it occurs. Obtaining Safety The experiment went well in terms of safety. There were no accidents apart form two broken test tubes the pieces were disposed of safely by the teacher. Everybody had eye protection on and so if shards of Pyrex had flown into our eyes we were all safe The solution didn t splash into everybody s eyes. ho accidents other than the test tubes breaking occurred. All the equipment was used safely especially the razor blades and cork borers Section 1 - Results I did a preliminary experiment of the ranges shown earlier in the plan. After the I retrieved the results of the experiment I noticed upon drawing the graph that osmosis occurred somewhere in the region of 0 to 0.6 moles of hacl solution. Therefore in the second experiment I did 0 to 0.5 moles of solution at 0.1 intervals. Both experiments had a good range of values. There were 6 tubes in each holding and 2 cylinders of potato per test tube. Section my results table. This rough table shows the values I collected after the experiment 2 This is a plan of Test Tube Concentration of Sodium Chloride solution /moles Mass Before /g Marked Unmarked Mass After /g Marked Unmarked Percentage Increase/Decrease Marked Unmarked

6 These are the final results showing the percentage increases and decreases for each potato cylinder. Test Concentration Mass Before /g Mass After /g Percentage Tube of Sodium Increase/Decrease Chloride 1 solution /moles Marked Unmarked Marked Unmarked Marked Unmarked These are the Values that I used to my Plot Graph Concentration of Sodium Chloride solution /moles Average Percentage Increase/Decrease /% Conclusion I have drawn a graph to show the average percentage increase and decrease in mass against the concentration. On the graph I have placed the concentration of the surrounding sodium chloride solution on the x-axis and then on the y-axis I have placed the percentage increase and decrease. I have drawn a sketch of the grtaph below in order to explain what it tells me.

7 o Between point A and B there is net movement into the potato as there has been an increase in mass of the potato. This means that the potato. This means that the potato was turgid after the experiment. o Point B is the point of equilibrium meaning that there is no net movement in or out of the cell. This also means that the concentration of salt inside is equal to the concentration outside of the potato cells. o Between points B and C there is net movement of water out of the potato as it has decreased in mass. Between the C and D and it is an almost horizontal line.

8 This means that this is the point where all the water in the cells of the potato have moved out of the cell and there is no solute left to leave. The actual graph show the percentage gain and loss in mass plotted against the molar concentration of sodium chloride (hacl). I have drawn a line of best fit onto the graph. The line is a negatively slopping curve. It doesn t pass through the origin which means that the percentage gain and loss in mass and the concentration are not directly proportional to each other. However there is a pattern on the graph. It shows the percentage gain and loss in mass is inversely proportional to the concentration i.e. as the concentration increases the percentage gain and loss in mass decreases.