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To investigate how varying the concentration of sucrose solutions affects the rate of osmosis between the solution and potato cylinders immersed in it.

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Osmosis in potato cells Section 1: Planning Aim: To investigate how varying the concentration of sucrose solutions affects the rate of osmosis between the solution and potato cylinders immersed in it. Hypothesis: A substance, when dissolved in water, has a natural desire to dilute itself by bonding with surrounding water molecules. When a semi permeable membrane, such as a cell membrane separates two solutions, pure water flows from the weaker solution to dilute the stronger one until they are both the same strength. The term used to describe the flow of water through a semi permeable membrane separating two solutions of different concentrations is osmosis. In other words, osmosis is the process in which water passes in and out of plant cells thus controlling their turgidity. It is in fact the diffusion of water from places of high water concentration to places of low water concentration and can take place only across a selectively permeable membrane - such as the cell membrane. The cell membrane allows small molecules such as those of water to diffuse through it. However, larger molecules such as those of sucrose can not pass through its pores. This movement of water will continue until equilibrium is attained and the concentration is equal on both sides of the cell membrane. An example of a region with high water concentration is distilled water or very dilute sucrose solution where there is a large number of free water molecules. Either of three things can happen if a plant cell is immersed in a solution. These are: 1. The net (overall) amount of water entering the cell may increase causing the cell to swell up and become turgid. This will happen if the medium surrounding the cell has a higher water concentration than the cell sap. As a result of this, the cell will gain water by osmosis. This is so because even though, the water molecules are free to pass across the cell membrane in both directions, more water comes into the cell than leave. ...read more.


After leaving the Petri dishes and their contents for 24 hours, I will take my new readings. In order to do so, I will remove each potato cylinder from the Petri dish labeled "0.125M", gently pat it with a tissue paper to remove any excess solution on it surface and weigh it. I will record the readings for each potato cylinder. I will then repeat the same procedure for the potato cylinders in the other Petri dishes. Precautions: To ensure that my test is a safe and fair one I will take the following precautions: 1. I will not hold my hand directly opposite to the place on the potato from which I expect the cork borer to immerge as it can injure my hand. 2. I will be exceptionally careful to ensure that I do not cut myself while using the extremely sharp scalpel. 3. I will ensure that my hands are clean and dry in order that I neither contaminate the potato nor pass excess water onto it. 4. I will ensure that I keep the factors I had mentioned earlier constant. 5. I will use a very sensitive weighing balance to denote the masses of the potato cylinders both before and after the experiment. I will reweigh each cylinder to ensure that I have denoted the correct reading. 6. I will ensure that the potato cylinders were completely immersed in the solutions. Unless all the potato cylinders are completely immersed in the solutions, the rate and amount of osmosis will not be equal and this would make my investigation an unfair one. 7. I will take care to ensure that my cylinders are of identical lengths before the experiment. I will do this by carefully measuring their lengths to the nearest millimeter. I will measure the length of each cylinder twice to ensure that my measurements are correct. I will repeat this process to measure the lengths of the cylinders after having immersed them in the various solutions. ...read more.


This would have added to their mass. 4. I may conduct the same experiment at different temperatures (for example at 15oC, 20oC,25oC,30oC and 35oC). In this way I will still obtain results for 0.125M.0.25M, 0.5M, 0.75M, 1.00M and distilled water. I expect the results to be the same as I think that temperature will only affect the sped at which osmosis takes place, and not the final amount of osmosis, which has taken place. I expect to obtain the same isotonic point for the concentration of cell sap no matter what temperature I carry the experiment at, since I intend to leave it for a longer period of time, thus allowing maximum osmosis to take place for all the solutions. 5. I can also use onions instead of potatoes to investigate the rate of osmosis. Here again I expect to observe more water loss when they are immersed in more concentrated solutions than in weaker ones. 6. I found the accurate cutting of the potato cylinders to be among the most difficult parts of the experiment. If I am to repeat the experiment I will try and use a device, which cuts the cylinders to the reset length? 7. I can also try and compare the rate of osmosis in different species of potatoes or in young and old potatoes. I expect to obtain similar isotonic point for each potato since they have almost identical structures and hence their cell saps have similar concentrations. In conclusion, I find my experiment quite successful. My procedures were shown to be safe and reliable and my method resulted in me obtaining a range of values I had expected. The experiment did not require the use of apparatus that is difficult or expensive to obtain and the materials were easily available. It was not time consuming and did not require extensive reparation in order to be carried out. ...read more.

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Here's what a teacher thought of this essay

3 star(s)

In this lengthy report (42 pages!) into an investigation concerning the effect of changing sucrose concentration on osmosis in potato tissue, the writer demonstrates a good understanding of the process of osmosis (at GCSE level) but undermines the good science carried out by using a very repetitive writing style. Both the method and data collected were of a good standard. The problem here is that in an attempt to be extremely thorough, the logical flow of ideas has been lost. A GCSE investigation of this nature can be adequately written up in around 1500 words (6 pages). Good scientific writing is concise and relevant and tells the reader exactly what he/she needs to know and no more. More is very often less! In this case, it certainly is.

3 stars

Marked by teacher Ross Robertson 10/05/2013

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