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How does the concentration of a sucrose solution affect the rate of Osmosis in Potato Cells?

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Introduction

Jennie Lace 10c How does the concentration of a sucrose solution affect the Rate of Osmosis in Potato Cells? PLAN I have chosen to investigate how the rate of osmosis taking place in potato cylinders varies when I change the concentration of the surrounding sucrose solution. I will vary the concentration of sucrose in the solution, and carry out various measurements on the potato cylinders before and after the experiment, to see what affect changing the concentration has had. I predict that with the lowest concentration of sucrose, the potato will increase in mass, because of water moving into the cells. The percentage increase will then get smaller each time I increase the concentration of sucrose in the solution. At some point, the mass of the potato will not increase, and will be the same as it was before (when there is no net flow of water particles in or out of the potato cells). After this, the mass of the potato will start to decrease as I keep on increasing the concentration of the sucrose solution. At some point the potato cylinders will lose all the water that they are able to, and the percentage mass loss will stop decreasing. This is what I expect the graph of my results to look like: Osmosis is the movement of water particles from a weak solution (a hypotonic solution) to a strong solution (a hypertonic solution. It happens through a selectively permeable membrane, and is a type of diffusion. In a potato, water particles travel through the cell membrane, which is a selectively permeable membrane. During osmosis in a plant cell, the water particles move through the cell membrane and into the large vacuole, where they are stored in a solution of sugars and salts. For this experiment, I will be using sucrose, which has large particles that cannot fit through the holes in the selectively permeable cell membrane, but water particles can fit through, as they are much smaller. ...read more.

Middle

Mass of potato before (g) Mass of potato after (g) Change in Mass (g) Change in mass (%) 0% 7.32 7.73 + 0.41 + 5.6% 5% 7.44 7.74 + 0.30 + 4.0% 10% 7.14 7.15 + 0.01 + 0.1% 15% 7.31 7.13 + 0.00 + 0.0% 20% 7.45 7.15 - 0.30 - 4.0% Experiment 3: Percentage Sucrose in Solution (%) Mass of potato before (g) Mass of potato after (g) Change in Mass (g) Change in mass (%) 0% 7.27 7.72 + 0.45 + 6.2% 5% 7.25 7.23 - 0.02 - 0.3% 10% 7.24 7.04 - 0.20 - 2.8% 15% 7.25 6.33 - 0.92 - 12.7% 20% 7.29 6.93 - 0.36 - 5.0% Averages: Percentage Sucrose in Solution (%) Mass of potato before (g) Mass of potato after (g) Change in Mass (g) Change in mass (%) 0% 7.33 7.71 + 0.38 + 5.18% 5% 7.44 7.53 + 0.09 + 1.21% 10% 7.21 7.11 - 0.10 - 1.39% 15% 7.32 6.86 - 0.46 - 6.28% 20% 7.45 7.00 - 0.45 - 6.04% I decided not to measure the temperature of each of the concentrations of water but I measured the temperature of the science lab hourly to see if an increase or decrease in temperature could explain any anomalies. Here are the hourly temperature measurements: Time Temperature (°C) 9:30 a.m. 23 10:30 a.m. 23 11:30 a.m. 25.5 12:30 p.m. 25.5 1:30 p.m. 29 Analysis and Conclusion I recorded the results that I obtained from doing the control experiments in a table, and I felt it was unnecessary to present them on a graph, as there were only two repeats. However, they did show me that there is not a very large increase or decrease in mass when the potato has been boiled, due to the cell membrane having been destroyed. The first potato cylinder lost 3.2% of its mass, which agreed with my prediction as I stated that the mass would decrease slightly, as some potato may break off, but no osmosis would take place. ...read more.

Conclusion

However, it was quite hard to try and keep all variables other than concentration the same, for example cutting the cylinders exactly the same size was rather difficult. It was hard to try and judge how much I should roll the cylinders on the paper towels before I weighed them, as I knew that they were so small that I bit of water could change the mass dramatically, and therefore compromise the reliability of my results. If I were to repeat the investigation I would not significantly change my method because I felt it was a good way of measuring osmosis. I would probably change the readings that I took, so that I had a lot more results to analyse and draw conclusions from. I would obviously need more time to take more readings, and the concentrations I would use are: * 0% sucrose * 2.5% sucrose * 5.0% sucrose * 7.5% sucrose * 10.0% sucrose * 12.5% sucrose * 15.0% sucrose * 17.5% sucrose * 20.0% sucrose * 22.5% sucrose * 25.0% sucrose I am unsure whether it is possible to have more than 20% sucrose, but I would like to try, as it would be interesting to see if the line produced on the graph would level out and become horizontal if the cells could not lose any more water. To support my conclusion by obtaining more evidence I could still investigate change in concentration of the solutions, but as well as measuring the percentage change in mass, I could measure the percentage change in length of the potato cylinders. I would use callipers to measure the change, and according to the theory behind osmosis, the cells would also expand and become turgid, causing an increase in size and length. I would plot a graph of my results, and like this investigation I could plot change in concentration against percentage change in length. This additional work would help make me more certain of my conclusion as it would show me that the cells expand in size as they take in water by osmosis. ...read more.

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Great attention to detail is shown in this write up together with the use of appropriate biological terminology throughout. A few small numerical errors.

Marked by teacher Adam Roberts 10/05/2013

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