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Osmosis Investigation

Extracts from this document...

Introduction

Skill Area P: Planning and Experimental Procedures Aim The aim of this experiment is to demonstrate the process of osmosis in potato tissue. To do this I will cut strips of potato, immerse them in various concentrations of sucrose solution and examine the change in length of the strips, if any. Preliminary Study To set up a range and interval for my experiment I carried out some preliminary work to give me an indication of what the change in length would be like. I used three concentrations, 0, 0.5 and 1.0 molar (1M) and immersed three 50mm strips in each solution for 15 minutes. The results are shown in the following table: - Molarity Length after immersion Average length after immersion Change in length Percentage change 1st strip 2nd strip 3rd strip 0 53mm 50mm 47mm 52mm 2mm 4% 0.5 51mm 49mm 49mm 50mm 0mm 0% 1.0 52mm 51mm 48mm 48mm -2mm -4% In this preliminary study, the equilibrium concentration, that is where there is no change in length, is approximately 0.5M. A range of concentrations either side of this value should therefore give me a satisfactory set of results. Apparatus/Materials 6 test tubes Test tube rack Scalpel Cutting board Vernier 20cm3 Graduated pipette Stopwatch Various sucrose solutions Potato Distilled water Planned method I will first obtain 5 sucrose solutions - 0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M. (These are all available from stock and made up by science technicians). ...read more.

Middle

As the difference between the water potential is less however, I think less water will be taken in than in the 0M solution and the increase in length will be smaller. In solutions stronger than 0.4M, I predict the potato strips will decrease in length. This is because above 0.4M the external solution will be of a higher concentration than the cell sap. The sucrose solution will have a more negative water potential than the cell. This means that water will leave the cell by ex-osmosis and give a plasmolysed state to the cell. A plasmolysed cell is one where the protoplast is completely pulled away from the cell wall (see diagram below). As the concentrations increase, the difference between the two water potentials of the cell and the solution will increase so I predict more water will leave the cell. Skill Area O: Obtaining Evidence I followed my original plan and did not make any changes to the proposed method. I used all the apparatus listed in the planning section. All safety procedures were carried out and no-one was injured during the experiment. All results obtained are shown in the table below. Results Molarity Length at end of experiment (mm) Average length after immersion (mm) Trial 1 Trial 2 Trial 3 0 53.46 53.24 52.40 53.03 0.2 51.52 52.63 52.24 52.13 0.4 49.96 50.71 49.68 50.12 0.6 48.64 48.13 48.84 48.54 0.8 48.45 48.39 48.15 48.33 1.0 47.80 48.09 48.23 48.04 Skill Area A: Analysing Evidence and Drawing Conclusions In the table below, ...read more.

Conclusion

If I were to redo this investigation I would suggest the following changes: * I would make sure a number of people were placing the strips in the solutions reducing the risk of one set of strips having more time for osmosis to occur. * I would use a cork borer as this would give me a uniform shape and keep the surface area constant. * To improve the reliability of my results, I could increase the number of concentrations by going up in intervals 0.1M not 0.2M (i.e. 0M, 0.1M, 0.2M, 0.3M...). This would give me more points to plot to improve my line of best fit. * If I didn't have time restrictions, I would leave the potato strips in the solutions for 24 hours, as this would give more time for osmosis to occur and stabilise. * Finally I would put strips in separate test tubes, as this would prevent the risk of strips touching and reducing the surface area for osmosis. To provide additional evidence for conclusions, I could repeat this investigation measuring the mass of the potato strips because this would mean that all over change was taken into account (change in all 3 dimensions). I could also investigate the effect of various concentrations of sucrose solution on different types of potatoes (e.g. sweet potato, new potato). Also I could compare the effect of various concentrations of sucrose solution on different types of vegetables (e.g. swede, parsnip, and carrot) and also see if the dynamic equilibrium varies with different tissues. ...read more.

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