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Investigation into the effect of osmosis on potatoes.

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Investigation into the effect of osmosis on potatoes Background Osmosis is a specialised form of diffusion where water molecules from a higher water concentration (hypotonic) diffuse through a partially permeable membrane to a lower water concentration (hypertonic). When a substance dissolves in water to form a solution that substance attracts water molecules therefore reducing the number of free water molecules in a solution which in effect reduces the concentration. Free water molecules can travel faster through partially permeable membranes than larger molecules of other substances that have a cloud of water molecules surrounding due to their smaller size Therefore when there are two solutions either side of a partially permeable membrane with a hypotonic solution on one side and a hypertonic solution on the other side the greater number of free water molecules in the hypotonic solution will move through the membrane faster than the higher number of large solute molecules with their water molecules surrounding them in the hypertonic solution and as there are less free water molecules in the hypertonic solution which are also moving more slowly, than those in the hypotonic solution due to the increased level of solute molecules, there will be a higher rate of movement of free water molecules from the hypotonic solution to the hypertonic solution and so the water concentration in the hypotonic solution will begin to decrease while the hypertonic solution's water concentration will increase until there is an equilibrium and the movement from either side to the other side is equal and then the water concentration on either side will not change unless more solute is added to either solution. ...read more.


Experiment planning In order to be able to check whether my main experiment would work well we conducted a preliminary experiment which we then evaluated to see how we could improve for my main experiment. Preliminary Experiment Method We placed three lengths of 3.5 cm of potato, which had been cut using a scalpel and measure using a ruler, into 5 Petri dishes. In one Petri dish we poured in 10cm3 of tap water and 10cm3 of 1 Mole/Litre sugar solution making a concentration of 0.5 moles/litre. We then used different ratios of water and 1M sugar solution to produce 20cm3 solutions with concentrations of 0.2M, 0.1M, 0.05M and 0M in the other four Petri dishes. We measured the volumes of the sugar solution and water using measuring cylinders. As this was the preliminary experiment we only waited two and a half hours before measuring the new lengths of the potatoes. Original concentration of sugar solution (Moles/Litre) Original potato cylinders' lengths (cm) New length of first potato cylinder (cm) New length of second potato cylinder (cm) New length of third potato cylinder (cm) Average new length of cylinders (cm) (2d.p.) Average % change in length of cylinders (2d.p.) 0.5 3.5 3.5 3.5 3.5 3.5 0 0.2 3.5 3.6 3.6 3.7 3.63 3.71 0.1 3.5 3.7 3.8 3.9 3.8 8.57 0.05 3.5 4.0 4.2 4.2 4.13 18 0 3.5 4.3 4.4 4.1 4.27 22 Preliminary Results Preliminary Conclusion These showed the expected trend though they can be made more accurate as will be explained next. ...read more.


This shows that water does travel from a higher water potential (the potato cell) to a lower water potential (the surrounding solution) as predicted. The opposite then happens when the original concentration of the sugar solution is above 0.405M because than the original water potential of the solution is higher than that of the potato and also pressure potential begins to play a role as pressure potential is positive when the potato is increasing in size and so this increases the potato's water potential meaning that if the original water potential of a solution is 5% more than the potato cell original water potential (about -1134KPa) the water will move in at a slower rate than if the original water potential of the solution is 5% less than that of the potato cell because of pressure potential as it will increase the water potential of the potato cell increasing in size. Evaluation We unfortunately were not able to use all of the suggestions I made in the evaluation after the preliminary experiment so the experiment could have been made more accurate by using distilled water, more sensitive length measuring equipment than a ruler, more accurate equipment to measure the volumes of the sugar solution and water and we could have also measure the volume of solution outside the potato when measuring the results to see whether there was a net gain or loss of water outside the potato. The results were all quite accurate showing no real inconsistencies. ...read more.

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