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Planning and Experimental Procedures.

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PLANNING AND EXPERIMENTAL PROCEDURES This experiment is aimed at finding out how different strengths of sucrose solution affect the percentage change in mass of potato cells, due to osmosis. Scientific Knowledge: Osmosis is a special kind of diffusion involving water molecules and is defined as 'the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane.' Osmosis can be seen in living cells: The cell membrane in cells is partially permeable and the vacuole contains a sugar/salt solution. Plant cells always have a strong cell wall surrounding them so that when they take up water by osmosis and start to swell, the cell wall prevents them from bursting. The pressure inside the cell rises and eventually the internal pressure is so high that no more water can enter the cell. Osmosis makes plant cells swell up if they are surrounded by weak solution and they become turgid. This is very useful for giving support to green plant tissue and for opening stomatal guard cells. Due to this, I have chosen to use solutions of a lower concentration ranging from a 0.2m solution to a 1.0m solution. This should provide a variety of results. The membrane of a potato has tiny holes in it which are so small that only water molecules can pass through them, and bigger molecules such as glucose become 'stuck' behind them. ...read more.


When there is more sugar, in ratio to the water, a decrease in mass occurs. This is because the water that is already present in the potato is passed through to the solution - plasmolysis (the reverse action to osmosis.) ANALYSING EVIDENCE AND DRAWING CONCLUSIONS This graph displays how the molar concentration of each solution affects the percentage change in mass. The last two points along the graph have little distance between them showing that there was only a small percent of mass change. The point of 0.20M gains approximately 4.90% in weight. This shows that the water potential of the sucrose solution in the dish is stronger than that of the potato disc. The next point, 0.40M, gains approximately 2.94% in mass, showing that the sucrose solution has an even weaker water potential than the 0.20M solution. This also proves that osmosis took place. This pattern carries on through the graph, until the concentration reaches above 0.80M, where there is no indication that the cell is increasing or decreasing further in size, suggesting that the cell is fully plasmolysed. Even though these results support the original prediction, the graph used only shows the mass change in percentage and not in grams which is more exact. This means that the 0.6M solution is illustrated to have no change in mass, yet the table of results (g) ...read more.


o Another investigation to prove that osmosis affects the percentage change in mass of potato cells due to different strengths of surrounding solutions, would be to carry out a flexibility test to measure how stiff the potato becomes when the surrounding solution is weaker than the concentration of the potato. Due to the water molecules passing from the surrounding solution into the potato cells, through osmosis, the cells would become turgid and therefore prevent movement. This investigation should show similar results to the actual investigation - just by a different means, which would make the conclusion more reliable. This investigation alone is not sufficient enough evidence to firmly support the conclusion. Although the results are reliable and accurate, this investigation only uses one method of showing that osmosis takes place, through measuring the percentage change in mass before and after osmosis has taken place. In order to state the conclusion and be sure that it is completely accurate and reliable, other investigations would have to be carried out; such as the one mentioned above, which measures the extent of turgidity and flaccidity of the potato disc. Only once more investigations have been carried out, and if the same results for each experiment are obtained, can the conclusion be correct and reliable as it is backed up by scientific evidence from more than one source. ?? ?? ?? ?? ...read more.

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