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# Osmosis GCSE Biology coursework

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Introduction

Aim The aim is to find the concentration (mole/dm3 (M)) of solute in a potato cell by using the process of osmosis and different concentrations of sucrose solution. Background information Osmosis is diffusion of water across a partially permeable membrane. It moves from a solution with less solute concentration (high water potential) to a solution with more solute concentration (low water potential). The one with a high water concentration is called a hypotonic solution and the low water concentration is called hypertonic solution, but these only depend on what type of concentration is on the other side of the partially permeable membrane. When more water passes through to one side of the membrane it is called net movement. (Toole + Toole "Essential AS level Biology") This is an example of the net movement through a partially permeable membrane. The right side is the more dilute solution, which makes it the hypotonic solution. The left side is the less dilute solution, which makes it the hypertonic solution. Net movement occurs through the more dilute to the less dilute so it goes from left to right though the partially permeable membrane. As you can see, the dotted line going horizontally through the middle of the two levels of solution shows the level of the solutions at the beginning when the two sides where given equal amounts of its solution. The horizontal solid lines show the level of the two sides after it was left for 2 hours. The right side has more solution in it because of the greater net movement of water going to the right from the left side by the process of osmosis. ...read more.

Middle

I wanted the all the surface area of the potato to be the same type of membrane and changing it will change the results. I did the cutting on a cutting tile to refrain the scalpel from cutting the table. Then I used the balance to weigh the potato cylinder in grams (to the nearest 2 decimal places). The amount I measured for each test tube was 30ml of solution. To make the various concentrations I had to dilute it with a curtain amount of distilled water (e.g. 15ml of solution and 15ml of distilled water to make a 0.5M solution). The range of concentrations I tested were 0M (water), 0.1, 0.2, 0.5, 0.6, 0.7, 0.8 and 1M. Once I put the potato cylinder in with the sucrose solution in the boiling tube, I started timing and put the boiling tube in the test tube rack. The time limit for the potato to be in the solution was 30 minutes because this is enough time for some significant change to happen and to get a good graph. When the 30 minutes was up I took the potato cylinder out of the boiling tube and weighed it again, making sure I dabbed the potato piece to get rid of excess solution so I don't weigh the solution with the potato. From the results (initial mass (at the beginning) and final mass (at the end)) I could work out the percentage change. This is because I could not make every potato piece the exact same weight and percentages are much easier to work with because when it is on a graph it is easier to see the trends and it is so widely used. ...read more.

Conclusion

I should have chose a smaller gauge for the potato cylinder to keep the results fair because I had to change the potato more often than if I was to use a smaller gauge (not enough potato to do all the initial and repeat experiments). I could have tested more concentrations like the ones in between 0.2M and 0.5M (0.3M and 0.4M) or maybe even more (0.25M) to get a better best fit line. Cutting the potatoes was the hardest part of the procedure as I had to get the length exactly right and make sure I took off all the skin as well. If it was available I would have used a machine to cut it accurately as to get it the same weight and length to get a very fair experiment for all the concentrations I tested. When I was ready to weigh the potato cylinders I could have accidentally dried some potatoes out more when they came out of the sucrose solution than others, therefore making a difference in weight. If I had the equipment and the time I could dry out the potatoes in a very fair and even way to ensure that they are all treated the same. Although the procedures could use some improving the experiment was a success. Extension Work For extension work I could change another variable like temperature of the sucrose solution to see how it affects water passing though a partially permeable membrane by osmosis. I could test much more concentrations (0.05M, etc.) and take more repeat experiments. It could be left for longer in the solution to get more net movement (if there is any) and get more curtain results over a large percentage change scale. ...read more.

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