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To observe the effect of varying salt concentrations outside plant tissue and how water movement by Osmosis is affected.

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Introduction

Aim: To observe the effect of varying salt concentrations outside plant tissue and how water movement by Osmosis is affected. Theory: Diffusion occurs from an area of high concentration to an area of low concentration. This results in a homogeneous distribution. This can be related to Osmosis in plants, as water will move from a less concentrated area to a more concentrated area. This occurs through a semi-permeable selective membrane. All plants have a Semi-permeable membrane and it is normally called the cell membrane. This means that the membrane surrounding the plant will let water through either way to balance the ratio of, e.g. salt. But nothing else can pass through the membrane apart from water because other molecules are too large to pass through. This process will continue as long as there is an imbalance in the concentration of the substance, e.g. salt. In plants, the wall of cellulose surrounds the cell membrane. The cellulose has no osmotic properties. It is freely permeable to all liquid and dissolved substances but the fact that it stretches very little limits the size to which plants cells can swell when they take in water by osmosis. If you have a solution A that is stronger than B that means A is hypertonic to B, B is hypertonic to A and if they are equal in strength they are isotonic. ...read more.

Middle

I will then use a knife/blade to cut the potato cylinders to exactly 20mm using a mm ruler. I will then mix the solutions up. To do this I will get the 20% salt solution (already mixed up) and put it into a measuring cylinder of 20cm3. I will then measure it up to 10cm3 and add water to the 20cm3 mark. This will make a 10% salt solution. I will keep doing this until I get a 5,10,15 and 20% solutions. These will be held in beakers and the solution at 20cm3. I will then put the potato cylinders into the different solutions and leave them for 2 hours. After 2 hours I will take the potato cylinders out of the solution, dry the surface with a paper towel and use the mm ruler to measure the lengths. I will do this three times with two batches of 20mm potato cylinders and one batch of 30mm potato cylinders. To plot the graph I will need the differences of weights/lengths. So, I will add the three values up for the start weight/length and divide by three. I will do that for the end weight/length as well. I will then subtract the average start weight/length from the end weight/length. Obtaining evidence: Table for the weight of potato core: Weight at start (g) ...read more.

Conclusion

I could extend the experiment to a more exact level by looking at the potato cylinders under a microscope, and then I would be able to see the cells in greater detail and draw some more observational results. If I were to find out more about osmosis I would do a demonstration of how +osmosis works more clearly. I would use Visking tubing to replace the plant cell wall and fill it with a sugar solution - Glucose. This is because the Visking tubing has tiny little holes in like a semi-permeable membrane. I will then place the tube in a beaker containing pure water. Diagram: Pure water Visking tubing containing Glucose Because there are more glucose molecules in the Visking tube, the water molecules pass through the semi-permeable membrane to 'dilute' the sugar concentration. There is a net flow of water molecules into the Visking tubing. The water molecules can travel both ways through the membrane. The Glucose cannot diffuse into the water because the molecules are too big to fit through the semi-permeable membrane. This causes the Visking tubing to fill up with water. Eventually the tubing will become so full up that it will burst. Graphs for the two experiments A graph to show the how the changing concentration of salt solution affects its mass in g A graph to show the how the changing concentration of salt solution affects its length in mm ...read more.

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