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The Consequences of Osmosis

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Introduction

The Consequences of Osmosis Firstly what happens to plant cells: Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight. When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight. Now to explain osmosis: When you put an animal or plant cell into a liquid containing water one of three things will happen. If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. ...read more.

Middle

This causes the vacuole to contract and causes the cytoplasm to contract as well. The cell membrane is drawn in as a result of this. The cell has now become flaccid and has decreased in mass as water is leaving the cell. Factors that affect the change in mass of potato is the water potential of the potato and of the solution outside the potato, the surface area of the potato and time. If the potato has a large surface area there are more pores in the membrane of the potato for the water to enter. Time is a factor as it affects the amount of water molecules that has left or entered in the end result. I believe that the graph will look like this because I predict that the higher the sugar concentration, the lower the percentage change. The line of the graph decrease quickly and then its descent slows down. To begin with when the water potential of the outside the potato is higher than that the potato, the percentage change in mass would increase as the net movement of water molecules would enter the potato. When the water potential of the sucrose solution is the same as the potato, there would be no change in mass as the number of water molecules entering the potato is the same as the number leaving the potato. When the water potential of the sucrose solution is lower than potato, the percentage change would decrease because the net movement of the water molecules is leaving the potato. Method The dependent variable that we changed was the water potential outside the potato chip, The pilot study showed that a range of 0% to 70% was suitable because after a period of time, two potato chips had sunk to the bottom and the other three were rising to the top. They showed a change in density of the potato chips. ...read more.

Conclusion

I changed this to 3cm because I couldn't find 10 5cm long chips. Also, I changed the amount of solution I would keep the potato chips in, because otherwise the water spilled out of the tube, and I only needed enough to cover the chip (it was 100ml). These changes were easy to make, as they were only minor, however the case of the stolen boiling tubes meant that I had to spend several lunchtimes taking my results again. I think I took enough results for the coursework. The range was big enough, but to fill in the gaps in my graph I could have taken different results, i.e. 1g, 1.2g, 1.4g etc. or even smaller. This way I could have found more accurate results. Also to make my experiment better I could have repeated it more, and possibly have worked with someone, as I was working on my own, or even with the whole class. I could have also cut the potatoes into doughnut shapes, because the cells in the middle of the potato might have different properties, making them react differently to osmosis. I could have also used a machine to cut the potato chips. I could also weigh each chip on a digital and more accurate scale, e.g. not to 0.00 but to 0.0000g. There was one main anomalous result, this is highlighted in red and is two sizes bigger (tube I), but there is one more, but it is not that far out (tube B). This may have been caused by human error, or one out of my ten results might have been inaccurate, and changed the average. Or perhaps the chip was not cut accurately, or I added the wrong amount of salt. concentration. I could extend my coursework by testing the same brand of chip using a different substance. By this I mean using a different thing instead of salt, i.e. sugar or coffee or even vimto. Then I could find out whether osmosis occurs differently with different things diluted in the water. I could even use different vegetables. ...read more.

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