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Osmosis: What is it and How Does It Occur in General Essay Sample

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Osmosis: What is it and How Does It Occur in General Essay Sample

Osmosis is the diffusion of free water molecules from a region of high concentration to a region of low concentration across a partially permeable membrane. Osmosis is complete when all of the water molecules have been evenly spread out and can take place either in plant cells or animal cells, so long as a partially permeable membrane is present. Osmosis can also be conducted in the visking tubing experiment, where the visking tubing is an artificial permeable membrane. So how does Osmosis take place? When you put an animal or plant cell into liquid containing water, one of three things can happen. In the 1st option, if the medium surrounding 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 leave, the net result is that water enters the cell; hence, the cell is likely to swell up. In a plant cell, there are many structures, which include a cellulose cell wall.

This outer structure gives the cell a fixed shape and can resist changes in pressure inside the cell. When the cell takes up water by osmosis and start to swell, the cell wall prevents it from bursting. Plant cells become ‘turgid’ when they are put in dilute solutions, which mean that it becomes swollen and hard. The pressure inside the cell rises, and eventually the internal pressure of the cell is so high that no more water can enter the cell. Turgidity is very important to plants because this is what makes the green parts of the plant ‘stand up’ into the sunlight. However, when animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. In the 2nd option, if the medium has the exact same water concentration as the cell, there will be no net movement across the cell membrane.

Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water and the cell will stay the same size. In the 3rd option, if the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it; therefore, the cell will shrink. When plant cells are placed in concentrated sugar solutions, they lose water by osmosis and they become ‘flaccid’ which is the exact opposite of ‘turgid’. If you look at these plant cells that have been placed into concentrated sugar solutions under the microscope, you would be able to see that the contents of the cells have shrunk and pulled away from the cell wall. A cell like this is called plasmolysed. In animal cells, water is sucked out of the cell by osmosis and the cell shrinks. Therefore, animal cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why we have kidneys.


LONGMAN GCSE BILOGY TEXTBOOKhttp://www.purchon.com/biology/osmosis.htm

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