osmosis project notes

Osmosis” is the process by which small molecules automatically cross a semi-permeable membrane, compensating for a difference in the concentration of those molecules on either side of the membrane.                                                                                                                       




When you put an animal or plant cell into a liquid containing water one of three things will happen.

1. 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. The net (overall) result is that water enters the cell. The cell is likely to swell up.
2. If the medium is exactly the same water concentration as the cell there will be no net movement of water 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. The cell will stay the same size.
3. 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.

Osmosis is a special case of diffusion. In this case, a large molecule like starch is dissolved in water. The starch molecule is too large to pass through the pores in the cell membrane, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semipermeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the starch, the starch molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter. Thus, more water flows into the side with the starch than flows out, and the starch gets diluted.

If the starch (or some other large molecule like a protein) is in a cell, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon, and if too much water enters the cell, the cell can burst, which kills the cell. So cells usually have some kind of mechanism for preventing too much water from entering or pumping the water out or simply making a tough outer coat that will not rupture.Things are more difficult when the starch or other large molecule is on the outside of the cell. Then the cell loses water faster than it comes in, and the cell shrinks, which might not be too bad except that the cell needs the water for the chemical reactions that take place inside that keep it alive. There are other ways that cells use to move materials across the cell membrane, most of which involve active transport, requiring the use of energy. The cell membrane also has other functions besides controlling the movement of materials into and out of the cell, and the membranes of specialized cells have very complex functions. So we see that the cell membrane is a very intricate and important component of the cell.