Osmosis is the diffusion of water through a membrane. The water diffuses from the side of the membrane where it is more concentrated to the opposite side. Water enters and leaves a cell by osmosis. For example, cells in the roots of a plant get water from the soil by osmosis. The water you drink enters your blood and all your body cells by osmosis.
Diffusion of water into a cell by osmosis builds up pressure in the cell. This pressure is called osmotic pressure. It pushes outward on the cell membrane and gives the cell shape. Osmosis pressure is similar to air pressure inside a balloon filled with air. Also, it is like water pressure inside a plastic bag filled with water.
Water can move out of a cell as well as into a cell by osmosis. If a cell is placed in a concentrated saltwater solution, water diffuses out of the cell. Loss of water causes the cells to shrink and become limp. For example, living plant cells can be placed in salt water and observed with a microscope. What dose the pbserver see? The cytoplasm shrivels and pulls away from the cell walls. The change is caused by water diffusing out of the cells. The shrinking of cytoplasm caused by the loss of water is called plasmolysis. Plasmolysis can cause the cells to die. A very large water loss from many cells can cause an organism to die.
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