Diffusion is the movement of molecules from a region of high concentration to a region of low concentration, this process is driven by the random motion of the molecules themselves. Some solute can move through the plasma membrane by diffusion although others are impeded. Osmosis is a special form of diffusion and is defined as the movement of water across a membrane. A difference in concentration in two regions forms a concentration gradient; under these conditions molecules will move about until they hav

The definition contains three important statements:

Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through.

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.

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.

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