In order for cells to interact with their environment, chemicals, including water, must be able to move across the cell membrane and across the cell. Movement within the cell occurs by a process known as diffusion. Molecules move across the cell membrane by a related process known as osmosis. Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. This happens because of random molecular motion. Molecules move around randomly until there is an even mixture throughout the container in which they are enclosed. The overall effect is that molecules move "down" a concentration gradient from a region of high concentration to a region of low concentration.
Osmosis is the movement of molecules down a concentration gradient and at the same time across a membrane. Cell membranes do not allow all molecules to cross them. They are said to be "selectively" or "differentially" permeable. Only certain molecules can cross the membrane into or out of a cell. For example, water can cross the membrane while sodium and chlorine ions (dissolved salt) cannot. If there is a concentration gradient across the membrane (if there is more salt and less water on one side than on the other), water will move across the membrane down the concentration gradient while the salt cannot. If there is more salt and less water inside a cell than outside, water will flow into the cell from the surrounding environment. This process is called osmosis.
When the environment outside a cell has a lower concentration of dissolved molecules than inside the cell, the solution is said to be hypotonic, and water will move from the solution into the cell. If the surrounding solution has a higher concentration of dissolved molecules than the cell, the solution is hypertonic. In that case, water will move from the cell out into the surrounding solution. An isotonic solution is one in which the concentration of dissolved molecules is the... [continues]
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