In order for cells to interact with their environment, molecules must be able to move through the cell membrane. Movement within the cell occurs by diffusion. Molecules move through the cell membrane by 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 cell and mixture. The overall effect is that molecules move down a concentration gradient from a region of high concentration to a region of low concentration which is passive transport. Osmosis is the movement of molecules down a concentration gradient and at the same time through a membrane. Selectively permeable membranes only allow certain molecules to go through the membrane into or out of a cell. For example, water can cross the membrane while a salt solution cannot. If there is a concentration gradient across the membrane, 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 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 when the concentration of dissolved molecules is the same inside and outside the cell, and there is no net movement of water across the membrane. When cells are placed in a hypertonic solution, water flows out of them and they shrivel up. When cells are placed in a hypotonic solution, water flows into them. If the cell...
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