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The plasma membrane is composed of two layers of phospholipids with proteins in between. It is selectively permeable. In terms of fluid balance, depending on the concentrations of water outside the membrane relative to the inside, water will move through the membrane by osmosis from high to low concentration. This is passive transport and requires no additional energy on the part of the cell to occur. Sometimes, there may be a high concentration of solute outside the cell, such as salt. If this is the case, the water inside the membrane would be in higher concentration relative to the outside and outward osmosis would occur. This results in plasmolysis of the cell and it will shrink drastically. Therefore, the plasma membrane constantly regulates the fluid balance of a cell.
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