what is the effect of the plasma membrane on fluid balance

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.