The plasma membrane of a cell is selectively permeable.
This means that the membrane specifically allows only certain molecules to move across it from the interior of the cell to the exterior or from the exterior of the cell to the interior.
The functions of such selective permeability of the membrane are:
It allows only those polar or non polar molecules or ions to traverse them and enter into the cell which are necessary for the cell metabolism or other functions of the cell.
It removes those ions or molecules from the cell interior to the cell exterior which are not necessary for the cells.
It prevents the movement of essential cellular components like non-polar or polar molecules, ions, amino acids, fatty acids, or carbohydrates from the cell interior to the cell exterior.
It prevents the movement of those molecules from the cell exterior to the cell interior which are not essential for the cell functioning.
Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for muscle movement
