The plasma membrane is the membrane that surrounds cells and
controls what comes in and what goes out of that cell. It is made
up of layers of phospholipids whos tails are nonpolar (no charge)
and the heads of the Phospholipids are Polar (have a charge).
Because of the charges of the phospholipids, some substances can
not come into the cell. The reason is their charges are different
then the layer and therefore, it repels the substance. However, if
the substance has the right charge or is small enough to slip
through membrane holes, the substance can get past the Phospholipid
bilayer and into the cell. Transport proteins can allow some
substances in that would normally not be let in.
The cell's outer membrane made up of a two layers of phospholipids with embedded proteins. It separates the contents of the cell from its outside environment, and it regulates what enters and exits the cell.
In animals the plasma membrane is the outermost covering of the cell whereas in plants, fungi, and somebacteria it is located beneath the [[cell wall].
Word origin: plasma: from Greek, from plassein, to mold.
Synonym: cell membrane, plasmalemma.
See also: fluid mosaic model.