Describe the composition of the plasma membrane.

The Plasma Membrane (formerly known as the cell membrane) forms the border of a neuron and acts to control the movement of substances into and out of the cell. This membrane is composed of two layers of lipid or fat molecules (phospholipids, in particular), with the water-repelling tails of these molecules directed toward the middle of the membrane and away from the fluid found outside and inside the cell. The water-attracting heads of the phospholipid molecules, therefore, are found in contact with extra - and intracellular fluid. Protein molecules are embedded within this bi-lipid membrane. Channel proteins serve as channels for controlling the rate of transport of specific molecules across the plasma membrane. Signal proteins respond to the binding of specific substances (often hormones) by sending messages into the cell to activate particular cellular mechanisms.