Pathophysiology - Definition and functions of the Plasma (Cell) Membrane.

In many respects, the plasma membrane (also called the
cell membrane) is one of the most important parts of the
cell. It acts as a semipermeable structure that separates
the intracellular and extracellular environments. It controls
the transport of materials from the extracellular fluids
to the interior of the cell, provides receptors for
hormones and other biologically active substances, participates
in the generation and conduction of electrical
currents that occur in nerve and muscle cells, and aids in
the regulation of cell growth and proliferation.
The cell membrane is a dynamic and fluid structure
consisting of an organized arrangement of lipids, carbohydrates,
and proteins (Fig. 1-2). A main structural
component of the membrane is its lipid bilayer. It is a
bimolecular layer that consists primarily of phospholipids,
with glycolipids and cholesterol. This lipid
bilayer provides the basic fluid structure of the membrane
and serves as a relatively impermeable barrier to
all but lipid-soluble substances. Approximately 75% of
the lipids are phospholipids, each with a hydrophilic
(water-soluble) head and a hydrophobic (water-insoluble)
tail. Phospholipid molecules along with the glycolipids
are aligned such that their hydrophilic heads face
outward on each side of the membrane and their
hydrophobic tails project toward the middle of the
membrane. The hydrophilic heads retain water and help
cells stick to each other. At normal body temperature,
the viscosity of the lipid component of the membrane is
equivalent to that of olive oil. The presence of cholesterol
stiffens the membrane.
Although the lipid bilayer provides the basic structure
of the cell membrane, proteins carry out most of the specific functions. The integral proteins span the entire lipid
bilayer and are essentially part of the membrane. Because
most of the integral proteins pass directly through the
membrane, they are also referred to as transmembrane
proteins. Other proteins, called the peripheral proteins,are bound to one or the other side of the membrane and
do not pass into the lipid bilayer.
The manner in which proteins are associated with the
cell membrane often determines their function. Thus,
peripheral proteins are associated with functions involving
the inner or outer side of the membrane where they
are found. Several peripheral proteins serve as receptors
or are involved in intracellular signaling systems. By contrast,
only the transmembrane proteins can function on
both sides of the membrane or transport molecules
across it. Many integral transmembrane proteins form
the ion channels found on the cell surface. These channel
proteins have a complex morphology and are selective
with respect to the substances they transmit.
A fuzzy-looking layer, called the cell coat or glycocalyx,
surrounds the cell surface. The structure of the cell
coat consists of long, complex carbohydrate chains
attached to protein molecules that penetrate the outside
portion of the membrane (i.e., glycoproteins); outwardfacing
membrane lipids (i.e., glycolipids); and carbohydrate-
binding proteins called lectins. The cell coat
participates in cell-to-cell recognition and adhesion. It
contains tissue transplant antigens that label cells as self
or nonself. The cell coat of a red blood cell contains the
ABO blood group antigens. An intimate relationship
exists between the cell membrane and the cell coat.

Figure 1-2. Structure of the plasma (cell) membrane, showing the hydrophilic (polar) heads and
the hydrophobic (fatty acid) tails (inset) and the position of the integral and peripheral proteins in
relation to the interior and exterior of the cell.