The Cell (Plasma) Membrane.
The cell is enclosed in a thin
membrane that separates the intracellular contents from the extracellular
environment. To differentiate it from the other cell membranes, such as the mitochondrial
or nuclear membranes, the cell membrane is often called the plasma membrane.
In many respects, the plasma 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 provides receptors for hormones and other
biologically active substances, participates in the electrical events 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.
4.8). 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.9 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. A second type of protein, the peripheral proteins, is
bound to one or the other side of the membrane and does not pass into the lipid
bilayer. Removal of peripheral proteins from the membrane surface usually
causes damage to the membrane.
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 intra- cellular 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. Mutations in these channel proteins, often called
channelopathies, are responsible for a host of genetic disorders. For
example, in cystic fibrosis, the primary defect resides in an abnormal
chloride channel, which results in increased sodium and water reabsorption that
causes respiratory tract secretions to thicken and occlude the airways. A
recent discovery showed there are specific water channels or pores called aquaporins
in the plasma membrane. It is now known that aquaporin disorders are
responsible for a number of diseases, including nephrogenic diabetes insipidus.
A fuzzy-looking layer surrounding
the cell surface is called the cell coat, or glycocalyx. The
structure of the glycocalyx consists of long, complex carbohydrate chains
attached to protein molecules that penetrate the outside portion of the
membrane (i.e., glycoproteins); outward-facing membrane lipids (i.e.,
glycolipids); and carbohydrate-binding proteins called lectins.
These proteins [lectins] are responsible for a variety of activities and have
antitumor, immunomodulatory, antifungal, and HIV-1 reverse transcriptase
inhibitory processes. 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. If the cell coat is enzymatically removed, the cell remains viable
and can generate a new cell coat, but damage to the cell membrane usually results in cell death.
