80
UNIT ONE
“tails,” consisting of fatty acid chains, make up the interior
of the membrane
(see f
gs. 3.3 and 3.6)
. The lipid molecules
can move sideways within the plane of the membrane, and
collectively they form a thin but stable fl
uid F
lm.
RECONNECT
To chapter 2, Lipids, page 62.
The interior of the cell membrane consists largely of the
fatty acid portions of the phospholipid molecules, so it is oily.
Molecules soluble in lipids, such as oxygen, carbon dioxide,
and steroid hormones, can pass through this layer easily; how-
ever, the layer is impermeable to water-soluble molecules, such
as amino acids, sugars, proteins, nucleic acids, and various
ions. Many cholesterol molecules embedded in the interior of
the membrane also help make it impermeable to water-soluble
substances. In addition, the relatively rigid structure of the
cholesterol molecules helps stabilize the cell membrane.
A cell membrane includes only a few types of lipid mol-
ecules but many types of proteins
(f
g. 3.7)
, which provide
specialized functions. Membrane proteins are classified
by shape, locations within the phospholipid bilayer, and
function
(table 3.1)
. A protein that spans the membrane is
termed an integral protein. A protein that projects from the
membrane’s outer surface is termed a peripheral protein. A
Cell membrane
Cell membrane
“Heads” of
phospholipid
“Tails” of
phospholipid
(a)
(b)
FIGURE 3.6
The cell membrane is a phospholipid bilayer. (
a
) A transmission electron micrograph of a cell membrane (600,000×); (
b
) the
framework of the membrane consists of a double layer of phospholipid molecules. In actuality, many other molecules are embedded in and extend
from the phospholipid bilayer.
protein that traverses the membrane and also extends from
the outer surface is both an integral and a peripheral protein.
A protein that extends outside the cell membrane at one end
and dips into the cytoplasm on the interior is termed a trans-
membrane protein. Many transmembrane proteins are tightly
coiled rods that function as
receptors.
They bind to speciF c
incoming molecules, such as hormones, triggering responses
from within the cell (see chapter 13, p. 483). CCR5, described
in the vignette on page 76, is such a receptor.
Certain compact and globular proteins span the cell
membrane and provide routes for small molecules and ions
to cross the otherwise impermeable phospholipid bilayer.
Some of these proteins form “pores” that admit water and
others are highly selective and form channels that admit only
particular ions. In nerve cells, for example, selective chan-
nels control the movements of sodium and potassium ions
(see chapter 10, p. 365). Clinical Application 3.1 discusses
how abnormal ion channels cause disease.
Peripheral proteins may also be enzymes (see chapter 4,
p. 117), and many are part of signal transduction pathways.
Other peripheral proteins function as
cellular adhesion mol-
ecules
(CAMs) that enable certain cells to touch or bind,
discussed at the end of this section. Carbohydrate groups
attached to peripheral proteins form glycoproteins that branch
from a cell’s surface, helping cells to recognize and bind to
each other. This is important as cells aggregate to form tis-
sues. Cell surface glycoproteins also mark the cells of an indi-
vidual as “self,” and distinguish particular differentiated cell
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