Cellular Structure, Proteins, and Metabolism
47
cellular membranes contain very little cholesterol. As described
in Chapter 2, cholesterol is slightly amphipathic because of a
single polar hydroxyl group (see Figure 2–12) on its nonpolar
ring structure. Like the phospholipids, therefore, cholesterol is
inserted into the lipid bilayer with its polar region at the bilayer
surface and its nonpolar rings in the interior in association with
the fatty acid chains. Cholesterol associates with certain classes
of plasma membrane phospholipids and proteins, forming orga-
nized clusters that work together to pinch off portions of the
plasma membrane to form vesicles that deliver their contents to
various intracellular organelles, as Chapter 4 will describe.
There are two classes of membrane proteins: integral
and peripheral.
Integral membrane proteins
are closely
associated with the membrane lipids and cannot be extracted
from the membrane without disrupting the lipid bilayer.
Like the phospholipids, the integral proteins are amphipa-
thic, having polar amino acid side chains in one region of
the molecule and nonpolar side chains clustered together
in a separate region. Because they are amphipathic, inte-
gral proteins are arranged in the membrane with the same
orientation as amphipathic lipids—the polar regions are at
the surfaces in association with polar water molecules, and
the nonpolar regions are in the interior in association with
nonpolar fatty acid chains (
Figure 3–7
). Like the membrane
lipids, many of the integral proteins can move laterally in the
plane of the membrane, but others are immobilized because
they are linked to a network of peripheral proteins located pri-
marily at the cytosolic surface of the membrane.
Most integral proteins span the entire membrane and
are referred to as
transmembrane proteins.
Most of these
transmembrane proteins cross the lipid bilayer several times
(
Figure 3–8
). These proteins have polar regions connected
by nonpolar segments that associate with the nonpolar regions
of the lipids in the membrane interior. The polar regions of
transmembrane proteins may extend far beyond the surfaces of
the lipid bilayer. Some transmembrane proteins form channels
through which ions or water can cross the membrane, whereas
others are associated with the transmission of chemical signals
across the membrane or the anchoring of extracellular and
intracellular protein fi laments to the plasma membrane.
Peripheral membrane proteins
are not amphipathic
and do not associate with the nonpolar regions of the lipids
in the interior of the membrane. They are located at the mem-
brane surface where they are bound to the polar regions of
the integral membrane proteins (see Figure 3–7). Most of the
peripheral proteins are on the cytosolic surface of the plasma
membrane where they are associated with cytoskeletal ele-
ments that infl uence cell shape and motility.
The extracellular surface of the plasma membrane con-
tains small amounts of carbohydrate covalently linked to some
of the membrane lipids and proteins. These carbohydrates
consist of short, branched chains of monosaccharides that
extend from the cell surface into the extracellular fl
uid, where
they form a layer known as the
glycocalyx.
These surface car-
bohydrates play important roles in enabling cells to identify
and interact with each other.
The lipids in the outer half of the bilayer differ somewhat
in kind and amount from those in the inner half, and, as we
have seen, the proteins or portions of proteins on the outer sur-
face differ from those on the inner surface. Many membrane
Extracellular fluid
Intracellular fluid
Carbohydrate
portion of
glycoprotein
Integral
proteins
Polar regions
Nonpolar regions
Peripheral
protein
Phospholipids
Transmembrane
proteins
Channel
Figure 3–7
Arrangement of integral and peripheral membrane proteins in
association with a bimolecular layer of phospholipids. Cholesterol
molecules are omitted for the sake of clarity.
NH
2
Extracellular
fluid
COOH
Phospholipid
bilayer
Transmembrane
nonpolar segment
Intracellular fluid
Figure 3–8
A typical transmembrane protein with multiple hydrophobic
segments traversing the lipid bilayer. Each transmembrane segment
is composed of nonpolar amino acids spiraled in an alpha helical
conformation (shown as cylinders).
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