Chapter 3
Cells: The Living Units
65
3
Phospholipids
Each lollipop-shaped phospholipid molecule
has a polar “head” that is charged and is
hydrophilic
(
hydro
5
water,
philic
5
loving), and an uncharged, nonpolar “tail” that
is made of two fatty acid chains and is
hydrophobic
(
phobia
5
fear). Te polar heads are attracted to water—the main constit-
uent of both the intracellular and extracellular fluids—and so
they lie on both the inner and outer surfaces of the membrane.
Te nonpolar tails, being hydrophobic, avoid water and line up
in the center of the membrane.
Te result is that all plasma membranes, indeed all biologi-
cal membranes, share a sandwich-like structure: Tey are com-
posed of two parallel sheets of phospholipid molecules lying
tail to tail, with their polar heads exposed to water on either
side of the membrane or organelle. Tis self-orienting prop-
erty of phospholipids encourages biological membranes to self-
assemble into closed, generally spherical, structures and to re-
seal themselves when torn.
With a consistency similar to olive oil, the plasma mem-
brane is a dynamic fluid structure in constant flux. Its lipid
molecules move freely from side to side, parallel to the mem-
brane surface, but their polar-nonpolar interactions prevent
them from flip-flopping or moving from one half of the bilayer
to the other half. Te inward-facing and outward-facing sur-
faces of the plasma membrane differ in the kinds and amounts
of lipids they contain, and these variations are important in de-
termining local membrane structure and function. Most mem-
brane phospholipids are unsaturated, a condition which kinks
their tails (increasing the space between them) and increases
membrane fluidity. (See the illustration of phosphatidylcholine
in Figure 2.16b, p. 45.)
Glycolipids
Glycolipids
(gli
0
ko-lip
9
idz) are lipids with at-
tached sugar groups. Found only on the outer plasma membrane
surface, glycolipids account for about 5% of total membrane lip-
ids. Teir sugar groups, like the phosphate-containing groups of
phospholipids, make that end of the glycolipid molecule polar,
whereas the fatty acid tails are nonpolar.
Cholesterol
Some 20% of membrane lipid is cholesterol. Like
phospholipids, cholesterol has a polar region (its hydroxyl
group) and a nonpolar region (its fused ring system). It wedges
its platelike hydrocarbon rings between the phospholipid tails,
stabilizing the membrane, while decreasing the mobility of the
phospholipids and the fluidity of the membrane.
Membrane Proteins
A cell’s plasma membrane bristles with proteins that allow it to
communicate with its environment. Proteins make up about half
of the plasma membrane by mass and are responsible for most of
the specialized membrane functions. Some membrane proteins
float freely. Others are “tethered” to intracellular structures that
make up the
cytoskeleton
and are restricted in their movement.
Tere are two distinct populations of membrane proteins,
integral and peripheral (Figure 3.3).
Integral Proteins
Integral proteins
are firmly inserted into
the lipid bilayer. Some protrude from one membrane face only,
but most are
transmembrane proteins
that span the entire mem-
brane and protrude on both sides. Whether transmembrane
or not, all integral proteins have both hydrophobic and hy-
drophilic regions. Tis structural feature allows them to interact
with both the nonpolar lipid tails buried in the membrane and
the water inside and outside the cell.
Some transmembrane proteins are involved in transport, and
cluster together to form
channels
, or pores, through which small,
water-soluble molecules or ions can move, thus bypassing the lipid
part of the membrane. Others act as
carriers
that bind to a sub-
stance and then move it through the membrane
(Figure 3.4a)
.
Some transmembrane proteins are enzymes (Figure 3.4d). Still oth-
ers are receptors for hormones or other chemical messengers and
relay messages to the cell interior—a process called
signal transduc-
tion
(Figure 3.4b).
Peripheral Proteins
Unlike integral proteins,
peripheral
proteins
(Figure 3.3) are not embedded in the lipid bilayer.
Instead, they attach loosely to integral proteins and are easily
removed without disrupting the membrane. Peripheral pro-
teins include a network of filaments that helps support the
membrane from its cytoplasmic side (Figure 3.4c). Some pe-
ripheral proteins are enzymes. Others are motor proteins in-
volved in mechanical functions, such as changing cell shape
during cell division and muscle cell contraction. Still others
link cells together.
Lipid Rafts
About 20% of the outer membrane surface contains
lipid
rafs
, dynamic assemblies of saturated phospholipids (which
pack together tightly) associated with unique lipids called
sphingolipids and lots of cholesterol. Te quiltlike lipid ra±s
are more stable and less fluid than the rest of the membrane,
and they can include or exclude specific proteins to various
extents. Because of these qualities, lipid ra±s are assumed to
be concentrating platforms for certain receptor molecules or
for protein molecules needed for cell signaling (discussed on
p.  81), membrane invagination (see endocytosis, p. 77), or
other functions.
The Glycocalyx
Many of the proteins that abut the extracellular fluid are gly-
coproteins with branching sugar groups. Te term
glycocalyx
(gli
0
ko-kal
9
iks; “sugar covering”) describes the fuzzy, sticky,
carbohydrate-rich area at the cell surface. Quite honestly, you
can think of your cells as sugar-coated. Te glycocalyx on each
cell’s surface is enriched both by glycolipids and by glycopro-
teins secreted by the cell.
Because every cell type has a different pattern of sugars in
its glycocalyx, the glycocalyx provides highly specific biological
markers by which approaching cells recognize each other (Fig-
ure 3.4f). For example, a sperm recognizes an ovum (egg cell)
by the ovum’s unique glycocalyx. Cells of the immune system
identify a bacterium by binding to certain membrane glycopro-
teins in the bacterial glycocalyx.
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