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Membranes:
Structure,
Function and
Chemistry
Chapter 7
Becker’s The World of Cell
The Functions of Membranes
Cells have a variety of membranes that
define the boundaries of the cell and its
internal compartments. All biological
membranes have the same general
structure: a fluid phospholipid bilayer
containing a mosaic of embedded
proteins.
While the lipid component of membranes
provides a permeability barrier, specific
proteins in the membrane regulate
transport of materials into and out of cells
and organelles.
Membranes serve as sites for specific
proteins and thus can have specific
functions. They can detect and transduce
external signals, mediate contact and
adhesion between neighboring cells, or
participate in cell-to-cell communication.
They also help to produce external
structures such as the cell wall or
extracellular matrix.
Models of Membrane
Structure:
An
Experimental
Perspective
Our current understanding of membrane
structure represents the culmination of
more than a century of studies, beginning
with the recognition that lipids are an
important membrane component.
■
Once proteins were recognized as
important components, Davson and
Danielli proposed their “sandwich”
model—a lipid bilayer surrounded on both
sides by layers of proteins. As membranes
and membrane proteins were examined
in more detail, however, this model was
eventually discredited.
In place of the sandwich model, Singer
and Nicolson’s fluid mosaic model
emerged and is now the universally
accepted description of membrane
structure. According to this model,
proteins with varying affinities for the
hydrophobic membrane interior float in
and on a fluid lipid bilayer.
■ We now know that lipids and proteins
are not distributed randomly in the
membrane but are often found in
microdomains known as lipid rafts that are
involved in cell signaling and other
interactions.
Membrane Lipids: The “Fluid”
Part of the Model
Prominent lipids in most membranes
include numerous types of phospholipids
and glycolipids. The proportion of each
lipid type can vary considerably
depending on the particular membrane
or monolayer.
In eukaryotic cells, sterols are also
important membrane components,
including cholesterol in animal cells and
phytosterols in plant cells. Sterols are not
found in the membranes of most
prokaryotes, but some bacterial species
contain similar compounds called
hopanoids.
Proper fluidity of a membrane is critical to
its function. Cells often can vary the
fluidity of membranes by changing the
length and degree of saturation of the
fatty acid chains of the membrane lipids
or by the addition of cholesterol or other
sterols.
Long-chain fatty acids pack together well
and decrease fluidity. Unsaturated fatty
acids contain cis double bonds that
interfere with packing and increase
fluidity.
Most membrane phospholipids and
proteins are free to move within the plane of the membrane unless they are specifically anchored to structures on the inner or outer membrane surface. Transverse diffusion, or “flip-flop,” between monolayers is not generally possible, except for phospholipids when catalyzed by enzymes called phospholipid translocators, or flippases.
As a result, most membranes are
characterized by an asymmetric
distribution of lipids between the two
monolayers and an asymmetric
orientation of proteins within the
membranes so that the two sides of the
membrane are structurally and
functionally dissimilar.
Membrane Proteins: The “Mosaic” Part of the Model
Proteins are major components of
all cellular membranes. Membrane
proteins are classified as integral,
peripheral, or lipid anchored, based
on how they are associated with
the lipid bilayer.
Proteins are major components of
all cellular membranes. Membrane
proteins are classified as integral,
peripheral, or lipid anchored, based
on how they are associated with
the lipid bilayer.
Integral membrane proteins have one or
more short segments of predominantly
hydrophobic amino acids that anchor the
protein to the membrane. Most of these
transmembrane segments are a-helical
sequences of about 20–30 predominantly
hydrophobic amino acids.
Peripheral membrane proteins are
hydrophilic and remain on the membrane
surface. They are typically attached to
the polar head groups of phospholipids
by ionic and hydrogen bonding.
Lipid-anchored proteins are also
hydrophilic in nature but are
covalently linked to the membrane
by any of several lipid anchors that
are embedded in the lipid bilayer.
Membrane proteins function as enzymes, electron
carriers, transport molecules, and receptor sites for
chemical signals such as neurotransmitters and
hormones. Membrane proteins also stabilize and shape
the membrane and mediate intercellular
communication and cell-cell adhesion.
Many proteins in the plasma
membrane are glycoproteins, with
carbohydrate side chains that
protrude from the membrane on
the external side, where they play
important roles as recognition
markers on the cell surface.
Thanks to current advances in SDS-PAGE, molecular
biology X-ray crystallography, affinity labeling, and the
use of specific antibodies, we are learning much about
the structure and function of membrane proteins that
were difficult to study in the recent past.