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Summary Molecular Biology of the Cell 2 (WBFA007-04)
Samenvatting Molecular Biology of the Cell Chapters 10 - 18
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Chapter 10 – Membrane StructureMEMBRANE STRUCTURE
Cell membranes are crucial to the life of the cell. The plasma membrane encloses the cell, defines its
boundaries, and maintains the essential differences between the cytosol and the extracellular
environment. Inside the eukaryotic cells, the membrane of many membrane-enclosed organelles
maintains the characteristic differences between the contents of each organelle and the cytosol. In all
cells, the plasma membrane also contains proteins that act as sensors of external signals, allowing the
cell to change its behaviour in response to environmental cues, these protein sensors, called
receptors, transfer information across the membrane.
All biological membranes have a common general structure: each
is a very thin film op lipid and protein molecules, held together
mainly by noncovalent interactions. Cell membranes are dynamic,
fluid structures. The lipid bilayer provides the basic fluid structure
of the membrane and serves as a relatively impermeable barrier
to the passage of most water-soluble molecules. Most membrane
proteins span the lipid bilayer and mediate nearly all of the other
functions of the membrane.
THE LIPID BILAYER
The lipid bilayer provides the basic structure for all cell membranes. It can contain different types of
lipid molecules and has some general properties.
PHOSPHOGLYCERIDE, SPHINGOLIPIDS, AND STEROLS ARE THE MAJOR LIPIDS IN CELL
MEMBRANES
All of the lipid molecules in cell
membranes are amphiphilic, they have
a hydrophilic or polar end and a
hydrophobic or nonpolar end. The
most abundant membrane lipids are
phospholipids. These have a polar head
group containing a phosphate group
and two hydrophobic hydrocarbon
tails, which are often fatty acids. One
tail is often unsaturated (double bond),
which causes a kink in the tail, while
the other is often saturated.
The most common lipid in animal cell
membranes are phosphoglycerides,
which have a three-carbon glycerol
backbone. By combining different fatty
acids and head groups, cells make many
different phosphoglycerides.
Another class of phospholipids are the
sphingolipids, which are built from
sphingosine, instead of glycerol. It is a
long acyl chain with an amino group and
two hydroxyl groups.
,Chapter 10 – Membrane Structure
In addition to phospholipids, the lipid bilayers in
many cell membranes contain glycolipids and
cholesterol. Glycolipids resemble sphingolipids,
but, instead of a phosphate-linked headgroup,
they have sugars attached. Eukaryotic plasma
membranes contain especially large amounts of
cholesterol, which is a sterol. It contains a rigid
ring structure, to which is attached a single polar
hydroxyl group and a short nonpolar hydrocarbon chain.
PHOSPHOLIPIDS SPONTANEOUSLY FORM BILAYERS
The shape and amphiphilic nature of the
phospholipid molecules cause them to form
bilayers spontaneously in aqueous environments.
The amphiphilic molecules bury their hydrophobic
tails in the interior, where they are shielded from
the water, and they expose their hydrophilic heads
to water. Depending on their shape, they can do
this in two ways:
1. They can form spherical micelles
2. They can form double-layered sheets, or
bilayers
The same forces that drive phospholipids to form bilayers also provide a self-sealing property. A small
tear in the bilayer creates a free edge with water; because this is energetically unfavourable, the lipids
tend to rearrange spontaneously to eliminate the free edge. A lipid bilayer has other characteristics
that make it an ideal structure for cell membranes. One of the most important of these is its fluidity,
which is crucial to many membrane functions.
THE LIPID BILAYER IS A TWO-DIMENSIONAL FLUID
Individual lipid molecules are able to diffuse freely within the plane of a lipid bilayer. Liposomes,
synthetic lipid bilayers in the form of a spherical vesicle, showed this.
Phospholipid molecules in synthetic bilayers very rarely
migrate from the monolayer on one side to that on the
other. This process, known as “flip-flop”, occurs on a time
scale of hours for any individual molecule. In contrast,
lipid molecules rapidly exchange places with their
neighbours within a monolayer. Individual lipid molecules
rotate very rapidly about their long axis and have flexible
hydrocarbon chains.
Despite the fluidity of the lipid bilayer, liposomes do not fuse spontaneously with one another when
suspended in water. Fusion does not occur because the polar lipid head groups bind water molecules
that need to be displaced for the bilayers of two different liposomes to fuse. All cell membranes fusion
events are catalysed by tightly regulated fusion proteins, which force appropriate membranes into
tight proximity, squeezing out the water layer that keeps the bilayers apart.
,Chapter 10 – Membrane Structure
THE FLUIDITY OF A LIPID BILAYER DEPENDS ON ITS COMPOSITION
The fluidity of cell membranes has to be precisely regulated. It depends on both its composition and
its temperature. A synthetic bilayer made from a single type of phospholipid changes from a liquid
state to a two-dimensional rigid crystalline state at a characteristic temperature. This change of state
is called a phase transition, and the temperature at which it occurs is lower if the hydrocarbon chains
are short or have double bonds. A shorter chain length reduces the tendency of the hydrocarbon tails
to interact with one another, in both the same and opposite monolayer, and cis-double bonds
produce kinks in the chains that make them more difficult to pack together, so that the membrane
remains fluid at lower temperatures.
Cholesterol modulates the properties of lipid bilayers. When mixed with phospholipids, it enhances
the permeability-barrier properties of the lipid bilayer. Cholesterol makes the lipid bilayer less
deformable and thereby decreases the permeability of the bilayer to small water-soluble molecules.
DESPITE THEIR FLUIDITY, LIPID BILAYERS CAN FORM DOMAINS OF DIFFERENT COMPOSITIONS
Because a lipid bilayer is a two-dimensional fluid, we might expect most types of lipid molecules in it
to be well mixed and randomly distributed in their own monolayer. The van der Waals attractive
forces between neighbouring hydrocarbon tails are not selective enough to hold groups of
phospholipid molecules together. There has been a long debate among cell biologists about whether
the lipid molecules in the plasma membrane of living cells similarly segregate into specialized domains,
called lipid rafts. Specific membrane proteins and lipids are seen to concentrate in a more temporary,
dynamic fashion facilitated by protein-protein interactions the allow the transient formation of
specialized membrane regions. Such clusters can be tiny nanoclusters on a scale of a few as the
caveolae involved in endocytosis.
LIPID DROPLETS ARE SURROUNDED BY A PHOSPHOLIPID MONOLAYER
Most cells store an excess of lipids in lipid droplets, from where they can be retrieved as building
blocks for membrane synthesis or as a food source. Fat cells are specialized for lipid storage. They
contain a giant lipid droplet that fills up most of their cytoplasm. Fatty acids can be liberated from lipid
droplets on demand and exported to other cells through the bloodstream.
THE ASYMMETRY OF THE LIPID BILAYER IS FUNCTIONALLY IMPORANT
The lipid compositions of the two monolayers of the lipid
bilayer in many membranes are strikingly different. Lipid
asymmetry is functionally important, especially in
converting extracellular signal into intracellular ones. Many
cytosolic proteins bind to specific lipid head groups found
in the cytosolic monolayer of the lipid bilayer.
Animals exploit the phospholipid asymmetry of their plasma membranes to distinguish between live
and dead cells. When animal cells undergo apoptosis, phosphatidylserine, which is normally confined
to the cytosolic (inner) monolayer of the plasma membrane lipid bilayer, rapidly translocates to the
extracellular monolayer.
GLYCOLIPIDS ARE FOUND ON THE SURFACE OF ALL EUKARYOTIC PLASMA MEMBRANES
Sugar-containing lipid molecules called glycolipids have the most extreme asymmetry in their
membrane distribution: these molecules, whether in the plasma membrane or in intracellular
membranes, are found exclusively in the monolayer facing away from the cytosol. They are made from
sphingosine and these intriguing molecules tend to self-associate, partly through hydrogen bonds
, Chapter 10 – Membrane Structure
between their sugars and partly through van der Waals forces
between their long and straight hydrocarbon chains. The sugars
are added in the lumen of the Golgi apparatus and are exposed
at the cell surface, where they have important roles in
interactions of the cell with its surroundings.
The most complex of the glycolipids, the gangliosides, contain
oligosaccharides with one or more sialic acid moieties, which give
gangliosides a net negative charge. Glycolipids help protects the
membrane, are important because of their electrical effects and
also function in cell-recognition processes.
MEMBRANE PROTEINS
Although the lipid bilayer provides the basic structure of biological membranes, the membrane
proteins perform most of the membrane’s specific tasks and therefore give each type of cell
membrane its characteristic functional properties. Membrane proteins vary widely in structure and in
the way, they associate with the lipid bilayer, which reflects their diverse functions.
MEMBRANE PROTEINS CAN BE ASSOCIATED WITH THE LIPID BILAYER IN VARIOUS WAYS
Membrane proteins are amphiphilic, having hydrophobic and hydrophilic regions. Many membrane
proteins extend through the lipid bilayer, and are therefore called transmembrane proteins, with part
of their mass on either side. Other membrane proteins are located entirely in the cytosol and are
attached to the cytosolic monolayer of the lipid
bilayer, either by an amphiphilic α helix exposed on
the surface of the protein or by one or more
covalently attached lipid chains.
Some proteins are made as single-pass membrane
proteins in the endoplasmic reticulum (ER). While
still in the ER, the transmembrane segment of the protein is cleaved off and a
glycosylphosphatidylinositol (GPI) anchor is added, leaving the protein bound to the noncytosolic
surface of the ER membrane solely by this anchor.
Membrane-associated proteins do not extend into the hydrophobic interior of the lipid bilayer at all;
they are instead bound to either face of the membrane by noncovalent interactions with other
membrane proteins.
LIPID ANCHORS CONTROL THE MEMBRANE LOCALIZATION OF SOME SIGNALING PROTEINS
How a membrane protein is associated with the
lipid bilayer reflects the function of the protein.
Only transmembrane proteins can function on
both sides of the bilayer. Cell-surface receptors
are usually transmembrane proteins that bind
signal molecules in the extracellular space and
generate different intracellular signals on the
opposite side of the plasma membrane.
Proteins that functions on only one side of the lipid bilayer, by contrast, are often associated
exclusively with either the lipid monolayer or a protein domain on that side. Membrane attachment
through a single lipid anchor is not very strong, therefore, a second lipid group is often added to
anchor proteins more firmly to a membrane.
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