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Summary Membranes
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Summary about the lecture and corresponding literature about membranes.
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Membranes (BOOK)Chapter 10 pp 565-590
Membrane structure
The lipid bilayer
The lipid bilayer provides the basic structure of all cell membranes. Lipid molecules constitute about
50% the mass of most animal cell membranes, nearly all the reminder being protein. All lipid
molecules in cell membranes are amphiphilic which means that they have a hydrophobic or
nonpolar end and a hydrophilic or polar end.
The most abundant membrane lipids are phospholipids. These have a polar head group containing a
phosphate group and two hydrophobic hydrocarbon tails. These tails are fatty acids and can differ in
length. One tail has one or more cis-double bonds which means that it is unsaturated while the other
tail doesn’t, which means it is saturated.
The main phospholipids in animal cell membranes are phosphoglycerides, which have a three-carbon
glycerol backbone. By combining several different fatty acids and head groups, cells make different
phosphoglycerides.
Another important class of phospholipids are the sphingolipids which are build from sphingosine (a
long acryl chain with an NH2 and two OH groups at one end) rather than glycerol.
The four major phospholipids in mammalian plasma membranes are:
In addition to phospholipids, most lipid bilayers contain glycolipids (spingolipids with a sugar instead
of a phosphate linked head group) and cholesterol (a sterol). These three are the three major classes
in cell membranes.
, Due to the shape and amphiphilic nature of phospholipid molecules,
they can form spontaneously bilayers in aqueous environments. The
hydrophobic portions of amphiphilic molecules cluster together so
that the smallest number of water molecules is affected. They can do
this in two ways, by forming a micelle or by forming a lipid bilayer:
Liposomes are lipid bilayers made from spherical vesicles.
Within synthetic bilayers, phospholipid rarely switch from monolayer, but do switch very often with
their neighbours within a monolayer. This gives rise to rapid lateral diffusion.
However, new phospholipids are only produced in one monolayer of a membrane so due to the rare
switching of monolayer this could form a problem for synthesis. This is fixed by a class of proteins
called membrane translocators or flippases which catalyze the rapid flip-flop of phospholipids
between monolayers.
The fluidity of a lipid bilayer depends on both is composition and its temperature. 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 chain that makes them more
difficult to be packet together, making the membrane fluid at lower temperature.
Cholesterol modulates the properties of lipid bilayers by enhancing the permeabilty-barrier
properties of the lipid bilayer. When the OH group is close to the polar head groups of the
phospholipids and they interact, the mobility of the first CH2 groups is decreased which makes the
lipid bilayer less permeable.
Lipid rafts are specialised domains in which the lipid molecules in the plasma membrane similarly
segregate.
Most cells store an excess of lipids in lipid droplets from where they can be retrieved when
necessary. Fat cells or adipocytes are specialised for lipid storage. They contain a lipid droplet from
which fatty acids can be liberated. The lipids in droplets are neutral lipids and do not contain
hydrophilic groups, lipid droplets therefore, are exlusively hydrophobic molecules —> 3D droplets
rather than bilayers.
The two monolayers in lipid bilayers are strikingly different in composition. This can also cause a
significant difference in charge between the monolayers. This lipid asymmetry is important
especially in converting extracellular signals into intracellular ones.
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