Lecture 5-6 Cell Membranes.doc

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BIO 1140: Lecture 5-6: Cell Membranes
Reading: Chapter 5
Fluid Mosaic Model:
Singer and Nicolson 1972.
Fluid lipid molecules in which proteins are embedded and freely floating. The
membrane is held together by non-covalent interactions.
Functions:
Because the membrane is made up of lipids, only molecules that are soluble
in lipids can cross the barrier. This is called “selectively permeable”.
Membranes provide a structure for organizing proteins.
Regulation of solute transport: in and out of the cell and the organelles.
E.g. the sodium pump.
Responses to external signals: is the first point of contact to the external
environment. Receptors and signal transduction occur on the membrane.
Cell-to-cell communication: recognition, adhesion and exchange of
materials are due to the membrane via gap junctions, and plasmodesmata.
Membranes:
Mitochondrial membranes have many proteins within them, more than
average has about 74% protein and 26% lipid.
Plasma membrane of Schwann cell (myelin sheath) has about 18% protein
and 82% lipid.
Lipid-protein assembly in which components are held together in a thin sheet
by non-covalent bonds. There are two fluid lipid layers, with a mosaic of
proteins within it. It is approximately 10 nm thick.
The unit membrane:
Gorter and Grendel, 1925.
The lipid bilayer is the key component of the permeability of the barrier.
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Composition:
oPhosphoglycerides: 16-18 carbons on each chain, both saturated
and unsaturated.
oGlycolipids: single sugars or oligosaccharides. They are attached to
the outer side of the membrane and linked to membrane lipids. Their
function is thought to be cell-to-cell recognition.
oSterols: cholesterol and phytosterols. They are absent from
prokaryotes. Cholesterol is embedded into the membranes of animals
cells with just the hydrophilic heads lined up with the heads of the
phospholipids.
Membrane asymmetry:
oLiquids distributed unequally between the two leaflets.
oEstablished during membrane biogenesis.
oFlip-flops versus rotation and lateral diffusion.
oThis reflects the different jobs of each side of the membrane. It is rare
for phospholipids to switch from one side to the other because of the
hydrophobic and hydrophilic interactions.
Active transport:
Carrier-mediated movement against gradient (uphill) via coupled input of
energy (“pumps”).
oThey are highly selective and will only transport certain molecules.
They are unidirectional meaning that they only go one way, no matter
which way the concentration gradient is.
Primary active transport:
oATP hydrolysis directly coupled to transport of solute. The pump flip-
flops between two stable conformations driven by the binding of the
solute and ATP.
oThere are several types including the P-type which move cations and
are reversibly phosphorylated. Another type of pumps are the V-type
pumps (vascuolar pumps) that move hydrogen ions into organelles
(specifically the vacuole).
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Secondary active transport:
oSimultaneous transport of two solutes. Downhill transport along
gradient established by the first active transport allows the uphill
transport of second solute. The energy stored in the proton gradient is
used. For example, the Na+ gradient in animal cells and the H+ gradient
in plant cells.
oThe mammalian kidney works by filtering the blood and then
reabsorbing the nutrients that are needed, the remainder leaves the
body as urine. The kidney tubules need to be very effective at
reabsorption and therefore have a lot of transport mechanisms.
Facilitated diffusion only moves one way, but secondary diffusion moves both
ways with, and against the concentration diffusion.
Homeoviscous Adaptation:
Changes in membrane composition that are designed to maintain membrane
fluidity as environmental temperature changes.
Compensates for changes in temperature while maintaining membrane
fluidity.
It is important to maintain membrane fluidity because if the fluidity decreases
(with cold) then it is very difficult to transport molecules across the
membrane. Membrane fluidity is also important in allowing cells to change
shape. Cells that secrete protons via endocytosis, exocytosis or vesicles can
only happen if the membrane is fluid.
If the membrane becomes too fluid then it no longer functions as a
permeability barrier and is no longer organized.
As temperature falls so does membrane fluidity because the fatty acids pack
together in a tighter conformation. To compensate for these changes, many
organisms will adjust the composition of their membranes as needed (these
are organisms that do not regulate their body temperatures).
oThey can play around with the lipid composition of the membrane
including moving from long chains of carbon in the fatty acids (which
pack together very nicely) to shorter chains or a mix of long and short
(which doesn’t give as rigid of a structure). Going from long chains to
short chains will help maintain membrane fluidity when the
temperature has dropped.
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Document Summary

Fluid mosaic model: singer and nicolson 1972. Fluid lipid molecules in which proteins are embedded and freely floating. The membrane is held together by non-covalent interactions. Functions: because the membrane is made up of lipids, only molecules that are soluble in lipids can cross the barrier. This is called selectively permeable : membranes provide a structure for organizing proteins, regulation of solute transport: in and out of the cell and the organelles. E. g. the sodium pump: responses to external signals: is the first point of contact to the external environment. Receptors and signal transduction occur on the membrane: cell-to-cell communication: recognition, adhesion and exchange of materials are due to the membrane via gap junctions, and plasmodesmata. Membranes: mitochondrial membranes have many proteins within them, more than average has about 74% protein and 26% lipid, plasma membrane of schwann cell (myelin sheath) has about 18% protein and 82% lipid.

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