exam readings part1.docx

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18 Apr 2012
Membrane structure
Plasma membrane
o Enclose cell, define boundaries
o Maintain essential differences between cytosol & extracellular environment
o Eukaryotic cell
Membranes of membrane-enclosed organelles (e.g. ER/ golgi apparatus/ mitochondria)
maintain characteristic differences between contents of each organelles & cytosol
o Ion gradients across membranes established by activities of specialized membrane proteins
used to synthesize ATP drive transmembrane movements of selected solutes
o Nerve/ muscle cell
Produce, transmit e- signals
Proteins -> sensors of external signals allow cell to change behavior in
response to environmental cues (e.g. Signals from other cells)
Transfer info across membrane
Very thin film of lipid & protein molecules
Non-covalent interacts
Dynamics, fluid
Move about in plane of membrane
Lipid (arranged as continuous double layer about 5nm thick membrane proteins embedded)
o 2D liquid constituent molecules free to move laterally
o 50% mass of membrane
o Lipid bilayer
Basic fluid structure of membrane
Relatively impermeable barrier to passage of water-soluble molecules
o All molecules are amphiphilic
o Most abundant membrane lipids- Phospholipids
Amphiphilic bilayer
1 Polar head group + 2 hydrophobic hydrocarbon tails (14- 24 C)
One tail with 1+ cis-double bonds unsaturated, one saturated
Each cis-bond creates small kink in tail
Composition [Length & saturation of fatty acid tails how phospholipid pack against
one another]; temp fluidity of membrane, vise versa
2 types of preparation
o Liposomes (bilayers made in form of spherical vesicles)
o Black membrane (planar bilayers formed across hole in partition
between 2 aq environment)
Measure motion of liquid molecules
o Construct lipid molecule
Fluorescent dye/ small gold particle attached to polar head
group follow diffusion
o Modify lipid head group to carry spin label
Nitroxyl group, contains unpaired e- spin creates paramagnetic
signal detected by e- spin resonance spectroscopy
Phospholipid molecules in synthetic bilayer rarely migrate from monolayer
(leaflet) on one side to other flip-flop (occurs <1 /month; except cholesterol)
Readily exchanges places within monolayer -> rise to rapid lateral diffusion
average lipid molecule diffuses length of large bacterial cell in 1s. Individual
lipid molecules rotate rapidly about long axis
Flexible hydrocarbon chains
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Fluidity has to be precisely regulated
Certain membrane transport processes & enzyme activities
Phase transition (change of state at freezing point) it is lower if hydrocarbon
chains are short or have double bonds (membrane becomes more difficult to
freeze | shorter chain length reduces tendency of hydrocarbon tails interact
with one another, cis-double bonds produce kinks in hydrocarbon chains make
more difficult to pack remains fluid at lower temp
Temp fluctuates with environment adjust fatty acid composition of lipids to
maintain constant fluidity (temp falls synthesize fatty acids with more cis-
double bonds avoid decrease in fluidity)
Main phospholipid- phosphoglycerides
Confined to monolayer as synthetic bilayers manufactured in only 1
monolayer of membrane (cytosolic monolayer of ER) if none newly made
molecules migrate promply to noncytosolic monolayer new lipid bilayer can
not be made solved by *transmembrane enzymes [phospholipid
translocators] catalyze rapid flip-flop of phospholipids from 1 monolayer to
3C glycerol backbone
2 long- chain fatty acids linked through ester bonds to adjacent C atoms of
3rd C attached to P group head group
Phosphatidylserine (-) charge
o Build from sphingosine (not glycerol)
o Long acyl chain with amino group (NH2) + 2 hydroxyl groups (OH) at
one end
o Fatty acid tail is attached to amino group, phosphocholine group
attached to terminal hydroxyl group (one end hydroxyl group free)
o Free hydroxyl group polar properties of adjacent head group form
hydrogen bonds with head group of neighboring lipid +
H2O/membrane protein
Hydrophilic molecules = dissolves readily in H2O (contain charged groups/
uncharged polar groups favorable electrostatic interactions / H bonds with
Hydrophobic molecules = insoluble (uncharged + nonpolar, force adjacent H2O
to reorganize into ice-like cages surround hydrophobic more ordered
increase free energy smallest # of H2O molecules least affected when
hydrophobic cluster together)
Lipid molecules spontaneously aggregate bury hydrophobic tail in interior,
expose hydrophilic heads to water form spherical micelles (tails inward) /
double layered sheets- bilayer (tails sandwiched between head groups)
Favorable arrangement: tails shielded from H2O in interior -> provide self-
healing property. Tear in bilayer freed edge with H2O energetically
unfavorable rearrange spontaneously to eliminate free edge
Consequence of prohibition of free edge only way for bilayer to avoid having
edges closing itself & forming sealed compartment
o Cholesterol
Large in eukaryotic PM, Sterol
Contains rigid ring attached to single polar hydroxyl group + short non-polar
hydrocarbon chain
Orient themselves in bilayer with hydroxyl group close to polar head groups of adjacent
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Modulates properties of bilayers
Enhances permeability-barrier properties when mixed with phospholipids
Inserts into bilayer with hydroxyl group close to polar head of phospholipids rigid,
platelike steroid rings interact + immobilize regions of hydrocarbon closest to polar
By decreasing mobility of first few CH2 groups of hydrocarbon chains of phospholipid
cholesterol makes lipid less deformable in region (decrease permeability of bilayer to
small water-soluble molecules)
Tightens packing of lipids in bilayer
Does not make membrane less fluid
Prevent hydrocarbon chains from coming together & crystallizing (at high[con])
Inositol phospholipids
In cytosolic leaflet of bilayer present in small quantities
Function in guiding membrane traffic & cell signaling
Local synthesis & destruction are regulated by large number of enzymes
create both small intracellular signaling molecules & lipid docking sites on
membrane recruit specific proteins from cytosol
Specific lipid kinases phosphorylate head group of lipids -> docking sites for
cytosolic signaling proteins specific phospholipid cleave certain I.P to generate
intracellular signaling molecules
Randomly distributed in own monolayer
Van der waals force between negboring tails are not selective enough to hold groups of
phospholipids together
Can create dynamic patch work of different domains with lipid mixtures
(Synthetic lipid bilayer, phosphatidylcholine, sphingomyelin, cholesterol)
Van der waals forces between long & saturated hydrocarbon chains of sphingomyelin
~strong enough to hold together
o Glycolipid
o Transmembrane protein
Spin lipid bilayer mediate nearly all of other functions of membrane
Transporting molecules across membrane (e.g. catalyzing membrane-associated
reactions, ATP synthesis)
In plasma membrane
Structure links connect cytoskeleton through lipid bilayer to extracellular
matrix/ adjacent cell
Receptors detect transduce chemical signals in cell environment
30% of proteins encoded in animal cell’s genome
Provide basic structure for all membranes
Structure properties of lipid molecule
Membrane protein (most membranes specific tasks -> give characteristic functional properties)
o ~Half mass = protein (lipid small more lipid)
o Myelin membrane
Electrical insulation for cell axons, <25% of membrane mass protein
o Membrane involved in ATP production
Internal membrane of mitochondria & chloroplast
o Extend through lipid bilayer, part of mass on either side
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