BIO130H1 Chapter Notes -Electron Paramagnetic Resonance, Lipid Bilayer, Membrane Transport Protein

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24 Apr 2012
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Week 1 (S2) Text Notes (pg.617-624, 629-640)
Membrane Structure
- plasma membrane – encloses cell, defines its boundaries, maintains difference
between cytosol and extracellular environment – contains proteins that act as
sensors of external signals to allow cell to change behavior in response to
environmental cues - receptors transfer information across the membrane
- eukaryotic cellsendoplasmic reticulum, Golgi apparatus, mitochondria, other
membrane-enclosed organelles
- ion gradients across membranes through membrane protein activity
synthesize ATP, drive the transmembrane movement of solutes, produce and
transmit electric signals (nerve and muscle cells)
- membrane general structure – thin lipid film and protein molecules held
together by noncovalent interactions – dynamic
- lipid molecules – arranged in continuous double layer 5 mm thick – bilayer
provides fluid structure of membrane – prevents passage of most water-soluble
molecules
- protein molecules span the lipid bilayer – mediate almost all membrane
functions
Lipid Bilayer
- provides basic structure for all cell membranes
- seen by electron microscopy
- structure of lipid molecules – assemble spontaneously into bilayers
Major Lipids in Cell Membranes
- lipid molecules – 50% of mass of most animal cell membranes, remainder is
mostly protein
- amphiphilic – hydrophilic/polar end and hydrophobic/nonpolar end
- phospholipids – most abundant – polar head group and two hydrocarbon
(hydrophobic) tails – tails are usually fatty acids, differ in length
- one tail usually has on or more cis-double bonds (unsaturated), and the other is
saturated (no double bonds) – cis-double bond creates a kink in tail structure
- phosphoglycerides – main phospholipids in animal cell membranes – have
three-carbon glycerol backbone
- two long chain fatty acids linked through ester bonds to adjacent carbon atoms
of glycol – third carbon atom is attached to phosphate group (which is linked to
a head group – several types of head groups)
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- combination of different fatty acids and head groups – many different
phosphoglycerides
- sphingolipods – another type of phospholipid – built into sphingosine, not
glycerol
- sphingosine – long acyl chain with amino group and two hydroxyl groups
- fatty acid tail attached to amino group, phosphocholine group is attached to
terminal hydroxyl group – leaves one hydroxyl free, which contribute to
polarity of adjacent head group (form hydrogen bonds)
- lipid bilayers in many cell membranes contain cholesterol and glycolipids
- eukaryotic plasma membranes contain large amounts of cholesterol (a sterol)
- cholesterol – rigid ring structure attached to a single polar hydroxyl group and
a short nonpolar hydrocarbon chain – orient themselves in the bilayer with
hydroxyl group close to the polar head groups of adjacent phospholipid
molecules
Phospholipid Bilayers
- phospholipid molecules – amphiphilic nature and shape cause them to form
bilayers – spontaneous in aqueous environments
- hydrophilic – dissolve in water – contain charged groups/uncharged polar
groups that create electrostatic interactions or hydrogen bonds
- hydrophobic – insoluble – atoms are uncharged/nonpolar – cannot form
energetically favourable interactions with water
- lipid molecules spontaneously aggregate to bury hydrophobic hydrocarbon
tails in the interior – expose hydrophilic heads to water
- either form spherical micelles or double layered sheets/bilayers
- micelle – cone shaped lipid molecules
- bilayer – cylinder shaped lipid molecules – phospholipids
- self-healing property of bilayer – tear in bilayer is energetically unfavourable –
lipids rearrange spontaneously to eliminate opening
Two-Dimensional Bilayer
- lipid molecules are able to diffuse freely within lipid bilayers
- liposomes – bilayers in the form of spherical vesicles – vary in size
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- black membranes – planar bilayers – formed across a partition between two
aqueous compartments
- measure lipid movement – construct a lipid molecule with a fluorescent dye or
gold particle on the head group – follow diffusion in a membrane
- alternative way to measure – modify lipid head to carry a spin label (i.e. a
nitroxyl group) that contains an unpaired electron – paramagnetic spin of the
electron can be detected by electron spin resonance spectroscopy – similar to
NMR
- phospholipid molecules do not often move from one side of the
monolayer/leaflet to the other – called flip-flop – only in cholesterol does this
occur rapidly
- lipid molecules often exchange places within a monolayer/leaflet – rapid lateral
diffusion – rotate very rapidly about their axis and have flexible hydrocarbon
chains
- lipid molecules are very disordered – variously spaced and oriented in bilayer
- confinement to their own monolayer provides problems for lipid synthesis
phospholipids are only synthesized in one monolayer of a membrane – of they
cannot migrate to noncytosolic monolayer, new lipid bilayer cannot be made
- phospholipid translocators – special class of transmembrane enzymes which
catalyze the flip-flop of phospholipids from one monolayer to the other
Composition and Fluidity of the Lipid Bilayer
- fluidity is precisely regulated
- viscosity increased beyond threshold – certain membrane transport processes
and enzyme activities cease
- fluidity – depends on composition and temperature
- synthetic bilayer – made from single type of phospholipid changes from liquid
to two-dimensional rigid crystalline/gel state – change of state called phase
transition
- temperature of phase transition lowered if hydrocarbon chains are short or
have double bonds – shorter chain reduces interactions, cis-double bond
produces kinks that make packing together difficult – remain fluid at lower
temperatures
- organisms whose temperatures fluctuate with the environment adjust fatty
acid composition of membrane lipids to maintain constant fluidity
- lower temperature – more cis-double bonded lipids to prevent freezing
- cholesterol mixed with phospholipids – enhances the permeability-barrier
properties of the lipid bilayer
- cholesterol inserts into the bilayer with hydroxyl group close to the polar head
groups of the phospholipids – steroid rings interact with and immobilize regions
of the hydrocarbon chain closest to polar head groups
- decreasing mobility of CH2 groups in hydrocarbon chains of phospholipid
molecules – lipid bilayer is less deformable – decreases permeability of the
bilayer to small water soluble molecules – however the membrane is not any
less fluid
Membrane Proteins
- perform most specific tasks in a cell – give the cell membrane its functional
properties
- amounts and types of proteins are highly variable
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