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BIO130H1 (167)

Lecture 1 and 2, Section 2 Text Notes

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University of Toronto St. George
Jane Mitchell

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 cells endoplasmic 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) - 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- 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 a
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