Chapter 10:
Membrane Structure
Membrane Structure
Membrane Function
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Plasma membrane defines cell and maintains differences btwn cytosol
and extracellular environment
Defines individual organelles
Establish ion gradients
Contain proteins act as sensors of external signals allow cell to adapt
to changing environment
Membrane Structure
General Membrane Structure
►Thin film of lipid and protein held together by noncovalent interactions
►lipid bilayer serves as basic fluid structure; impermeable barrier
►Proteins mediate all other functions of membrane
The Lipid Bilayer
Lipids of Membrane
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50% of total membrane mass, 5 x 106/um2
Amphipathic
Self-sealing
Phosphlipids, cholestrol, glycolipids
Phospholipids= most abundant lipid in membrane
Spontaneously aggregate to bury hydrophobic tail
The Lipid Bilayer
Fluidity of Lipid Bilayer
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Rapid lateral diffusion of phospholipids 10-8cm/sec
Change places 107 times/sec
Rarely flip-flop
Fluidity depends on composition (phospholipid and cholesterol) and
temperature
Phase transition= the temperature at which there is a change of state
from liquid to solid
The Lipid Bilayer
Fluidity and length and saturation of FA hydrocarbon chains
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Short hydrocarbon chain lengths
Double bonds fluidity
fluidity
The Lipid Bilayer
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Fluidity and cholesterol content
Cholesterol fluidity
Provides mechanical stability
1 cholesterol/phospholipid in eucaryotes
No cholesterol in procaryotes;
mechanical stabililty imparted by cell
wall
The Lipid Bilayer
Major Membrane Phospholipids
The Lipid Bilayer
The Lipid Bilayer
Lipid Rafts
Microdomains enriched in sphingolipids, cholesterol and membrane
proteins
► Long saturated FA chain of sphingolipids = attractive forces that hold
adjacent molecules together
► Thicker than other parts of bilayer
► able to accommodate membrane proteins concentrating for transport or to
enable proteins to function together
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The Lipid Bilayer
Membrane asymmetry
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Phospholipid distribution:
phosphatidylcholine and sphingomyelin confined to outer monolayer
phosphatidylethanolamine and phosphatidylserine are on inner monolayer
Charge
Proteins
Impt to function (ie, apoptosis and translocation of phosphatidylserine)
glycolipids
The Lipid Bilayer
Glycolipids
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Sugar containing lipid molecules w/ most
extreme assemmetry in distribution
Found exclusively on noncytoplasmic
monolayer
On surface of all plasma membranes
gangliosides= most complex, sialic acid
containing oligosaccharides, net negative
chg, most abundant in pm of nerve cells
Function- protection, cell recognition,
transmission of electrical impulses
Membrane Proteins
Proteins Carry Out Specific Functions of Membrane
►Give ea type of membrane characteristic functional properties
►Amts and types of proteins highly variable
►By mass proteins represent 50% lipids 50%; lipids small thus 1 protein/
50 lipid molecules
►Associate w/ membrane in various ways depending on function:
transmembrane, integral, peripheral
Membrane Proteins
Transmembrane Proteins Typically Cross as Alpha Helix
► Unique orientation reflection syn and insertion in ER and function
► Membrane spanning doamin comprised of nonpolar aa 20-30
► Alpha helix is predominate conformation but beta shees form closed
beta barrels that can span membrane as well
► Can predict membrane spanning regions via hydropathy plot
Membrane Proteins
Beta Barrels form Transmembrane Channels
Tend to be more rigid
► Comprised of 8-22 strands
► Some pore forming proteins generating water-filled channels for select
hydrophilic solutes
► Polar side chains line channel on inside; nonpolar side chains project out
interact w/hydrophobic core of lipid bilayer
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Membrane Proteins
Many Membrane Proteins are Glycolslated
►Majority of transmembrane proteins in
animal cells are glycolsylated
►Sugars added in lumen of ER and golgi
►Oligosaccharide chains always present on
noncytosolic side
Membrane Proteins
Membrane Proteins can be Solubilized and Purified in Detergents
 Distrupt hydrophobic interactions
 Bind to hydrophobic regions of membrane and displace lipid molecules
 SDS, triton
Membrane Proteins
Red Blood Cells
Model system for studying membranes
Available in lg numbers
Easy to isolate uncontaminated from other cell
types
No nucleus or internal organelles
Can prepare ghosts and in-side-out vesicles
Membrane Proteins
Ghosts and InSide-Out RBCs
Ghosts= empty RBCs
prepared by placing cells in soln of low salt to cause water to move into and lyse cells
can reseal or be studied while leaky
Inside-Out Vesicles
Membrane Proteins
Use of Sealed and Unsealed RBC ghosts
 Demonstrated some proteins extend across lipid bilayer
 Enabled sidedness of proteins to be determined
 Label intact sealed ghosts and inside-out vesicles w/ water solutble label
that cannot penetrate lipid bilayer– perform SDS PAGE
 Exposed internal or external surface to proteolytic enzymes that are
membrane impremeant (transmembrane protein will be partially digested
from both sides
 Labeled antibodies
Membrane Proteins
Studies of RBC Plasma Membrane Proteins by SDS PAGE
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15 major protein bands btwn 15,000-250,000 daltons
3 most prominent comprose more than 65% protein mass; ea
arranged differently in membrane
 Spectrin
 Glycophorin
 Band 3
Membrane Proteins
Spectrin= Cytoskeletal Protein Assoc. w/ Cytosolic Side of Membrane
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Most proteins are peripheral and assoc w/ cytosolic side
Spectrin most abundant
Long, thin, flexible rod 100 nm length, constitutes 25% protein mass
2.5 x 105 copies/cell
Principle component of cytoskeleton underlying RBC, maintains structural
integrity and biconcave shape
Heterodimer formed from 2 lg structurally similar subunits that assoc head to
head to form 200 nm long tetramer
Tail ends of 4-5 tetramers linked by binidng short actin filaments and other
cytoskeletal components forming meshwork
Abnormalities in spectrin results in anemia and spherical shaped RBCs that are
fragile
Ankyrin binds spectrin and band 3 to membrane
Membrane Proteins
Spectrin
Membrane Proteins
Glycophorin
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sm singlepass glycoprotein of 131 aa
Most of mass on external surface of
membrane
100 sugar residues on 16 diff side chains
which account for 60% protein mass
>90% sialic acid most of negative chg on
surface of RBC
1 million molec/cell
Function unknown
Membrane Proteins
Band 3
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Multipass membrane protein
Catalyzes couple transport of anions
930 aa thought to extend across bilayer 12X
Allows HCO3 to cross membrane in exchg for Cl- increasing amt of
CO2 delivered to lungs
Membrane Proteins
Bacteriorhodopsin is a Proton Pump
Halobacterium salinarum “purple membrane”
7 transmembrane helices of 25 aa
Retinal light absorbing chromophore linked to
lys side chain
Light excites chromophore causing conf chg
which results in transfer of H+ from inside to
outside cell
Electrochemical gradient used to syn ATP
Can pump up to 100 H+/sec
Membrane Proteins
Membrane Proteins Often Function as Lg Complexes
Membrane Proteins
Membrane Proteins Diffuse in Plane of
Membrane but Do Not Flip-Flop
Membrane Proteins
Techniques for Measuring the Lateral
Diffusion of Membrane Proteins
Membrane Proteins
Confining Proteins and Lipids to Specific
Regions of the Membrane
Membrane Proteins
Cell surface Coated with Sugar Residues
Glycocalyx= CHO rich region on cell surface
Proteoglycans= long polysaccharide chains linked to protein core
Oligosahharides also linked to proteoglycans
Possbile functions: protection, receptors, electrical conductivity