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Chapter 10: Membranes
Know the terminology:
Phospholipid, phosphoglyceride, sphingolipid,
cholesterol, steroids, phosphotide, polar head
group, fatty acid, glycerol, glycoprotein,
proteoglycan
bilayer, fluidity, homeoviscous adaptation,
integral membrane protein, transmembrane
domain, peripheral protein, lipid raft, hydropathy
plot
Membranes allow compartmentation
Biological membranes are composed of a:
(i) Lipid bilayer
(ii) Proteins
Lipid components of the bilayer
Phospholipids
• Phosphoglycerides: glycerol, 2 fatty acids and
polar head group
• Sphingolipids: sphingosine, 1 fatty acid and polar
head group
Other lipids:
• Steroids (cholesterol mainly)
• Fatty acids: aliphatic chains with carboxylic acid
group
Protein components of the bilayer
Simple proteins
Glycoproteins:
Proteins with carbohydrate chains
Proteoglycans:
Proteins with glycosaminoglycan chains
Phosphoglycerides
Composed of:
(i) A glycerol backbone (with 3 positions)
(ii) 2 long chain fatty acids
(iii) A polar head group
Lipid bilayer
Sphingolipids
(e.g. sphongomyelin) Composed of:
(i) A sphingosine backbone
(ii) 1 long chain fatty acid
(iii) A polar head group
Sphingolipids and phosphoglycerides
Cholesterol
Cholesterol
Cholesterol increases
“tightness” of
membranes but
increases fluidity
Membranes are heterogeneous
(1) Inner and outer leaflets are distinct in
composition
Membranes are heterogeneous
(2) Regions of membranes can be enriched in
specific lipids such as cholesterol (lipid rafts)
Membrane fluidity
Membranes composed of phospholipids are highly
mobile.
Membrane fluidity depends upon lipid
composition
Phospholipid movement depends upon:
(i) Fatty acid chain length
(ii) Saturation
(iii) Polar head group
(iv) Physical conditions
Membrane fluidity also depends upon presence of
other macromolecules:
(i) Cholesterol
(ii) Glycolipids
Homeoviscous adaptation
Environmental conditions (temperature, salt
concentration) can alter membrane fluidity
Cells adjust lipid profiles to maintain constant
fluidity
Reduced temperature “solidifies” membranes
Cell increase fluidity by:
• using shorter fatty acids
• introducing double bonds into fatty acids
• altering polar head groups
Membrane proteins
Many membranes are primarily protein
(e.g. mitochondrial inner membrane is about
~80% protein, 20% lipid)
Proteins can be associated with the membrane many
different ways:
(1) Integral proteins are embedded within the
membrane
(2) Peripheral proteins are only associated
with the membrane (via various connections)
Topography and membrane proteins
Topography and membrane proteins
Transmembrane proteins
Many proteins cross
completely through the
membrane one or more
times
Typically an alpha-helix
with hydrophobic amino
acids (Figs. 10-19, 1020)
Surface hydrophocity of a-helices
Hydrophobic amino acids-green
Polar-blue,
Charged-red
Predicting membrane-spanning domains
Membrane spanning domains can be predicted from
primary structures using hydropathy plots.
Some integral proteins are b-barrels
Many large pores are composed
of beta-sheets arranged into a
barrel.
Controlling protein location
In a naked cell, proteins
are free to move within
membranes but often cells
restrict movement of
proteins.
(1) Some proteins
interact with each other
(self-assembly) (Fig 1043)
Controlling protein location
(2) Others can interact directly with the
cytoskeleton (or via linkers)
Controlling protein location
(3) Some interact via external domains (e.g.
carbohydrate)
Controlling protein location
(4) Inter-cellular interactions may prevent
movements.
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