Structure and Composition
Dr. Abhishek Roy
Junior Resident (2nd Year)
Dept. of Biochemistry
Grant Govt. Medical College
& Sir J.J. Group of Hospitals, Mumbai
• Viewed in cross section, all cell membranes share a
characteristic trilaminar appearance. This
erythrocyte was stained with osmium tetroxide &
viewed with Electron Microscope.
• The plasma membrane appears as a three-layer
structure, 5 to 8 nm (50 to 80 Å) thick. The
trilaminar image consists of two electron-dense
layers (the osmium, bound to the inner and outer
surfaces of the membrane) separated by a less
dense central region.
• The functional specialization of each membrane type
is reflected in its unique lipid composition.
• Cholesterol is prominent in plasma membranes but
barely detectable in mitochondrial membranes.
• Cardiolipin is a major component of the inner
mitochondrial membrane but not of the plasma
• Phosphatidylserine, phosphatidylinositol, and
phosphatidylglycerol are relatively minor
components (yellow) of most membranes but serve
critical functions; phosphatidylinositol and its
derivatives, for example, are important in signal
transductions triggered by hormones.
Lipid composition of the plasma membrane
and organelle membranes of a rat hepatocyte.
Peripheral, integral, and amphitropic proteins
• Most peripheral proteins are released by changes in
pH or ionic strength, removal of Ca2+ by a chelating
agent, or addition of urea or carbonate.
• Integral proteins are extractable with detergents,
which disrupt the hydrophobic interactions with the
lipid bilayer and form micelle-like clusters around
individual protein molecules.
• Integral proteins covalently attached to a membrane
lipid, such as a glycosyl phosphatidylinositol (GPI), can
be released by treatment with phospholipase C.
Transbilayer disposition of glycophorin in an erythrocyte.
• A segment of 19 hydrophobic residues (residues 75 to 93)
forms an α-helix that traverses the membrane bilayer.
• The segment from residues 64 to 74 has some
hydrophobic residues and probably penetrates the outer
face of the lipid bilayer
Integral membrane proteins.
• Types I and II have a single transmembrane helix; the amino-terminal
domain is outside the cell in type I proteins and inside in type II.
• Type III proteins have multiple transmembrane helices in a single
• In type IV proteins, transmembrane domains of several different
polypeptides assemble to form a channel through the membrane.
• Type V proteins are held to the bilayer primarily by covalently linked
• Type VI proteins have both transmembrane helices and lipid (GPI)
Bacteriorhodopsin, a membrane-spanning protein.
• The single polypeptide chain folds into seven hydrophobic
α-helices, each of which traverses the lipid bilayer roughly
perpendicular to the plane of the membrane.
• The seven transmembrane helices are clustered, and the
space around and between them is filled with the acyl
chains of membrane lipids.
Lipid annuli associated with two integral membrane
a) The crystal structure of sheep aquaporin (PDB ID 2B6O),
a transmembrane water channel, includes a shell of phospholipids
positioned with their head groups (blue) at the expected positions
on the inner and outer membrane surfaces and their hydrophobic
acyl chains (gold) intimately associated with the surface of the
protein exposed to the bilayer. The lipid forms a “grease seal”
around the protein, which is depicted as a green surface
b) The crystal structure of the Fo integral protein complex of the V-
type Na+ -ATPase from Enterococcus hirae (PDB ID 2BL2) has 10
identical subunits, each with four transmembrane helices,
surrounding a central cavity filled with phosphatidyl-glycerol
(PG). Here five of the subunits have been cut away to reveal the PG
molecules associated with each subunit around the interior of this
Tyr and Trp residues of membrane proteins clustering at the water-lipid interface.
• Residues of Tyr (orange) and Trp (red) are found predominantly where the nonpolar region of acyl chains
meets the polar head group region.
• Charged residues (Lys, Arg, Glu, Asp; shown in blue) are found almost exclusively in the aqueous phases.