BC368
Biochemistry of the Cell II
Biological Membranes
Chapter 11: Part 1
February 10, 2015
Plasma Membrane
“Possibly the decisive step [in the origin
of life] was the formation of the first cell,
in which chain molecules were enclosed
by a semi-permeable membrane which
kept them together but let their food in.”
J. B. S. Haldane,
1954
Plasma Membrane
Plasma Membrane
Membrane is composed of:
A. Lipids
 Phospholipids
 Sterols
B. Proteins
 Integral
 Peripheral
C. Carbohydrates
 Glycolipids
 Glycoproteins
Plasma Membrane
Variable
components in different membrane types
Membrane Lipids
Amphiphilic
lipids
Major types:
phospholipids,
glycolipids, sterols
sphingosine
Glycolipid
glycerophospholipid
sphingophospholipid
Phospholipids
Two classes:
glycerophospholipids
(aka
phosphoglycerides)
and
sphingophospholipids
Fig 10-7
Phospholipids
Two classes:
glycerophospholipids
(aka
phosphoglycerides)
and
sphingophospholipids
Membrane Lipids: 1A. Glycerophospholipids
fatty acids; phosphate and polar “head group”
on glycerol.
Two
Vary
in the FA’s and head group.
Membrane Lipids: 1B. Sphingophospholipids
Named
for the enigmatic Sphinx
Common in nerve and brain cell membranes
Membrane Lipids: 1B. Sphingophospholipids
Named
for the enigmatic Sphinx
Sphingosine
replaces glycerol, so only 1 FA tail
note amide
linkage
Membrane Lipids: 1B. Sphingophospholipids
Example:
sphingomyelin
Head group = phosphocholine or
phosphoethanolamine
Glycolipids
Two classes:
glycosphingolipids
and galactolipids
Fig 10-7
Membrane Lipids: 2A. Glycosphingolipids
Sphingolipids with carbohydrate head
group; common on cell surfaces
Examples:
cerebrosides and gangliosides
Glucose or galactose
Ganglioside
Sugar
Sugar
Membrane Lipids: 2B. Galactolipids
Diglycerides with galatose groups
Common
in plant (thylakoid) membranes
Membrane Lipids: 3. Sterols
Cholesterol and
cholesterol-like
compounds
Lipid Components of Membranes
Lipid
composition
varies across
different
membranes.
Fig 11-2
Lipid Components of Membranes
Lipid composition
varies across the two
leaflets of the same
membrane.
Turnover of Membrane Lipids
Fig 10-16
Defects in Membrane Turnover
Deposits of
gangliosides in Tay
Sachs brain
Lipid Aggregates
 Lipids spontaneously
aggregate in water as
a result of the
Hydrophobic Effect.
Lipid Aggregates
Amphiphilic lipids form
structures that solvate
their head groups and
keep their hydrophobic
tails away from water.
Above the critical
micelle concentration,
single-tailed lipids form
micelles.
Fig 11-4
Lipid Aggregates
Fig 11-4
 Bilayers can form vesicles
enclosing an aqueous cavity
(liposomes).
Double-tailed lipids
form bilayers, the basis
of cell membranes.
Fig 11-4
Membrane Proteins
Integral proteins
(includes lipid-linked):
need detergents to
remove
Peripheral proteins:
removed by salt, pH
changes
Amphitropic proteins:
sometimes attached,
sometimes not
Single Transmembrane Segment Proteins
Usually alpha-helical,
~20-25 residues, mostly
nonpolar.
Example: glycophorin of
the erythrocyte.
Fig 11-8
Multiple Transmembrane Segment Proteins
7 alpha-helix
motif is very
common.
Example:
bacteriorhodopsin
Fig 11-10
Beta Barrel Transmembrane Proteins
 Multiple
transmembrane
segments form β
sheets that line
a cylinder.
 Example: porins.
Lipid-Linked Membrane Proteins
 Attached lipid
provides a
hydrophobic
anchor.
 An important lipid
anchor is GPI
(glycosylated
phosphatidylinositol.
Fig. 11-14
Membrane Carbohydrates
 On exoplasmic face
only
Membrane Carbohydrates
 On exoplasmic face
only
 An example is the blood
group antigens
glycosphingolipids
Membrane Dynamics
At its transition
temperature (TM), the
bilayer goes from an
ordered crystalline state
to an a disordered fluid
one.
Fig 11-16
Membrane Dynamics
Phospholipids in a bilayer have free lateral diffusion.
Fig 11-17
Membrane Dynamics
Phospholipids in a bilayer have restricted movement
between the two faces.
Fig 11-17
Membrane Dynamics
Flippases, floppases, and scramblases catalyze
movement between the two faces.
Fluid Mosaic
Fluorescent Recovery
After Photobleaching
Fluorescent tag is attached
to a membrane component
(lipid, protein, or
carbohydrate).
Fluorescence is bleached
with a laser.
Recovery is monitored over
time.
Fluorescent Recovery
After Photobleaching
FRAP Movie
Protein Mobility in the Membrane
Some membrane
proteins have
restricted
movement.
May be anchored
to internal
structures (e.g.,
glycophorin is
tethered to
spectrin).
Fig. 11-20
Protein Mobility in the Membrane
Lipid rafts are
membrane
microdomains
enriched in
sphingolipids,
cholesterol, and
certain lipid-linked
proteins.
Fig. 11-21
Thicker and less
fluid than neighboring
domains.
Protein Mobility in the Membrane
Lipid rafts are
membrane
microdomains
enriched in
sphingolipids,
cholesterol, and
certain lipid-linked
proteins.
Lipid Rafts
Thicker and less
fluid than neighboring
domains.
Nature Reviews Molecular Cell
Biology 4, 414-418 (May 2003)
Domains of gel/fluid lipid segregation in a model membrane vesicle, which is a mixture of
fluid dilaurylphosphatidylcholine phospholipids with short, disordered chains and gel
dipalmitoylphosphatidylcholine phospholipids with long, ordered chains. A red fluorescent
lipid analogue (DiIC18) partitions into the more ordered lipids, whereas a green fluorescent
lipid analogue (BODIY PC) partitions into domains of more fluid lipids. These domains in a
model membrane are much larger than the domains of cell membranes.