Lipids, Biological Membranes,
and Membrane Transport
Chapters 9 and 10
The lipid bilayer
• The thickness of a bilayer is usually
up to around 60 Å.
• Is the barrier that keeps ions,
proteins and other molecules where
they are needed.
• Are impermeable to most watersoluble (hydrophilic) molecules.
• Particularly impermeable to ions,
which allows cells to regulate salt
concentrations and pH by pumping
ions across their membranes using
proteins called ion pumps.
Biological Membranes
• Typically include several types of lipids
other than phospholipids.
• A particularly important example in
animal cells is cholesterol, which helps
strengthen the bilayer and decrease its
permeability
• When closed into `bubbles', bilayers
provide a barrier between `inside' and
`outside'; i.e. they define closed
`compartments'.
• Large bubbles (microns in diameter) are
often called `vesicles'
The Plasma Membrane
• Composed of a phospholipid
bilayer and proteins.
• The phospholipid sets up
the bilayer structure
• Phospholipids have
• hydrophilic heads and
fatty acid tails.
• The plasma membrane is
fluid--that is proteins move
in a fluid lipid background
The Fluid Mosaic Model
• Originally proposed by S.
Jonathan Singer and Garth
Nicolson in 1972.
• Allows for dynamic nature of
membrane
• Little transition of lipids can
take place without specific
enzymes to mediate transfer
- flippase.
Flippase
• Enzymes located in
the membrane responsible
for aiding the movement
of phospholipid molecules
between the two leaflets
that compose a cell's
membrane
• Two types:
– Transverse
– Lateral
Transverse Diffusion
• Or flip-flop involves the
movement of a lipid or
protein from one membrane
surface to the other.
• Is a fairly slow process due
to the fact that a relatively
significant amount of energy
is required for flip-flopping
to occur.
Transverse Diffusion
• Most large proteins do not
flip-flop due to their
extensive polar regions,
which are unfavorable in the
hydrophobic core of a
membrane bilayer.
• This allows the asymmetry of
membranes to be retained
for long periods, which is an
important aspect of cell
regulation.
Lateral Diffusion
• Refers to the lateral
movement of lipids and
proteins found in the
membrane.
• Membrane lipids and proteins
are generally free to move
laterally if they are not
restricted by certain
interactions.
• Is a fairly quick and
spontaneous process
The Endo-membrane system
• Proteins or lipids made in the ER
contained in transport vesicles
fuse with the Golgi.
• The Golgi modifies proteins and
lipids from the ER, sorts them and
packages them into transport
vesicles.
• This transport vesicle “buds off”
and moves to the cytoplasm.
• Fuse with plasma membrane.
•
Flippase
• Potential role of ATPdependent lipid flippases in
vesicle formation.
• ATP-dependent lipid
translocation might help
deform the membrane by
moving lipid mass towards
the cytoplasmic leaflet
Flippase
• This area asymmetry will increase
the spontaneous curvature of the
bilayer, and may thus help deform
the membrane during vesicle
budding.
• Lem3-Cdc50 proteins regulate the
localization and activity of P4ATPases.
• P4-ATPases play a pivotal role in the
biogenesis of intracellular transport
vesicles, polarized protein transport
and protein maturation.
•
Flippase
• Interaction of P4-ATPases with
peripheral guanine nucleotideexchange factors (GEFs) might cause
activation of small GTPases.
• GTPases subsequently bind to the
membrane and facilitate the
assembly of coat proteins (if
required)
• And thus, the endo-membrane system
allows gene expression, posttranslational modification, and
secretion to occur!
Membrane Structure and Dynamics
Membrane functions - physical barrier from entry and exit
form cell and organelles
Membranes - Lipid, protein and carbohydrate
Membrane
% Protein
% Lipid
Plasma membrane
46
54
Mitochondria
76
24
% Carbohydrate
2-4
1-2
• Phospholipids:
• Two fatty acids covalently
linked to a glycerol, which
is linked to a phosphate.
• All attached to a “head
group”, such as choline, an
amino acid.
• Head group POLAR – so
hydrophilic (loves water)
• Tail is non-polar –
hydrophobic
• The tail varies in length
from 14 to 28 carbons.
Membrane components • 60 to 70% of mammalian lipids are phospholipids
• Bacteria have almost no PC and are mostly PE
• Neuronal tissue (myelin) PI > PC
Lipid
P-Choline
P-Ethanolamine
P-Insositol
P-Serine
Sphingosine
plasma membrane
35
19
7
9
18
golgi
45
17
9
4
12
mito
50
23
13
5
3
nuclei
62
23
9
4
3
• Alterations in lipid composition - permeability, fluidity, exocytosis,
neural transmission and signaling potential
Membrane Asymmetry
– P-ethanolamine and P-serine predominately faces inside of cell
– P-choline faces outside of membrane and inside of organelles
– carbohydrates of glycoproteins facing outside
•
During apoptosis there is a re-arraignment of lipids where
phosphatidyl serine moves to the exterior face of the membrane.
•
One of the key signals of cell death
• Proteins - Add function and structure to membrane
• Extrinsic proteins (peripheral)
– Loosely attached to membrane
– ionic bonds with polar head groups and carbohydrates
– hydrophobic bonds with lipid
– proteins have lipids tails
– easily displaced from membrane
– salt, pH, sonication
Integral proteins
- tightly bound to membrane - span both sides
Protein has both polar and hydrophobic sections removed only through disrupting
membrane with detergents
detergents disrupt lipid bilayer and incorporate proteins and some lipids into
detergent micelles
 allows for purification of membrane proteins
 reconstitute into specific vesicles for study
Transmembrane
• So designated because they are
both structurally and functionally
an integral component of a
membrane.
• Example:
•
Human erythrocyte glycophorin A
• Involved in interactions with the
Red Blood Cell cytoskeleton that
may modulate membrane rigidity.
• The extracellular portion of the
protein also serves as the
receptor for the influenza virus
proteins
Transmembrane
• Has a total molecular weight of
about 31,000 and is approximately
40% protein and 60%
carbohydrate.
• The primary structure consists of
a segment of 19 hydrophobic
amino acid residues with a short
hydrophilic sequence on one end
and a longer hydrophilic sequence
on the other end.
• The 19-residue sequence is just
the right length to span the cell
membrane if it is coiled in the
shape of an α-helix.
• The large hydrophilic sequence
includes the amino terminal
residue of the polypeptide chain.
proteins
Transmembrane
• General “Rules of thumb”
• takes about 20 aa to cross
membrane

many proteins cross many
times

odd # of transmembrane
regions,
 -COOH terminal usually
cytosolic
+
 -NH
3 terminal
extracellular

can be predicted by amino acid
sequence

high % of side chains will be
hydrophobic
proteins
membrane transport
• The term refers to the collection of mechanisms that
regulate the passage of solutes such as ions and
small molecules through biological membranes namely lipid
bilayers that contain proteins embedded in them.
• The regulation of passage through the membrane is due to
selective membrane permeability.
• The movements of most solutes through the membrane are
mediated by membrane transport proteins which are
specialized to varying degrees in the transport of specific
molecules.
An example of membrane transport
• Cholesterol
– waxy steroid metabolite found in the cell membranes and
transported in the blood plasma of all animals.
– Essential structural component of mammalian cell membranes,
required to establish proper membrane permeability and fluidity
– transported in the circulatory system within lipoproteins
• LDL molecules are the major carriers of cholesterol in the
blood
–
each one contains approximately 1,500 molecules of cholesterol
• Recognized by the LDL receptor
An example of membrane transport
• Upon binding many LDL receptors become localized
in clathrin-coated pits.
• Both the LDL and its receptor are internalized
by endocytosis to form a vesicle within the cell.
•
The vesicle then fuses with a lysosome, which has an
enzyme called lysosomal acid lipase that hydrolyzes the
cholesterol.
• Now within the cell, the cholesterol can be used for
membrane biosynthesis or esterified and stored within
the cell, so as to not interfere with cell membranes.
Cholesterol Uptake
• Cells destined to take up
cholesterol possess surface
receptors for the LDL
particle.
• Receptor Binding &
Activation: that LDL receptor
binds to Apo-B protein on the
LDL particle
• Coated Pit Formation:
– Clathrin forms cage around
forming endosome
• Clathrin-Coated Vesicle
Budding
– Uncoating of the Vesicle
Cholesterol Uptake
• Early Endosome associates
with other vesicle
• Formation of CURL
(Compartment for Uncoupling
of Ligand and Receptor) or
Late Endosome
• Recycling of the Receptor to
the cell surface
• Fusion of Transport Vesicle
with Lysosome
• Digestion of the LDL to
Release Cholesterol
CURL Formation & Lysosome Digestion
• CURL = Compartment for
Uncoupling of Receptor &
Ligand
• pH drops to acidic (pH 5)
• Conformational change in
Receptor releases LDL
• Receptor recycles to cell
membrane
• Late Endosome fuses with
lysosomal vesicles
• LDL is degraded; Esterases
digest esters Cholesterol is
released into the cytoplasm
• Familial Hypercholesterolemia (FH): high levels of blood
cholesterol and other characteristics
• Leads to an increase in blood LDL (cholesterol)
• Risk of Atherosclerosis & Heart Disease
• Atherosclerosis: buildup of cholesterol deposits lead to
plaques & clog arteries
• Contributes to heart attacks at early age
• One human mutation is due to a defect in LDL receptor
(e.g., in adapter binding site; can't form coated pit
for LDL uptake) which causes the buildup of LDL
particles at the cell surface leading to plaque
formation.
Other types of
Membrane Transport
Summary of membrane transport
• Three types of membrane transporters enhance the
movement of solutes across plant cell membranes
– Channels – passive transport
– Carriers – passive transport
– Pumps- active transport
Channels
• Transmembrane proteins that
work as selective pores
– Transport through these passive
• The size of the pore determines
its transport specifity
• Movement down the gradient in
electrochemical potential
• Unidirectional
• Very fast transport
• Limited to ions and water
Channels
• Sometimes channel transport
involves transient binding of the
solute to the channel protein
• Channel proteins have
structures called gates.
– Open and close pore in response to
signals
• Light
• Hormone binding
• Only potassium can diffuse
either inward or outward
– All others must be expelled
by active transport.
The aquaporin channel protein
• There is some diffusion of
water directly across the bilipid membrane.
• Aquaporins: Integral
membrane proteins that form
water selective channels –
allows water to diffuse faster
– Facilitates water movement in
plants
• Alters the rate of water flow
across the plant cell
membrane – NOT direction
Carriers
• Do not have pores that extend
completely across membrane
• Substance being transported is
initially bound to a specific site
on the carrier protein
– Carriers are specialized to carry a
specific organic compound
• Binding of a molecule causes the
carrier protein to change shape
– This exposes the molecule to the
solution on the other side of the
membrane
•
Transport complete after dissociation of
molecule and carrier protein
• Moderate speed
Carriers
– Slower than in a channel
• Binding to carrier protein is like
enzyme binding site action
• Can be either active or passive
• Passive action is sometimes called
facilitated diffusion
• Unidirectional
Example: GLUT1 glucose carrier
• GLUT1 is a large integral protein,
predicted via hydropathy plots to include
12 transmembrane a-helices
•
Transporter exists in 2 conformations,
T1 with glucose binding site exposed on outer
surface of plasma membrane, and T2, with
binding site exposed on inner surface.
•
D-Glc binding on T1 triggers change to T2.
•
Glc is released into cytosol, triggering
conformational change back to T1, ready to
pick up another glucose from the outside.
•
Process is fully reversible, and as
[S]in approaches [S]out, rates of entry and
exit become equal.
Active transport
• To carry out active transport:
– The membrane transporter must couple the
uphill transport of a molecule with an energy
releasing event
• This is called Primary active transport
– Energy source can be
• The electron transport chain of mitochondria
• The electron transport chain of chloroplasts
• Absorption of light by the membrane transporter
• Such membrane transporters are called
PUMPS
Primary active transport• Movement against the
electrochemical
gradient
• Unidirectional
• Very slow
• Significant interaction
with solute
• Direct energy
expenditure
Pumps
pump-mediated transport against the
gradient (secondary active transport)
• Involves the coupling of the
uphill transport of a
molecule with the downhill
transport of another
• (A) the initial conformation
allows a proton from outside
to bind to pump protein
• (B) Proton binding alters the
shape of the protein to allow
the molecule [S] to bind
pump-mediated transport against the
gradient (secondary active transport)
• (C) The binding of the
molecule [S] again alters
the shape of the pump
protein. This exposes the
both binding sites, and the
proton and molecule [S] to
the inside of the cell
• (D) This release restores
both pump proteins to their
original conformation and
the cycle begins again
pump-mediated transport against the
gradient (secondary active transport)
• Two types:
• (A) Symport:
– Both substances move in
the same direction across
membrane
• (B) Antiport:
– Coupled transport in which
the downhill movement of
a proton drives the active
(uphill) movement of a
molecule
Example: Lactose permease
H+ symport carrier
pump-mediated transport against the
gradient (secondary active transport)
• The proton gradient required for secondary
active transport is provided by the activity
of the electrogenic pumps
• Membrane potential contributes to
secondary active transport
• Passive transport with respect to H+
(proton)
Summary
The end