Membrane structure
Membrane Transport of Small
Molecules
Electrical Properties of Membranes
Importance of cell membranes
•Plasma membrane separates cellular components from
the environment.
•Allows organelles to execute specialized functions by
keeping the contents of the organelle separate from
the rest of the cell.
•Provides boundaries that establish electrochemical
gradients. These are used to make ATP and to
generate nerve impulses.
Biological membranes are thin films composed mainly of
amphipathic lipids and proteins. Most of the molecules
are held together by non-covalent interactions.
Polar head
Nonpolar tail
phosphatidylcholin
(phospholipid)
Hydrocarbon tail
derived from
fatty acid.
Unsaturated
hydrocarbon
tail
saturated hydrocarbon tail
Fatty Acids
*
*
*
*
*
Phospholipids
Amphipathic molecules pack so as
to minimize the interaction
between water and the nonpolar
parts. The two hydrocarbon tails
give phospholipids a cylindrical
shape that causes the molecules
to pack as a bilayer in water.
Cholesterol
•Another type of lipid
called cholesterol
decreases fluidity because
it restricts the movement
of the hydrocarbon chains.
Important chemical
characteristics of
cholesterol:
•Hydroxyl group constitutes
the polar head group.
•OH is attached to rigid
steroid ring.
•One hydrocarbon tail.
Cholesterol
Membrane fluidity
Fluidity of the Plasma Membrane
Growth of Membranes
FRAP
(Fluorescence Recovery After Photobleaching)
FLIP
(Fluorescence Loss In Photobleaching)
Membrane polarity
Phosphatidylcholin
Sphingomyelin
Glycolipids
Cholesterol
Phosphatidylserin
Phosphatidylethanolamin
Phosphatidylinositol
Membrane polarity
Glycolipids
Polarity of Plasma Membrane
Detergents
Membrane proteins
Transport
Cell-Cell Communication
Enzymatic activity
Membrane proteins
Alpha Helix
Amino Acids
Transmembrane alpha helix
The hydrophobic core of the bilayer
is 3 nm thick.
One turn of the helix spans 0.54 nm
and there are 3.5 a.a./turn.
Hence, an transmembrane domain
composed composed of an alphahelix is around 20 amino acids long.
Hydropathy plots
Hydropathy plots
Glycophorin
Additional characteristics of
many alpha helical
transmembrane proteins:
•SH groups facing the
cytoplasm are reduced and
those facing outside the cell
are oxidized to S-S.
•Sugars are covalently
attached to certain amino acid
side chains in regions that
face outside the cell.
*
Protein 3 is known as a beta barrel and is composed of a beta sheet curved
into a barrel
•Much rarer than the alpha helix transmembrane domain - limited to the
outer membranes of bacteria, chloroplasts, and mitochondria.
•One well-known representative is a protein called Porin. The inside of the
barrel is lined with polar amino acid side chains and the outside of the
barrel is lined with nonpolar amino acid side chains.
•The polar groups in the peptide backbone are “hidden” by hydrogen bonding
with antiparallel strands.
Beta Sheet
Beta Barrels
transporter
receptor
enzyme
channel
Mild nonionic detergent that
dissolves membranes
without unfolding proteins.
Detergents
Both are
amphipathic
Ionic detergent that
dissolves membranes
and unfolds proteins.
SDS-PAGE
(Polyacrylamide Gel Electrophoresis)
After electrophoretic separation, proteins in the
gel can be stained with a dye (Coomassie blue)
which binds to protein.
- Binds to most proteins (hydrophobic interactions?) in
amounts roughly proportional to the molecular weight of
the protein (~ 1 SDS molecule for every 2 aa residues).
- Bound SDS contributes large -ve charge >> intrinsic charge
of protein, so proteins are separated almost exclusively
on the basis of molecular weight.
Detergents
A major advance in the study
of membrane proteins
involved in transport was the
discovery that the proteins
would spontaneously
assemble with purified lipids
in vesicles sometimes called
“proteoliposomes”.
“Liposome” indicates a
vesicle made of lipids and
“proteo” indicates that
proteins are present.
Membrane proteins form complexes
eg. “photosynthetic reaction center”
(from the bacterium Rhodopseudomonas)
- has 4 subunits, 3 of which contain
a-helical segments that span the
membrane (total: 11)
- helical segments are rich in
non-polar aa, whose hydrophobic
side chains interact with the
membrane lipids that surround
the complex
[In red: non-protein prosthetic groups and electoncarrying isoprenoid compounds called quinones]
Fluorescence recovery
after photobleaching
provides evidence
that many proteins
undergo lateral
diffusion.
Cellular cortex
Compartmentalization of membrane proteins
Red Blood Cells (SEM)
The world of ghosts
Membrane Transport
of Small Molecules
Permeability of Plasma Membrane
Transport of molecules that are impermeable to the lipid bilayer is
achieved by two main classes of membrane transport proteins.
Conformational change “carries”
the solute across the
membrane.
Aqueous pore provides passage
way so solute can diffuse through
the membrane.
Four ways of molecules and ions across the membrane
Electrochemical gradient
Carrier Proteins
A carrier protein binds solute on one side of a membrane,
undergoes a conformational change, and releases the solute on the
other side of the membrane.
A carrier protein resembles an enzyme
Enzyme
Binding sites for substrates
Chemically transforms substrate
Reaction rate saturate at high
substrate concentration
Carrier protein
Binding sites for solute
Transports solute
Transport rate saturates at high
solute concentration
By coupling the conformational change to a source of energy, a
carrier protein can perform active transport.
Depending on how many solute molecules are transported and in
what direction, carrier proteins are dubbed uniporters, symporters,
or antiporters.
Na+-K+ pump (aka Na+-K+ ATPase) in the plasma membrane is an
antiporter that performs active transport.
This protein establishes a concentration gradient for Na+ that is
low inside the cell and high outside, and a concentration gradient
for K+ this high inside the cell and low outside.
Direct active transport: The Na+-K+ pump (a P-class pump)
The Na+ gradient generated by the Na+ - K+ ATPase powers the
transport of glucose into the cell by a Na+ -driven glucose symporter.
The energetically favorable
movement of Na+ down its
electrochemical gradient is
coupled to the energetically
unfavorable transport of
glucose up its concentration
gradient. Hence, glucose is
being subjected to active
transport.
Glucose transport: analogy with enzyme
“ENZYME”
Can be described by:
k1
Sout + T
SUBSTRATE
k2
Sout.T
k-1
k3
Sin.T
k-2
PRODUCT
Sin + T
k-3
Kinetics: rate equations give an expression analogous to Michaelis-Menten equation:
V0 =
Vmax [S]out
Kt + [S]out
V0 = accumulation of glucose inside cell when its concentration outside is [S]out
Kt = measure of affinity of transporter for S
Three carrier proteins, appropriately positioned in the plasma
membrane, function to transport glucose across the intestinal
epithelium.
The S.R. calcium pump
Osmotic pressure
Osmotic pressure - homeostasis
Maintaining osmotic balance is very important
Ion Channels
and
Membrane Potential
Ion channels are ion-selective because of narrow
region called the “selectivity filter”.
side view
R
top view - each circle
represents an end-on
view of an alpha-helix
and the R-groups
radiating outwards are
amino acid side chains.
R
R
R
R
R
R
R R
R
R
R
R
R
R
Stryer Fig. 13.27 Selectivity of a sodium channel.
How can a K+ channel discriminate between K+ and Na+ since they
both have the same charge and Na+ is smaller than K+?
The answer: as the ion
passes through the
selectivity filter, the ion
must shed water.
Carbonyl oxygens with
partial negative charge
can take the place of
water for K+ but Na+ is
too small. Hence, Na+
favors remaining
associated with water and
hydrated ion is too large
to fit through the
selectivity filter.
Ion channels fluctuate between open and closed state to
regulate ion flow.
•Voltage-gated channels respond to the membrane potential.
•Ligand-gated channels respond to association of small molecules called ligands.
•Mechanically gated channels respond to movement.
Establishing a resting membrane potential in animal cells results from
the coordinated action of carrier proteins and ion channels.
1. Na+-K+ ATPase concentrates K+ inside the
cell and Na+ outside (active transport).
2. K+ leak channels allow K+ to diffuse out of
the cells, down the concentration gradient
(passive transport).
3. Negative charge left behind in the
cytoplasm counteracts the efflux of K+ so only
a very small amount (1/100,000) K+ leak out.
4. The efflux of the K+ is sufficient to
generate a membrane potential of
approximately -100 mV - positive outside and
negative inside.
The resting potential is defined as the membrane potential occurring when
there is no net flow of ions. Cells typically have resting potentials
between -20 and -200 mV. The major contributor is the K+ leak channel,
but other channels account for the range of potentials observed.
Resting Membrane Potential: The Nernst equation
Nerve cell (Neuron)
Dendrites - cell protrusions that receive signals from axons.
Cell body - location of nucleus.
Axon - single long protrusion that sends signal away from the cell body.
A nerve impulse results from electrical disturbances in the plasma membrane that
spread from one part of the cell to another. The electrical disturbance is called an
action potential and it consists of a wave of membrane depolarization that moves down
the axon.
Membrane depolarizations spread
passively only short distances
Voltage-gated channels are the key to an action potential.
Depolarization to a
specific membrane
potential causes the
channels to open. This
specific membrane
potential can be
thought of as the
threshold that must
be reached to get the
channel to open.
The structure and function of the voltage-gated
Na+ channel
Figure 21-13
A threshold depolarization initiates the action potential.
When a nerve receives a signal, a
modest depolarization of the
membrane occurs. If this
depolarization reaches the
threshold, all the voltage-gated
Na+ channels experiencing this
threshold depolarization will
open simultaneously. Na+ rushes
into the cell causing a rapid and
large depolarization of the
membrane. This rapid and large
depolarization is the action
potential.
The channel cycle of closed, opened, inactivated along the axon
results in propagation of the action potential.
Initiation of an action potential requires that something
cause the membrane to depolarize to the threshold
potential. The process begins at the synapse.
presynaptic cell
postsynaptic cell
Neurotransmitters are small molecules that
transmit impulses at chemical synapses
Excitation vs. Inhibition Synapses
A single nerve cell combines the excitatory and inhibitory signals received from
numerous other neurons to control the frequency with which action potentials
are generated.
The presynaptic terminals are derived
from numerous other neurons. Ligandgated ion channels are concentrated at
the synapses on the postsynaptic cell
(colored yellowish-brown).
Neurotransmitters released into each
synapse will generate a postsynaptic
potential (PSP).
The voltage-gated Na+ channels are
concentrated at the axon hillock.
An action potential is initiated at the
axon hillock when the sum of excitatory
and inhibitory PSP depolarize the axon
hillock to the threshold potential.
The greater the “combined PSP”, the more frequent action potentials are
generated at the axon hillock. Note that the magnitude of the change in
membrane potential during an action potential is constant because the
membrane potential is limited by the Na+ concentration.
Myelination increases the speed and efficiency of action potential
propagation.
Glial cells (diagrammed in red)
surrounding the axon produce an
electrical insulation rich in
glycolipids.
Nodes of Ranvier located at regular
intervals are openings in the
insulation where Na+ channels are
concentrated.
Influx of Na+ at one node results in
depolarization at the next node due
to the rapid diffusion of Na+ in the
cytoplasm. This triggers an action
potential that leads to rapid
depolarization of the next node.
The action potential jumps from node
to node by a process called saltatory
conduction.
Action potentials travel rapidly from one
node to the next
Figure 21-18
The neuromuscular junction is an example of how the action potential
from the nerve triggers a response in another cell.
Electric synapses
Comparison of action potential transmission across electric
and chemical synapses
Figure 21-36
The End