Chapter 11 Biomembranes
and Transport Across
Biomembranes
A cell body
A celium
mitochondrion
A digestive vacuole
The endoplasmic reticulum
A secretory vesicle
1,Biomembranes are organized,sheetlike assemblies consisting mainly of proteins and
lipids
1.1 The functions carried out by membranes are diverse and indispensable for life.
1.1.1 Biomembranes give cells their individuality by separating them from the
environment.
1.1.2 Biomembranes are highly selective permeability barriers with specific protein
channels and pumps regulating the molecular and ionic compositions of the intracellular
medium (transport across membranes).
1.1.3 Biomembranes control the flow of information between cells and their environment
through specific receptors on the plasma membrane (signal transduction).
1.1.4 Eukaryotic cells have an extensive internal membrane system dividing cells into
various compartments (forming different
organelles).
1.1.5 The two most important energy conversion processes,photosynthesis (occurring in
the inner membranes of chloroplasts) and oxidative phosphorylation (ocurring in the inner membranes
of mitochondria) are carried out by membrane
systems.
1.1.6 Certain biosynthesis (e.g.,synthesis of
lipids and some proteins,smooth ER?) occur on biomembranes.
1.2 Many common features underlie the diversity of biological membranes.
1.2.1 All membranes share a,three-layer”,sheet-like appearance (of 6 to 10 nm thick) under
electron microscopic examination,with two electron-dense layers separated by a less dense
central region.
1.2.2 All membranes are closed entities.
1.2.3 Membranes consist mainly of lipids and proteins with mass ratio ranges between 1:4
to 4:1 (membranes with different functions have different proteins,some lipids and proteins have
covalently linked carbohydrates).
1.2.4 Membranes contain specific proteins (e.g.,pumps,channels,receptors,energy
transducers,and enzymes) to mediate their distinctive functions.
1.2.5 All membrane lipids are amphipathic that form bilayer structures spontaneously in
aqueous media.
1.2.6 Membranes are cooperative
noncovalent assemblies.
1.2.7 Membranes are always asymmetric
with two different faces.
1.2.8 Membranes are fluid two-dimensional
structures (the fluid mosaic model) with oriented proteins and lipids (which form bilayer structures)
that can diffuse laterally but not vertically,(fig and video).
1.2.9 Most membranes are electrically polarized with inside negative (~-70 mV) (the
membrane potential plays a key role in transport,energy conversion,and excitability).
1.3 Proteins attach to membranes in different ways.
1.3.1 Some proteins,called integral proteins,span the lipid bilayer (e.g.,glycophorin in
erythrocyte,bacteriorhodopsin in halobacterium halobacterium).
Fluid mosaic model
1.3.2 Release of integral proteins from membranes requires detergents to interfere
the hydrophobic interactions between proteins and surrounding lipids.
1.3.3 Released integral proteins are usually water-insoluble (easily precipitate to
form insoluble aggregates) when the detergents are removed,
1.3.4 Integral proteins usually have one or more domains rich in hydrophobic amino
acid residues.
1.3.5 Transmembrane domains of a protein can be predicted with reasonable
accuracy through hydropathy plotting.
1.3.6 The three-dimensional structure of the photosynthetic reaction center of a purple
bacterium was the first integral membrane protein to have its atomic structure determined
by X-ray crystallography,where hydrophobic residues are on the exterior interacting with the
lipid bilayer.
1.3.7 Some proteins,called peripheral proteins,are bound to membranes loosely and
reversibly,
1.3.8 Peripheral proteins can be released from membranes by relatively
mild treatments and once released are generally water soluble.
1.3.9 Membrane proteins covalently
attached to lipids of several kinds are anchored to the lipid bilayer (e.g.,fatty
acyl chain,farnesyl chain and others).
4,Membrane lipids are amphipathic and form ordered structures spontaneously in water
4.1 All membrane lipids contain a polar (hydrophilic) head and a nonpolar (hydrophobic)
tail.
4.1.1 Membrane lipids are usually
represented by a circle head and one or two attached wavy or straight lines as the tails.
4.2 The amphipathic membrane lipids form ordered structures in water.
4.2.1 Amphipathic lipids form oriented monolayers at air-water interfaces,
(experiment?).
4.2.2 Fatty acids,lysophospholipids (glycerophospholipids lacking one fatty
acyl group) forms the globular micelles in water.
4.2.3 Phospholipids and glycolipids in aqueous media favorably form
bimolecular sheets (lipid bilayers) rather
than micelles,This is because the two fatty acyl tails are too bulky to fit into the
interior of a micelle,(cylindrical-shaped versus wedge-shaped).
4.2.4 The formation of lipid bilayers from phospholipids and glycolipids is rapid and
spontaneous,stabilized by the full array of weak interactions.
4.2.5 Lipid bilayers have an inherent tendency to be extensive (due to diffusion?
function?).
4.2.6 Lipid bilayers tend to close themselves (to limit the amount exposed
hydrocarbon chains),generating artificial structures called liposomes.
4.2.7 Lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavorable
(driven by hydrophobic interaction and diffusion).
4.3 Liposomes can be used to carry membrane
impermeable substances into cells.
4.3.1 Water-soluble substances (e.g.,proteins,
nucleic acids,drugs) can be encapsulated into liposomes.
4.3.2 Liposomes can fuse with cell plasma membranes (a lipid bilayer),releasing substances
into cells (can be used as drug delivery tools).
4.3.3 Liposomes are used as model systems to
study membrane permeability (or membrane protein reconstitution).
2,Proteins facilitate ions and solutes to move across the hydrophobic membranes in various
ways.
2.1 Simple diffusion of ions and polar molecules in living organisms is impeded by selectively
permeable biomembranes.
2.1.1 Only relatively nonpolar molecules (like
O2,N2) cross biomembranes by simple diffusion(i.e.,move from higher concentration area to lower
one until they become evenly distributed).
2.1.2 Water,though polar,diffuses rapidly across biomembranes by mechanisms not fully
understood,High concentration (55M) may be the reason.
2.2 Most ions and polar solutes move across
biomembranes by carrier-mediated
transport.
2.2.1 Carriers are usually proteins
(also called pumps (active),transporters or
permeases （透 (性 )酶） (passive)).
2.2.2 Carrier proteins are similar to
enzymes,lowering the activation energy of
simple diffusion process,by providing an
alternative hydrophilic transmembrane
pathway.
2.2.3 Initial rate kinetics of protein-mediated transport is very similar to
Michaelis-Menton kinetics of enzymes (showing a hyperbolic curve with similar
saturation effect).
2.2.4 An equation similar to the Michaelis-Menton equation describes
carrier-mediated solute transport across biomembranes,(fig.12-16,p.412)
2.2.5 Carrier proteins are often stereospecific,like enzymes (e.g.,the widely
existing glucose transporter is highly specific for D-glucose 1.5mM,D-mannose/D-galactose
20-30mM,L-glucose 3000mM,Ktransport
reflects the affinity to substrate).
2.3 In facilitated diffusion (also called passive transport) solutes move downhill its concentration
gradient.
2.3.1 Facilitated diffusion usually results in
equilibrium of a solute across the biomembrane leading to no net accumulation of the solute (unless
the solute has a significant different solubility on
the two sides or the solutes has a net charge),(electrochemical potential).
2.3.2 Glucose transporter in some animal cells mediate facilitated diffusion of glucose across the
plasma membranes (GluT1 from the blood plasma to erythrocytes,GluT2 from hepatocytes to the
blood plasma).
2.3.3 Chloride-bicarbonate exchanger (also called anion exchange protein or band 3 protein)
on the erythrocyte membrane mediates bidirectional exchange of HCO3- and Cl- by
facilitated diffusion (not active transport),It is
also called cotransporter (may be active).
2.3.4 Facilitated diffusion process is
independent of metabolic energy.
2.4 In active transport,solutes move against the concentration gradient (uphill) resulting in the
accumulation of a solute on one side of a membrane.
2.4.1 Active transport is thermodynamically unfavorable (endergonic) and occurs only when
an energy source (exergonic process) is coupled.
2.4.2 In primary active transport,energy is provided directly by the hydrolysis of ATP (as with
the Na+-K+ ATPase on vertebrate plasma membranes),by electrons flowing down an electron
transport system (as with H+ pumping out of the
mitochondria inner membranes),or by absorption of sunlight (as with the light-driven H+ pumping of
bacteriorhodopsin in halobacterium).
Directly coupled to an energy source or a
chemical reaction.
2.4.3 In secondary active transport,ion gradients across the membrane,themselves created
by active transport systems (initially by the primary active transports),are used to drive the
concentration uptake of other ions or metabolites.
2.4.4 Many cells contain secondary transport systems that couple the spontaneous,downhill flow
of H+ or Na+ to the simultaneous uphill pumping of another ion,sugars,or amino acids.
2.4.5 Lactose is transported into E.coli cells
through a secondary active transport involving
symport of H+ and lactose by the galactoside
permease.
2.4.6 Glucose and certain amino acids are
transported into intestinal and kidney cells by
symport with Na+ (while the Na+ gradient is
established by the Na+-K+ ATPase).
2.4.7 Some antibiotics act by shuttling ions
across membranes,valinomycin (a cyclic peptide
of 12 residues) binds K+ and moves across the
membrane,while gramicidin A (a linear small
peptide of 15 residues) form a static pore in the
membrane through which cations can pass.
2.4.8 These ion shuttling antibiotics (also
called ionophores（离子载体）,hydrophobic on
the outside) kill bacterial cells by disrupting
secondary transport processes and energy-
conserving reactions,(destroys the ion gradients).
2.5 Na+-K+ ATPase is responsible to maintain the Na+ and K+ gradients across animal cell
plasma membranes.
2.5.1 Most animal cells have a high
concentration of K+ (145 mM) and a low one of Na+ (5 mM) relative the external medium (K+,
5mM; Na+,150 mM?),(fig.)
2.5.2 The Na+-K+ gradient in animal cells controls cell volume,renders nerve and muscle
cells electrically excitable,and drives the active transport of sugars and amino acids.
2.5.3 Jens Skou identified (1950s) an enzyme from crab nerves that hydrolyzes ATP to
ADP only in the presence of both Na+ and K+.2.5.4 This Na+-K+ ATPase was
subsequently shown to be responsible to
pumping Na+ and K+ across membranes against their concentration gradients (Skou shared the
1997 Nobel Prize in Chemistry for this discovery).
2.5.5 For each molecule of ATP hydrolyzed (to ADP and Pi),two K+ ions are pumped
inward,and three Na+ pumped outward across
the plasma membrane.
2.5.6 More than a third of the ATP consumed by a resting animal is used by
this ATPase to pump Na+ and K+.
2.5.7 Ouabain (a cardiotonic steroid
isolated from a plant called foxglove) was found specifically bound to the ATPase
from the extracellular face,blocking both
the ion pumping and ATP hydrolysis,indicating the obligatory coupling of these
three processes as catalyzed by a single enzyme (elaborate mechanism?)
2.5.8 It has been demonstrated that K+ has a much higher affinity for the outside surface of
the enzyme,while Na+ binds much more tightly to the cytoplasmic side.
2.5.9 It has also been an established fact that Na+ triggers phosphorylation,whereas K+
triggers dephosphorylation of the enzyme
(ATPase).
2.5.10 A model has been proposed on the mechanism of Na+-K+ pumping of the enzyme,
mainly supposing that the enzyme cycles between two conformations,a phosphorylated one binding
to K+ with high affinity (from the outside face of
the cell,and to Na+ with low affinity,thus releasing Na+ ions to the outside face),and a
dephosphorylated one binding to Na+ with high affinity (from the inside face of the cell,and to K+
with low affinity,thus releasing K+ ions to the
inside face).
2.6 Ion-selective channels enable specific ions to flow through biomembranes rapidly downhill its
concentration gradient.
2.6.1 Ion channels form pores that open and
close quickly.
2.6.2 The opening and closing of the ion
channel pores can be regulated by ligands or voltage change (called ligand-gated and voltage-
gated channels,respectively).
2.6.3 Acetylcholine（乙酰胆碱） receptor,sodium channel and potassium channel are among
the best studied ion channels.
2.6.4 The sodium and potassium channels
mediate the transimission of nerve impulse (or atction potential) down the motor neuron
(triggering a wave of depolarization).
2.6.5 Acetylcholine receptor mediates the transmission of nerve signals across synapses,
binding of acetylcholine (released from the presynaptic neuron) to the receptor opens the
ion channels,allowing Na+ and K+ (Ca2+) to
diffuse through quickly down their concentration gradients (thus depolarizing the
postsynaptic neurons or muscle cells).
2.6.6 Rate of ion movement across the opened ion channel is not saturable in the
way that carrier-mediated translocation is saturable in the transporting of ions or
solutes (this kinetic property distinguishes
ion channels from carrier mediated transport across membranes),(different
molecular mechanims).
2.6.7 The flow of ions through a single channel and the transition between
different states were monitored with a time resolution of microseconds using the
revolutionary patch-clamp（膜片钳）techniques by Erwin Neher and Bert
Sakmann in the 1970s (they won the 1991
Nobel Prize in medicine or physiology for such high resolution studies on ion
channels).