MC 2 LAB│KEYFEJ C. RAMOS, R.CH.

JANUARY 31, 2019

BIOLOGICAL MEMBRANES
AND TRANSPORT
•The first cell probably came into being when a membrane formed, enclosing a
small volume of aqueous solution and separating it from the rest of the
universe.
•Membranes define the external boundaries of cells and regulate the
molecular traffic across that boundary, in eukaryotic cells, they divide the
internal space into discrete compartments to segregate processes and
components.
•Membranes are flexible, self-sealing, and selectively permeable to polar
solutes. Because membranes are selectively permeable, they retain certain
compounds and ions within cells and within specific cellular compartments,
while excluding others.
BIOLOGICAL MEMBRANES
AND TRANSPORT
•They include an array of proteins specialized for promoting or catalyzing various
cellular processes.
•At the cell surface, transporters move specific organic solutes and inorganic ions
across the membrane; receptors sense extracellular signals and trigger molecular
changes in the cell; adhesion molecules hold neighboring cells together.
•Within the cell, membranes organize cellular processes such as the synthesis of lipids
and certain proteins, and the energy transductions in mitochondria and chloroplasts.
MEMBRANE COMPOSITION
The relative proportions of protein and lipid vary with the type of membrane,
reflecting the diversity of biological roles.
The protein composition of membranes from different sources varies even more
widely than their lipid composition, reflecting functional specialization.
FUNDAMENTAL PROPERTIES OF MEMBRANES
•Membranes are impermeable to most
polar or charged solutes, but
permeable to nonpolar compounds; they
are 5 to 8 nm (50 to 80 Å) thick and
appear trilaminar when viewed in cross
section with the electron microscope.

•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.
FLUID MOSAIC MODEL
•Phospholipids form a bilayer in which the nonpolar regions of the lipid molecules in
each layer face the core of the bilayer and their polar head groups face outward,
interacting with the aqueous phase on either side.
•Proteins are embedded in this bilayer sheet, held by hydrophobic interactions
between the membrane lipids and hydrophobic domains in the proteins.
•Some proteins protrude from only one side of the membrane; others have domains
exposed on both sides.
•The orientation of proteins in the bilayer is asymmetric, giving the membrane
“sidedness”: the protein domains exposed on one side of the bilayer are different
from those exposed on the other side, reflecting functional asymmetry.
FLUID MOSAIC MODEL
•The individual lipid and protein units in a membrane form a fluid mosaic with a
pattern that is free to change constantly.
•The membrane mosaic is fluid because most of the interactions among its components
are noncovalent, leaving individual lipid and protein molecules free to move laterally
in the plane of the membrane.
MEMBRANE PROTEINS
•Integral proteins are very firmly associated with the membrane, removable only by
agents that interfere with hydrophobic interactions, such as detergents, organic
solvents, or denaturants.
•Peripheral proteins associate with the membrane through electrostatic interactions
and hydrogen bonding with the hydrophilic domains of integral proteins and with the
polar head groups of membrane lipids. They can be released by relatively mild
treatments that interfere with electrostatic interactions or break hydrogen bonds; a
commonly used agent is carbonate at high pH. Peripheral proteins may serve as
regulators of membrane-bound enzymes or may limit the mobility of integral proteins
by tethering them to intracellular structures.
MEMBRANE DYNAMICS
•One remarkable feature of all biological membranes is their flexibility—their ability
to change shape without losing their integrity and becoming leaky.
•The basis for this property is the noncovalent interactions among lipids in the bilayer
and the motions allowed to individual lipids because they are not covalently
anchored to one another.
•The structure and flexibility of the lipid bilayer depend on temperature and on the
kinds of lipids present.
MEMBRANE DYNAMICS
•The rigid planar structure of the steroid nucleus, inserted between fatty acyl side
chains, reduces the freedom of neighboring fatty acyl chains to move by rotation
about their carbon–carbon bonds, forcing acyl chains into their fully extended
conformation.
•The presence of sterols therefore reduces the fluidity in the core of the bilayer, thus
favoring the liquid-ordered phase, and increases the thickness of the lipid leaflet.
•At relatively low temperatures, the lipids in a bilayer form a
semisolid gel phase, in which all types of motion of individual
lipid molecules are strongly constrained; the bilayer is
paracrystalline.
•At relatively high temperatures, individual hydrocarbon chains
of fatty acids are in constant motion produced by rotation
about the carbon–carbon bonds of the long acyl side chains. In
this liquid-disordered state, or fluid state, the interior of the
bilayer is more fluid than solid and the bilayer is like a sea of
constantly moving lipid.
TRANSBILAYER MOVEMENT OF LIPIDS REQUIRES
CATALYSIS
•At physiological temperature, transbilayer—or “flipflop”— diffusion of a lipid
molecule from one leaflet of the bilayer to the other occurs very slowly if at all in
most membranes.
•Transbilayer movement requires that a polar or charged head group leave
itsaqueous environment and move into the hydrophobic interior of the bilayer, a
process with a large, positive free-energy change.
•Flippases, a family of proteins facilitates flipflop diffusion, providing a
transmembrane path that is energetically more favorable and much faster than the
uncatalyzed movement.
MEMBRANE FUSION
•A remarkable feature of the biological membrane is its ability to undergo fusion with
another membrane without losing its continuity.
•Within the eukaryotic endomembrane system (which includes the nuclear membrane,
endoplasmic reticulum, Golgi, and various small vesicles), the membranous
compartments constantly reorganize.
•Vesicles bud from the endoplasmic reticulum to carry newly synthesized lipids and
proteins to other organelles and to the plasma membrane.
•Exocytosis, endocytosis, cell division, fusion of egg and sperm cells, and entry of a
membrane-enveloped virus into its host cell all involve membrane reorganization in
which the fundamental operation is fusion of two membrane segments without loss of
continuity.
Specific fusion of two membranes requires that (1) they
recognize each other; (2) their surfaces becom closely
apposed, which requires the removal of water molecule
normally associated with the polar head group of lipids;
(3) their bilayer structures become locally disrupted
resulting in fusion of the outer leaflet of eac membrane
(hemifusion); and (4) their bilayers fuse to form a single
continuous bilayer. Receptor mediated endocytosis, or
regulated secretion, also requires that (5 the fusion
process is triggered at the appropriate time or in response
to a specific signal. Integral proteins called fusion
proteins mediate these events, bringing abou specific
recognition and a transient local distortion of the bilayer
structure that favors membrane fusion.
SOLUTE TRANSPORT ACROSS MEMBRANES
When two aqueous compartments containing
unequal concentrations of a soluble compound
or ion are separated by a permeable divider
(membrane), the solute moves by simple
diffusion from the region of higher
concentration, through the membrane, to the
region of lower concentration, until the two
compartments have equal solute concentrations.
SOLUTE TRANSPORT ACROSS MEMBRANES
•When ions of opposite charge are separated by a
permeable membrane, there is a transmembrane
electrical gradient, a membrane potential, Vm
(expressed in volts or millivolts).
•This membrane potential produces a force opposing
ion movements that increase Vm and driving ion
movements that reduce Vm. Thus the direction in which
a charged solute tends to move spontaneously across
a membrane depends on both the chemical gradient
(the difference in solute concentration) and the
electrical gradient (Vm) across the membrane.
Together, these two factors are referred to as the
electrochemical gradient or electrochemical
potential.
SOLUTE TRANSPORT ACROSS MEMBRANES
•Membrane proteins lower the activation energy for
transport of polar compounds and ions by
providing an alternative path through the bilayer
for specific solutes.
•Proteins that bring about this facilitated diffusion,
or passive transport, are not enzymes in the usual
sense; their “substrates” are moved from one
compartment to another, but are not chemically
altered. Membrane proteins that speed the
movement of a solute across a membrane by
facilitating diffusion are called transporters or
permeases.
SOLUTE TRANSPORT ACROSS MEMBRANES
Active Transport results in the accumulation
of a solute above the equilibrium point.
Active transport is thermodynamically
unfavorable (endergonic) and takes place
only when coupled (directly or indirectly) to
an exergonic process such as the absorption
of sunlight, an oxidation reaction, the
breakdown of ATP, or the concomitant flow
of some other chemical species down its
electrochemical gradient.
SOLUTE TRANSPORT ACROSS MEMBRANES
Active Transport results in the accumulation
of a solute above the equilibrium point.
Active transport is thermodynamically
unfavorable (endergonic) and takes place
only when coupled (directly or indirectly) to
an exergonic process such as the absorption
of sunlight, an oxidation reaction, the
breakdown of ATP, or the concomitant flow
of some other chemical species down its
electrochemical gradient.
AQUAPORINS
A family of integral proteins
discovered by Peter Agre, the
aquaporins (AQPs), provide
channels for rapid movement of
water molecules across all
plasma membranes.
ION-SELECTIVE CHANNELS
Ion-selective channels provide another mechanism for moving inorganic
ions across membranes. Ion channels, together with ion pumps such as the
Na+K+ ATPase, determine a plasma membrane’s permeability to specific
ions and regulate the cytosolic concentration of ions and the membrane
potential.
ligand-gated channels (which are generally oligomeric), binding of an
extracellular or intracellular small molecule forces an allosteric transition
in the protein, which opens or closes the channel. In voltage-gated ion
channels, a change I transmembrane electrical potential (Vm) causes a
charged protein domain to move relative to the membrane, opening or
closing the ion channel.
REFERENCES
Lehninger Principles of Biochemistry, Nelson and Cox, 4th edition