Passive Transport Is Facilitated by Membrane Proteins

Passive Transport Is Facilitated by
Membrane Proteins
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 (Fig. 11–27a). When ions
of opposite charge are separated by a permeable membrane,
there is a transmembrane electrical gradient, a
membrane potential, Vm (expressed in millivolts).
This membrane potential produces a force opposing
ion movements that increase Vm and driving ion movements
that reduce Vm (Fig. 11–27b). 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.
This behavior of solutes is in accord with the second
law of thermodynamics: molecules tend to spontaneously
assume the distribution of greatest randomness
and lowest energy.
To pass through a lipid bilayer, a polar or charged
solute must first give up its interactions with the water
molecules in its hydration shell, then diffuse about
3 nm (30 Å) through a substance (lipid) in which it is
poorly soluble (Fig. 11–28). The energy used to strip
away the hydration shell and to move the polar compound
from water into lipid, then through the lipid
bilayer, is regained as the compound leaves the membrane
on the other side and is rehydrated. However,
the intermediate stage of transmembrane passage is a
high-energy state comparable to the transition state in
an enzyme-catalyzed chemical reaction. In both cases,
an activation barrier must be overcome to reach the
intermediate stage (Fig. 11–28; compare with Fig. 6–3).
The energy of activation (DG‡) for translocation of a
polar solute across the bilayer is so large that pure
lipid bilayers are virtually impermeable to polar and
charged species over periods relevant to cell growth
and division.
Membrane proteins lower the activation energy for
transport of polar compounds and ions by providing an
alternative path across the membrane for specific solutes.
Proteins that bring about this facilitated diffusion, or
FIGURE 11–27 Movement of solutes across a permeable membrane.
(a) Net movement of an electrically neutral solute is toward the side of
lower solute concentration until equilibrium is achieved. The solute concentrations
on the left and right sides of the membrane are designated C1
and C2. The rate of transmembrane solute movement (indicated by the
arrows) is proportional to the concentration ratio. (b) Net movement of
an electrically charged solute is dictated by a combination of the electrical
potential (Vm) and the ratio of chemical concentrations (C2/C1) across the
membrane; net ion movement continues until this electrochemical potential
reaches zero.
FIGURE 11–26 Summary of transporter types. Some types (ionophores,
ion channels, and passive transporters) simply speed transmembrane
movement of solutes down their electrochemical gradients, whereas
others (active transporters) can pump solutes against a gradient, using
ATP or a gradient of a second solute to provide the energy.
Sout
Sin
Sin
Sout
Sin
Sin
Ion
Ion
Ion
Ion Ion
Ion
Sout
Sout
ATP
ADP + Pi
Facilitated diffusion
(down electrochemical
gradient)
Simple diffusion
(nonpolar
compounds
only, down
concentration
gradient)
Primary active
transport (against
electrochemical
gradient driven
by ATP)
Secondary active
transport (against
electrochemical
gradient, driven by
ion moving down
its gradient)
Ion channel (down
electrochemical
gradient; may be
gated by a ligand
or ion)
Ionophoremediated
ion
transport (down
electrochemical
gradient)
C1 >> C2
Before
equilibrium
Net flux
C1
C1 = C2