Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

PRINCIPLES OF MEMBRANE TRANSPORT
599
Figure 11–2 Permeability coefficients for the passage of various
molecules through synthetic lipid bilayers. The rate of flow of a solute
across the bilayer is directly proportional to the difference in its concentration
on the two sides of the membrane. Multiplying this concentration difference (in
mol/cm 3 ) by the permeability coefficient (in cm/sec) gives the flow of solute in
moles per second per square centimeter of bilayer. A concentration difference
of tryptophan of 10 –4 mol/cm 3 (10 –4 mol / 10 –3 L = 0.1 M), for example, would
cause a flow of 10 –4 mol/cm 3 × 10 –7 cm/sec = 10 –11 mol/sec through 1 cm 2
of bilayer, or 6 × 10 4 molecules/sec through 1 μm 2 of bilayer.
high permeability
O 2
10 2
1
blood; the resulting accumulation of cystine in the urine leads to the formation of
cystine stones in the kidneys.
All membrane transport proteins that have been studied in detail are multipass
transmembrane proteins—that is, their polypeptide chains traverse the lipid
bilayer multiple times. By forming a protein-lined pathway across the membrane,
these proteins enable specific hydrophilic solutes to cross the membrane without
coming into direct contact with the hydrophobic interior of the lipid bilayer.
Transporters and channels are the two major classes of membrane transport
proteins (Figure 11–3). Transporters (also called carriers, or permeases) bind the
specific solute to be transported and undergo a series of conformational changes
that alternately expose solute-binding sites on one side of the membrane and
then on the other to transfer the solute across it. Channels, by contrast, interact
with the solute to be transported much more weakly. They form continuous pores
that extend across the lipid bilayer. When open, these pores allow specific solutes
(such as inorganic ions of appropriate size and charge and in some cases small
molecules, including water, glycerol, and ammonia) to pass through them and
thereby cross the membrane. Not surprisingly, transport through channels occurs
at a much faster rate than transport mediated by transporters. Although water can
slowly diffuse across synthetic lipid bilayers, cells use dedicated channel proteins
(called water channels, or aquaporins) that greatly increase the permeability of
their membranes to water, as we discuss later.
Active Transport Is Mediated by Transporters Coupled to an
Energy Source
All channels and many transporters allow solutes to cross the membrane only
passively (“downhill”), a process called passive transport. In the case of transport
of a single uncharged molecule, the difference in the concentration on the two
sides of the membrane—its concentration gradient—drives passive transport and
determines its direction (Figure 11–4A). If the solute carries a net charge, however,
both its concentration gradient and the electrical potential difference across
the membrane, the membrane potential, influence its transport. The concentration
gradient and the electrical gradient combine to form a net driving force, the
electrochemical gradient, for each charged solute (Figure 11–4B). We discuss
electrochemical gradients in more detail later and in Chapter 14. In fact, almost all
plasma membranes have an electrical potential (i.e., a voltage) across them, with
the inside usually negative with respect to the outside. This potential favors the
entry of positively charged ions into the cell but opposes the entry of negatively
charged ions (see Figure 11–4B); it also opposes the efflux of positively charged
ions.
H 2 O
urea
glycerol
tryptophan
glucose
CI _
K +
Na +
10 _ 2
10 _ 4
10 _ 6
10 _ 8
10 _ 10
10 _ 12
10 _ 14
low permeability
permeability coefficient (cm/sec)
MBoC6 m11.02/11.02
lipid
bilayer
(A) TRANSPORTER
solute-binding site
solute
(B) CHANNEL PROTEIN
Figure 11–3 Transporters and channel
proteins. (A) A transporter alternates
between two conformations, so that
the solute-binding site is sequentially
accessible on one side of the bilayer
and then on the other. (B) In contrast, a
channel protein forms a pore across the
bilayer through which specific solutes can
passively diffuse.