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

600 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
lipid
bilayer
(A)
simple
diffusion
OUTSIDE
channelmediated
transportermediated
PASSIVE TRANSPORT
+ + + +
+ + + +
ENERGY
ACTIVE TRANSPORT
+ + + +
+ + + +
concentration
gradient
Figure 11–4 Different forms of
membrane transport and the influence
of the membrane. Passive transport
down a concentration gradient (or an
electrochemical gradient—see B below)
occurs spontaneously, by diffusion, either
through the lipid bilayer directly or through
channels or passive transporters. By
contrast, active transport requires an input
of metabolic energy and is always mediated
by transporters that pump the solute
against its concentration or electrochemical
gradient. (B) The electrochemical gradient
of a charged solute (an ion) affects its
transport. This gradient combines the
membrane potential and the concentration
gradient of the solute. The electrical and
chemical gradients can work additively to
increase the driving force on an ion across
the membrane (middle) or can work against
each other (right).
INSIDE
– – – –
– – – –
– – – –
– – – –
(B)
concentration
gradient (with no
membrane potential)
electrochemical gradient with
a membrane potential
As shown in Figure 11–4A, in addition to passive transport, cells need to be
able to actively pump certain solutes across the membrane “uphill,” against their
electrochemical gradients. Such active transport is mediated by transporters
whose pumping activity is directional because it is tightly coupled to a source of
metabolic energy, such as an ion gradient or ATP hydrolysis, as discussed later.
Transmembrane movement of small molecules mediated by transporters can be
either active or passive, whereas that mediated by channels is always passive (see
Figure 11–4A).
MBoC6 m11.04/11.04
Summary
Lipid bilayers are virtually impermeable to most polar molecules. To transport
small water-soluble molecules into or out of cells or intracellular membrane-enclosed
compartments, cell membranes contain various membrane transport proteins,
each of which is responsible for transferring a particular solute or class of
solutes across the membrane. There are two classes of membrane transport proteins—transporters
and channels. Both form protein pathways across the lipid
bilayer. Whereas transmembrane movement mediated by transporters can be
either active or passive, solute flow through channel proteins is always passive. Both
active and passive ion transport is influenced by the ion’s concentration gradient
and the membrane potential—that is, its electrochemical gradient.
TRANSPORTERS AND ACTIVE MEMBRANE
TRANSPORT
The process by which a transporter transfers a solute molecule across the lipid
bilayer resembles an enzyme–substrate reaction, and in many ways transporters
behave like enzymes. By contrast to ordinary enzyme–substrate reactions, however,
the transporter does not modify the transported solute but instead delivers it
unchanged to the other side of the membrane.
Each type of transporter has one or more specific binding sites for its solute
(substrate). It transfers the solute across the lipid bilayer by undergoing reversible