8
1 Cell and Membrane Physiology
C. Carriers
Larger solutes, such as sugars and amino acids, are typically as-
sisted across the membrane by carriers. Carriers can be considered
enzymes that catalyze movement rather than a biochemical reaction.
Translocation involves a binding step, which slows transport rate con-
siderably compared with pores and channels (see Table 1.2). There
are three principal carrier modes:
facilitated diffusion
,
primary
active transport
, and
secondary active transport
.
1. Transport kinetics:
Carriers, like enzymes, show substrate
specificity, saturation kinetics (Michaelis-Menten kinetics), and
susceptibility to competition. A general scheme for carrier-medi-
ated transport envisions a solute-binding step, a change in car-
rier conformation that reveals a conduit through which the solute
may pass, and then release on the opposite side of the mem-
brane (Figure 1.13). When solute concentrations are low, carrier-
mediated transport is more efficient than simple diffusion, but a
finite number of solute binding sites means that a carrier can satu-
rate when substrate concentrations are high (Figure 1.14). The
transport rate at which saturation occurs is known as the
trans-
port maximum
(
T
m
) and is the functional equivalent of V
max
that
defines maximal reaction velocity catalyzed by an enzyme.
1
2. Facilitated diffusion:
The simplest carriers use electrochemi-
cal gradients as a motive force (facilitated diffusion) as shown in
Figure 1.15A.They simply provide a selective pathway by which or-
ganic solutes, such as glucose, organic acids, and urea, can move
across the membrane down their electrochemical gradients. The
binding step ensures selectivity of passage. Common examples of
such carriers includes the GLUT family of glucose transporters and
the renal tubule urea transporter (see 27·V·D). The GLUT1 trans-
porter is ubiquitous and provides a principal pathway by which all
cells take up glucose. GLUT4 is an insulin-regulated glucose trans-
porter expressed primarily in adipose tissue and muscle.
3. Primary active transport:
Moving a solute uphill against its
electrochemical gradient requires energy.
Primary active trans-
porters
are
ATPases
that move or
“pump”
solutes across mem-
branes by hydrolyzing adenosine triphosphate (ATP) as shown
in Figure 1.15B. There are three main types of pump, all related
P-type ATPase family members: a
Na -K
ATPase
, a group of
Ca
2
ATPases
, and a
H -K
ATPase
.
a. Na -K ATPase
: The Na -K ATPase (
Na -K
exchanger
or
Na -K
pump
) is common to all cells and uses the energy
of a single ATP molecule to transport three Na out of the
cell, while simultaneously bringing two K back from the ECF.
Movement of both ions occurs uphill against their respective
electrochemical gradients. The physiologic importance of the
Na -K ATPase cannot be overstated. The Na and K gradi-
ents it establishes permit electrical signaling in neurons and
Figure 1.13
Model for transport by a carrier protein.
A solute binds to a site within the
carrier protein on one side of the
membrane.
Intracellular
fluid
Carrier
Binding site
Conformational change reveals a
hydrophilic path to the opposite
side of the membrane.
Solute is released. Carriers work
in the reverse direction also.
1
2
3
Figure 1.14
Carrier saturation kinetics.
1
For further discussion of enzymatic maximal velocity, see
LIR Biochemistry, 5e, p. 56.
INFO
LINK
Concentration
Simple
diffusion
Carrier-
mediated
transport
Transport rate
Transport maximum (T
m
)
Preston_CH01.indd 8
5/21/12 2:43:42 PM
