Ganong's Review of Medical Physiology, 23rd Edition

48 SECTION ICellular & Molecular Basis of Medical Physiology

SECONDARY ACTIVE TRANSPORT

In many situations, the active transport of Na+ is coupled to
the transport of other substances (secondary active trans-
port). For example, the luminal membranes of mucosal cells
in the small intestine contain a symport that transports glu-
cose into the cell only if Na+ binds to the protein and is trans-
ported into the cell at the same time. From the cells, the

glucose enters the blood. The electrochemical gradient for

Na+ is maintained by the active transport of Na+ out of the

mucosal cell into ECF. Other examples are shown in Figure 2–

19. In the heart, Na,K ATPase indirectly affects Ca2+ trans-
    port. An antiport in the membranes of cardiac muscle cells
    normally exchanges intracellular Ca2+ for extracellular Na+.
    Active transport of Na+ and K+ is one of the major energy-
    using processes in the body. On the average, it accounts for

FIGURE 2–17 Diagrammatic representation of the pore-forming subunits of three ion channels. The α subunit of the Na+ and Ca2+
channels traverse the membrane 24 times in four repeats of six membrane-spanning units. Each repeat has a “P” loop between membrane spans
5 and 6 that does not traverse the membrane. These P loops are thought to form the pore. Note that span 4 of each repeat is colored in red, rep-
resenting its net “+” charge. The K+ channel has only a single repeat of the six spanning regions and P loop. Four K+ subunits are assembled for a
functional K+ channel. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. McGraw-Hill, 2000.)

Extracellular

side

Na+ channel I

Cytoplasmic

side

(^12345) P (^612345) P (^612345) P (^612345) P 6
Ca2+ channel
NH 2 COOH
NH 2
NH 2
COOH
COOH
12345 6 12345 6 12345 6 12345 6
K+ channel
12345 6
II III IV
P P P P
P