Page 39 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

and, thus, for maintenance of the cell mem-
Active Transport
brane potential. During each transport cycle
Active transport occurs in many parts of the (! A1, A2), 3 Na and 2 K are “pumped” out of
+
+
body when solutes are transported against and into the cell, respectively, while 1 ATP
their concentration gradient (uphill transport) molecule is used to phosphorylate the carrier
and/or, in the case of ions, against an electrical protein (! A2b). Phosphorylation first
potential (! p. 22). All in all, active transport changes the conformation of the protein and
occurs against the electrochemical gradient or subsequently alters the affinities of the Na +
+
potential of the solute. Since passive transport
and K
binding sites. The conformational
Fundamentals and Cell Physiology task. Active transport requires the expenditure the membrane (! A2b–d). Dephosphoryla-
mechanisms represent “downhill” transport
change is the actual ion transport step since it
(! p. 20 ff.), they are not appropriate for this
moves the binding sites to the opposite side of
tion restores the pump to its original state
of energy. A large portion of chemical energy
+
+
(! A2e–f). The Na /K pumping rate increases
provided by foodstuffs is utilized for active
transport once it has been made readily avail-
when the cytosolic Na concentration rises—
+
+
able in the form of ATP (! p. 41). The energy
due, for instance, to increased Na influx, or
+
rises. Therefore,
when the extracellular K
created by ATP hydrolysis is used to drive the
+
transmembrane transport of numerous ions,
Na ,K -activatable ATPase is the full name of
+
+ +
metabolites, and waste products. According to
the pump. Na- K -ATPase is inhibited by
Secondary active transport occurs when
pended in these reactions produces order in
cells and organelles—a prerequisite for sur-
uphill transport of a compound (e.g., glucose)
via a carrier protein (e.g., sodium glucose
1 the laws of thermodynamics, the energy ex- ouabain and cardiac glycosides.
vival and normal function of cells and, there-
fore, for the whole organism (! p. 38 ff.). transporter type 2, SGLT2) is coupled with the
In primary active transport, the energy pro- passive (downhill) transport of an ion (in this
+
duced by hydrolysis of ATP goes directly into example Na ; ! B1). In this case, the electro-
+
ion transport through an ion pump. This type chemical Na gradient into the cell (created by
+
+
of ion pump is called an ATPase. They establish Na -K -ATPase at another site on the cell mem-
the electrochemical gradients rather slowly, brane; ! A) provides the driving force needed
e.g., at a rate of around 1 µmol ! s –1 ! m –2 of for secondary active uptake of glucose into the
+
membrane surface area in the case of Na -K - cell. Coupling of the transport of two com-
+
ATPase. The gradient can be exploited to pounds across a membrane is called cotrans-
achieve rapid ionic currents in the opposite port, which may be in the form of symport or
direction after the permeability of ion chan- antiport. Symport occurs when the two com-
nels has been increased (! p. 32 ff.). Na can, pounds (i.e., compound and driving ion) are
+
for example, be driven into a nerve cell at a rate transported across the membrane in the same
of up to 1000µmol ! s –1 ! m –2 during an action direction (! B1–3). Antiport (countertrans-
potential. port) occurs when they are transported in op-
ATPases occur ubiquitously in cell mem- posite directions. Antiport occurs, for example,
+
+
+
branes (Na -K -ATPase) and in the endo- when an electrochemical Na gradient drives
+
plasmic reticulum and plasma membrane H in the opposite direction by secondary ac-
2+
(Ca -ATPase), renal collecting duct and stom- tive transport (! B4). The resulting H gradient
+
ach glands (H ,K -ATPase), and in lysosomes can then be exploited for tertiary active sym-
+
+
+
+
+
(H -ATPase). They transport Na , K , Ca 2+ and port of molecules such as peptides (! B5).
+
H , respectively, by primarily active mecha- Electroneutral transport occurs when the
+
nisms. All except H -ATPase consist of 2 α-sub- net electrical charge remains balanced during
units and 2 "-subunits (P-type ATPases). The transport, e.g., during Na /H antiport (! B4)
+
+
–
+
α-subunits are phosphorylated and form the and Na -Cl symport (! B2). Small charge sep-
ion transport channel (! A1). aration occurs in electrogenic (rheogenic)
+
+
+
Na -K -ATPase is responsible for main- transport, e.g., in Na -glucose 0 symport
26 tenance of intracellular Na and K homeostasis (! B1), Na -amino acid 0 symport (! B3),
+
+
+
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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