Chapter 3
Cells: The Living Units
73
3
diffuse according to
electrochemical gradients
, thereby recog-
nizing the effect of both electrical and concentration (chemical)
forces. Hence, the electrochemical gradients maintained by the
Na
1
-K
1
pump underlie most secondary active transport of nu-
trients and ions, and are crucial for cardiac, skeletal muscle, and
neuron function.
Figure 3.10
on p. 74,
Focus on Primary Active Transport: ±e
Na
1
-K
1
Pump
, describes the operation of the Na
1
-K
1
pump.
Make sure you understand this process thoroughly before mov-
ing on to the topic of secondary active transport.
Secondary Active Transport
A single ATP-powered pump,
such as the Na
1
-K
1
pump, can indirectly drive the
secondary
active transport
of several other solutes. By moving sodium
across the plasma membrane against its concentration gradient,
the pump stores energy (in the ion gradient). ±en, just as water
pumped uphill can do work as it flows back down (to turn a
water wheel, for instance), a substance pumped across a mem-
brane can do work as it leaks back, propelled “downhill” along
its concentration gradient. In this way, as sodium moves back
into the cell with the help of a carrier protein, other substances
are “dragged along,” or cotransported, by the same carrier pro-
tein
(Figure 3.11)
. ±is is a symport system.
For example, some sugars, amino acids, and many ions are
cotransported via secondary active transport into cells lining
the small intestine. Because the energy for this type of transport
is the concentration gradient of the ion (in this case Na
1
), Na
1
has to be pumped back out of the cell to maintain its diffusion
gradient. Ion gradients can also drive antiport systems such as
those that help regulate intracellular pH by using the sodium
gradient to expel hydrogen ions.
Regardless of whether the energy is provided directly (pri-
mary active transport) or indirectly (secondary active trans-
port), each membrane pump or cotransporter transports only
specific substances. Active transport systems provide a way for
the cell to be very selective in cases where substances cannot
pass by diffusion. No pump—no transport.
Vesicular Transport
In
vesicular transport
, fluids containing large particles and
macromolecules are transported across cellular membranes in-
side membranous sacs called
vesicles
. Like active transport, ve-
sicular transport moves substances into the cell (endocytosis)
and out of the cell (exocytosis). It is also used for combination
processes such as
transcytosis
, moving substances into, across,
and then out of the cell, and
vesicular trafficking
, moving sub-
stances from one area (or membranous organelle) in the cell
to another. Vesicular transport processes are energized by
ATP (or in some cases another energy-rich compound,
GTP
guanosine triphosphate).
Endocytosis, Transcytosis, and Vesicular Trafficking
Virtu-
ally all forms of vesicular transport involve an assortment of
protein-coated vesicles of three types and, with some excep-
tions, all are mediated by membrane receptors. Before we get
specific about each type of coated vesicular transport, let’s look
at the general scheme of endocytosis.
Whenever a cell uses energy to move solutes across the mem-
brane, the process is referred to as
active
. Substances moved
actively across the plasma membrane are usually unable to pass
in the necessary direction by passive transport processes. ±e
substance may be too large to pass through the channels, inca-
pable of dissolving in the lipid bilayer, or unable to move down
its concentration gradient.
±ere are two major means of active membrane transport:
active transport and vesicular transport.
Active Transport
Like carrier-mediated facilitated diffusion,
active transport
requires carrier proteins that combine
specifically
and
revers-
ibly
with the transported substances. However, facilitated diffu-
sion always follows concentration gradients because its driving
force is kinetic energy. In contrast, active transporters or
solute
pumps
move solutes, most importantly ions, “uphill”
against
a concentration gradient. To do this work, cells must expend
energy.
Active transport processes are distinguished according to
their source of energy:
In
primary active transport
, the energy to do work comes
directly from hydrolysis of ATP
.
In
secondary active transport
, transport is driven indirectly
by
energy stored in ionic gradients
created by primary active trans-
port pumps. Secondary active transport systems are all
coupled
systems
; that is, they move more than one substance at a time.
In a
symport system,
the two transported substances move
in the same direction (
sym
5
same). In an
antiport system
(
anti
5
opposite, against), the transported substances “wave to each
other” as they cross the membrane in opposite directions.
Primary Active Transport
In
primary active transport
, hy-
drolysis of ATP results in the phosphorylation of the transport
protein. ±is step causes the protein to change its shape in such a
manner that it “pumps” the bound solute across the membrane.
Primary active transport systems include calcium and hy-
drogen pumps, but the most investigated example of a pri-
mary active transport system is the
sodium-potassium pump
,
for which the carrier, or “pump,” is an enzyme called
Na
1
-K
1
ATPase
. In the body, the concentration of K
1
inside the cell is
some 10 times higher than that outside, and the reverse is true
of Na
1
. ±ese ionic concentration differences are essential for
excitable cells like muscle and nerve cells to function normally
and for all body cells to maintain their normal fluid volume.
Because Na
1
and K
1
leak slowly but continuously through leak-
age channels in the plasma membrane along their concentra-
tion gradient (and cross more rapidly in stimulated muscle and
nerve cells), the Na
1
-K
1
pump operates almost continuously
as an antiporter. It simultaneously drives Na
1
out of the cell
against a steep concentration gradient and pumps K
1
back in.
Earlier we said that solutes diffuse down their concentration
gradients. ±is is true for uncharged solutes, but only partially
true for ions. ±e negatively and positively charged faces of the
plasma membrane can help or hinder diffusion of ions driven
by a concentration gradient. It is more correct to say that ions
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