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

606 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes
ions
or K + or Na + or Ca ++
small molecule
H + H + H +
H + H +
lipid
bilayer
CYTOSOL
P
ADP
ATP
ATP
ADP
+
P i
ATP
ADP +
P i
ATP
ADP +
P i
P i +
ADP
ATP
P-type pump
ABC transporter
V-type proton pump
F-type ATP synthase
There Are Three Classes of ATP-Driven Pumps
ATP-driven pumps are often called transport ATPases because they hydrolyze ATP
to ADP and phosphate and use the energy released to pump MBoC6 ions n11.100/11.12
or other solutes
across a membrane. There are three principal classes of ATP-driven pumps (Figure
11–12), and representatives of each are found in all prokaryotic and eukaryotic
cells.
1. P-type pumps are structurally and functionally related multipass transmembrane
proteins. They are called “P-type” because they phosphorylate
themselves during the pumping cycle. This class includes many of the ion
pumps that are responsible for setting up and maintaining gradients of
Na + , K + , H + , and Ca 2+ across cell membranes.
2. ABC transporters (ATP-Binding Cassette transporters) differ structurally
from P-type ATPases and primarily pump small molecules across cell
membranes.
3. V-type pumps are turbine-like protein machines, constructed from multiple
different subunits. The V-type proton pump transfers H + into organelles
such as lysosomes, synaptic vesicles, and plant or yeast vacuoles (V = vacuolar),
to acidify the interior of these organelles (see Figure 13–37).
Structurally related to the V-type pumps is a distinct family of F-type ATPases,
more commonly called ATP synthases because they normally work in reverse:
instead of using ATP hydrolysis to drive H + transport, they use the H + gradient
across the membrane to drive the synthesis of ATP from ADP and phosphate (see
Figure 14–30). ATP synthases are found in the plasma membrane of bacteria, the
inner membrane of mitochondria, and the thylakoid membrane of chloroplasts.
The H + gradient is generated either during the electron-transport steps of oxidative
phosphorylation (in aerobic bacteria and mitochondria), during photosynthesis
(in chloroplasts), or by the light-driven H + pump (bacteriorhodopsin) in
Halobacterium. We discuss some of these proteins in detail in Chapter 14.
For the remainder of this section, we focus on P-type pumps and ABC transporters.
Figure 11–12 Three types of ATP-driven
pumps. Like any enzyme, all ATP-driven
pumps can work in either direction,
depending on the electrochemical
gradients of their solutes and the ATP/ADP
ratio. When the ATP/ADP ratio is high, they
hydrolyze ATP; when the ATP/ADP ratio is
low, they can synthesize ATP. The F-type
ATPase in mitochondria normally works in
this “reverse” mode to make most of the
cell’s ATP.
A P-type ATPase Pumps Ca 2+ into the Sarcoplasmic Reticulum
in Muscle Cells
Eukaryotic cells maintain very low concentrations of free Ca 2+ in their cytosol
(~10 –7 M) in the face of a very much higher extracellular Ca 2+ concentration (~10 –3
M). Therefore, even a small influx of Ca 2+ significantly increases the concentration
of free Ca 2+ in the cytosol, and the flow of Ca 2+ down its steep concentration
gradient in response to extracellular signals is one means of transmitting these
signals rapidly across the plasma membrane (discussed in Chapter 15). It is thus