Chemistry Reference
In-Depth Information
Chapter 9
Sodium and Potassium
e
Channels
and Pumps
Introduction e Transport Across Membranes
177
Sodium versus Potassium
178
Potassium Channels
180
Sodium Channels
184
The SodiumePotassium ATPase
184
Active Transport Driven by Na
D
Gradients
187
Sodium/Proton Exchangers
190
Other Roles of Intracellular K
D
191
INTRODUCTION e TRANSPORT ACROSS MEMBRANES
Before examining the important roles of the alkali metals sodium and potassium, we should briefly review how ions
are transported across membranes. As we pointed out in Chapter 3, the phospholipid bilayer of biological
membranes is essentially impermeable to polar molecules and to ions
the permeabilities of Na
þ
and K
þ
are of the
e
order of 10
12
cm/sec
the corresponding value for H
2
Oisaround10
2
. So, simple diffusion would not suffice to
explain the msec transmission of nerve impulses. Transport across membranes is conferred by two classes of
membrane proteins, namely channels and pumps. Channels allow ions to flow down a concentration gradient by
a process known as passive transport or facilitated diffusion. Of course, channels cannot remain open all of the time,
and so they are usually gated, which simply means that, like regular garden gates, they usually remain shut, and can
only be opened, either by the binding of a ligand (ligand-gated) or by changes in the membrane potential (voltage-
gated). Ligand-gated channels, like the acetylcholine receptors in post-synaptic membranes, are opened by the
binding of the neurotransmitter acetylcholine, whereas the voltage-gated sodium and potassium channels, which
mediate the action potentials in neuronal axons described below, are opened by membrane depolarisation.
In contrast, pumps use energy, in the form of ATP or light, to drive the unfavourable uphill transport of ions or
molecules against a concentration gradient; in other words, they are involved in active transport. There are two
types of ATP-driven pumps, so-called P-type ATPases and ABC (ATP-binding cassette) transporters, both of
which use conformational changes induced by ATP binding and its subsequent hydrolysis to transport ions across
the membrane. The (Na
þ
-K
þ
)-ATPase described below is one of the P-type ATPases, which achieves uphill
exchange of cytoplasmic Na
þ
ions for extracellular K
þ
ions using ATP-mediated phosphorylation, followed by
autodephosphorylation, to drive conformational changes that allow access to the binding sites of the pump from
only one side of the membrane at a time. Another mechanism of active transport, which uses the electrochemical
gradient of one ion to drive the counter-transport of another, will be illustrated by the Na
þ
/H
þ
exchanger, crucial
among other things for the control of intracellular pH. Yet another example, discussed in Chapter 11, is the
Na
þ
/Ca
2
þ
exchanger, which plays an important role in removing Ca
2
þ
from cells.
e
