This calculation ignores the effect of the proton (pH) and other ions that may
be present — mainly sodium ion since the concentrations of calcium and mag-
nesium ions usually are much lower (see Table 5.2 ).
5.3.2 Facilitated Diffusion,
Facilitated diffusion within organisms takes place when carriers or proteins
residing within membranes — ion channels, for instance — organize the move-
ment of ions from one location to another. This diffusion type is a kinetic, not
thermodynamic, effect in which EA for the transfer is lowered and the rate of
diffusion is accelerated. Facilitated diffusion channels organize ion movements
in both directions, and the process can be inhibited both competitively and
noncompetitively. It is known that most cells maintain open channels for K +
most of the time and closed channels for other ions. Potassium - ion - dependent
enzymes include Na + /K + ATPases (to be discussed in Section 5.4.1 ), pyruvate
kinases, and dioldehydratases (not to be discussed further).
5.3.2.1 Gated Channels. The usual situation for gated channels is that the
gate has a “ receptor ” species for an “ effector ” species. The receptor binds in
the channel, generating a binding energy that causes a conformation change
in the effector to open the gate. The gate opens, ions enter, and a change in
concentration ( C ) or electrolytic potential ( ψ ) sends the desired message.
Voltage - gated channels that open or close by voltage changes, as well as chemi-
cally gated channels that open or close by concentration changes, exist. The
proteins forming gates are usually helical bundles and therefore have primary
sequences that favorα - helix formation. Membranes containing the channels
have shapes controlled by fi lamentous networks or structures within the cell.
Two important types of membranes for our discussions are (1) cytoplasmic
membranes and (2) vesicle or organelle membranes for entities within the
cell.
5.3.3 Active Transport — Ion Pumps
The action of so - called active transport, also known as ion pumps, facilitates
larger Na + /K + gradients than those possible considering calculations of Donnan
equilibria. For instance, the concentration of K + in red blood cells equals
92 mM versus 10 mM in blood plasma. Calculation of the membrane potential
using equation 5.11 would lead to a large negative potential:
TABLE 5.2 Concentration of Some Ions in Living Cells (mM)
System Na + K + Ca 2+ Mg 2+ Cl − HPO 42 −
Red blood cells 11 92 10 − 4 2.5 50 3
Blood plasma 160 10 2 2 100 ∼ 3
Source : Reference 2.
MOVEMENT OF MOLECULES AND IONS ACROSS MEMBRANES 197
