Page 67 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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100 Ion Transport Across Biological Membranes

of specific inorganic ions across the membrane. Invariably,
this transport of inorganic ions across the cell membrane
is accompanied by changes in the transmembrane volt-
age. The equilibrium transmembrane potential for a spe-
cific inorganic ion, for instance K , is given by the Nernst
+
equation:
+
RT [K ] o
E K = ln . (1)
+
ZF [K ] i
+
R, T, and F are the molar gas constant, absolute tempera-
ture, and Faraday constant, respectively. Z represents the
FIGURE 1 An environmental stimulus, for instance, light, acti-
valence of the inorganic ion and the subscripts o and i
vates a protein-mediated reaction in the eye, leading to the trans-
indicate whether the ion is outside or inside the cell mem-
membrane flux of inorganic ions, a change in the transmembrane
voltage, and neurotransmitter release. The neurotransmitter dif- brane. The transmembrane potential of neurons is around
fuses across a junction between the nerve terminal of the axon −60 mV. In the nervous system, changes in the transmem-
and the cell body of the adjacent cell about 20–40 nm in length, brane potential due to a change in the flux rate of inorganic
called synapse. The neurotransmitter binds to receptors in the ions can be propagated rapidly and over distances as long
membrane of the postsynaptic cell. Excitatory neurotransmitters
( ) activate receptors that form cation-specific transmembrane as several feet via the axon, a long projection of many
channels. Inhibitory neurotransmitters (•) activate receptors that nerve cells (Fig. 1). At the axonal terminal, the voltage
form anion-specific transmembrane channels. Once the trans- change initiates a process leading to the flux of calcium
membrane voltage of the cell is changed by a critical amplitude ions into the nerve terminal. This results in the secre-
and sign (by ∼+20 mV), an all-or-none process occurs. Trans- tion of chemical signals, neurotransmitters, which bind to
+
membrane Na and K channels in the axonal membrane open
+
transiently, resulting in an electrical signal that travels down the membrane-bound proteins, neurotransmitter receptors, on
axon and neurotransmitter is again secreted. This process re- adjacent cells. Upon binding specific neurotransmitters,
peats itself and is terminated when the neurotransmitter is re- the receptors transiently open transmembrane channels.
leased adjacent to receptors on the surface of muscle cells. In the The channels are permeable to Na ,K ,orCl , depend-
+
−
+
case of muscle cells, the receptor is the muscle nicotinic acetyl- ing on the receptor. The resulting changes in the trans-
choline receptor and the neurotransmitter acetylcholine. The volt-
age change in the muscle cell membrane initiates muscle con- membrane voltage may lead to propagation of a signal to
traction. (From Hess, G. P., and Grewer, C. (1998). “Methods in an adjacent cell. Thus, this interplay between chemical
Enzymology” (G. Marriott, ed.), Vol. 291, pp. 443–474, Academic reactions and transmembrane voltage changes plays a de-
Press, New York.) The resulting flow of inorganic ions through the cisive role in the rapid communication between nerve (and
membrane of the muscle cell results in a change of its transmem-
nerve and muscle) cells and in nervous system function.
brane voltage V m and muscle contraction.
In 1890, Max Planck derived the relationship between
the rate of movement of inorganic cations and anions
(3) the binding of specific ligands to a channel-forming across a porous barrier and the resulting electric field.
protein. (4) Some proteins use the energy liberated by If one assumes a constant electric field and constant in-
the hydrolysis of ATP to transport inorganic ions against a organic ion concentration, the Planck equation is easily
concentration gradient. Only one example of each of these integrated and can be used to estimate the transmembrane
various proteins will be mentioned. In each case, the pro- voltage change, V m , that results from the flow of inor-
tein chosen is the one about which we have the most infor- ganic ions across cell membranes. The resulting Goldman
mation. The proteins that facilitate inorganic ion transport equation is
across biological membranes are discussed in an order that
+
+
RT P K (K ) o + P Na (Na ) o + P Cl [Cl ] i
−
illustrates their function in the life of an organism. V m = ln . (2)
F P K (K ) i + P Na (Na ) i + P Cl [Cl ] o
+
−
+
P K ,P Na , and P Cl represent the permeability coefficient of
−
I. RELATIONSHIP BETWEEN the membrane for K ,Na , and Cl , respectively. [K ],
+
+
+
−
TRANSMEMBRANE INORGANIC ION [Na ], [Cl ] represent the molar concentrations of the
−
+
FLUX AND TRANSMEMBRANE ions, and the subscript o or i indicates whether the ions are
POTENTIAL outside or inside the cell membrane. As usual, R, T, and F
represent the molar gas constant, the absolute temperature,
Membranes surround the individual cells of animals and and the Faraday constant respectively.
organelles within the cell. They are composed of lipids and How is the rate of ion movement through a protein-
proteins. Specific proteins are responsible for the transport formed channel across a cell membrane related to the