The membrane potential

A potential difference exists across plasma membranes
that makes the inside of the membrane electrically
negative and the outside positive. The magnitude
of this membrane potential is infl uenced by
the composition of the fl uid bathing the membrane.
When mucosal strips of rabbit ileum are bathed by a
physiological electrolyte solution, the membrane potential
across the brush border averages –36 mV (cell
interior negative) (Rose & Schultz, 1971). We will see
that the membrane potential plays an essential role in
the transport of substances across the membrane.
Figure 3.7 shows very simply how a membrane
potential is established. The membrane separating
the two aqueous compartments is permeable to K+
but not to anions. The K+ concentration of the intracellular
compartment is 10-fold that of the extracellular
compartment. Potassium ions tend to diffuse by
random motion in both directions across the membrane.
However, because the K+ concentration is initially
higher in the intracellular compartment, there
is greater net movement of K+ from inside to outside.
This diffusion of K+ down the concentration gradient,
unaccompanied by anions, produces separation of
charge and therefore the membrane becomes electrically
charged, with the inside negative with respect to
the outside. This build-up of electronegativity along
the inside of the membrane will tend to retard the
outward diffusion of K+. The membrane potential
is the point at which the electric potential across the
membrane becomes great enough to prevent further
net diffusion of K+ to the exterior, despite the high K+
concentration gradient.
In an actual plasma membrane, the membrane
potential is established in a similar manner by an interaction
between permanently open potassium leak
channels and a sodium-potassium pump located in
the membrane. As described later, K+ is pumped into
the cell in exchange for the outward removal of Na+,
creating the high intracellular concentration of K+.
The net outward diffusion of K+ through the potassium
leak channels down the concentration gradient
generates the inside-negative membrane potential
aided by a small contribution by the sodium-potassium
pump. Membranes are not in fact solely permeable
to K+; they are also permeable to Cl–, and are not
totally impermeable to Na+.
The separation of electric charges that creates the
membrane potential occurs only in the immediate
INTRACELLULAR EXTRACELLULAR
100 mM K+ 10 mM K+
K+
K+
–
–
–
–
+
+
+
+
anion
K+-selective membrane
anion
Fig. 3.7 Diagram showing the principle of how a membrane
potential is established. A membrane separates two compartments.
The left (intracellular) compartment contains 100 mM K+ and the right
(extracellular) compartment contains 10 mM K+. The membrane is freely
permeable to K+ but not to the accompanying anion. Net K+ movement
from left to right creates a membrane voltage, left side negative. This buildup
of electronegativity will tend to retard the movement of K+ from left to
right until further net movement is prevented. The membrane voltage at
this point defi nes the membrane potential.
20 Vitamins: their role in the human body
vicinity of the membrane, therefore the bulk of the
cytosol and extracellular fl uid remains electrically
neutral. The charge separation involves only a minute
fraction of the total number of positive and negative
charges that exist in the cell, so ion concentrations are
practically unaffected.
3.1.4 Protein-mediated membrane
transport systems
The term ‘transport’ refers to solute translocation
that is mediated by a transmembrane protein (transporter).
Most transporters are multisubunit protein
complexes made up of identical or structurally similar
polypeptides held together noncovalently. Transporters
exert their effect through a change in their threedimensional
shape (conformational change), and it
is this change that limits the rate of transport. Each
transporter is responsible for the translocation of a
specifi c type of molecule, or a group of closely related
molecules. Specifi city is imparted by the tertiary and
quaternary structures of the transporter molecule
– only if a solute’s spatial confi guration fi ts into the
protein will the solute be transferred across the membrane.
The rapidity of protein-mediated transport
is due to the fact that the transported molecules are
prevented from entering the hydrophobic core of
the membrane bilayer, and are therefore not slowed
down.
Transporters fall into two main classes: carriers and
ion channels. Ion pumps are a type of carrier protein
that is also an enzyme.
Dozens of different transport proteins have been
identifi ed. Some of these proteins, called uniporters,
transport a single substance from one side of a
membrane to the opposite side. Others couple the
movement of two substances to one another. When
two coupled substances are moved in the same direction,
the transport protein is called a symporter; if the
two substances are moved in opposite directions, the
protein is termed an antiporter. While channel proteins
are always uniporters, carrier proteins can be
uniporters, symporters or antiporters.
The movement of ions across the plasma membrane
is also mediated by transport proteins. Symporters
and certain antiporters co-transport ions together
with specifi c small molecules, whereas ion channels,
ion pumps and certain antiporters transport only
ions. In all cases, the rate and extent of ion transport
across membranes is infl uenced not only by the ion
concentration gradient, but also by the membrane
potential. The combination of the ion concentration
gradient and the membrane potential is referred to as
the electrochemical gradient.