Human Physiology, 14th edition (2016)

150 Chapter 6

heartbeat, and the generation of nerve impulses. To understand

these processes, we must first examine the electrical properties

of cells.

Equilibrium Potentials

There are many inorganic ions in the intracellular and extra-

cellular fluid that are maintained at specific concentrations.

The extent to which each ion contributes to the potential

difference across the plasma membrane—or membrane

potential —depends on (1) its concentration gradient, and

(2) its membrane permeability. Because the plasma mem-

brane is usually more permeable to K^1 than to any other

ion, the membrane potential is usually determined primarily

by the K^1 concentration gradient.

Thus, we can ask a hypothetical question: What would

be the voltage of the membrane potential if the membrane

were permeable only to K^1? In that hypothetical case, K^1

would distribute itself as shown in figure  6.25. The fixed

anions would cause the intracellular K^1 concentration to

become higher than the extracellular concentration. However,

once the concentration gradient (ratio of concentrations out-

side and inside the cell) reached a particular value, net move-

ment of K^1 would cease. If more K^1 entered the cell because

of electrical attraction, the same amount would leave the

cell by net diffusion. Thus, a state of equilibrium would be

Cellular proteins and the phosphate groups of ATP and
other organic molecules are negatively charged at the pH of the
cell cytoplasm. These negative ions ( anions ) are “fixed” within
the cell because they cannot penetrate the plasma membrane.
As a result, these anions attract positively charged inorganic
ions ( cations ) from the extracellular fluid that can pass through
ion channels in the plasma membrane. In this way, fixed anions
within the cell influence the distribution of inorganic cations
(mainly K^1 ,  Na^1 , and Ca^2 1 ) between the extracellular and
intracellular compartments.
Because the plasma membrane is more permeable to
K^1 than to any other cation, K^1 accumulates within the cell
more than the others as a result of its electrical attraction
for the fixed anions ( fig.  6.24 ). So, instead of being evenly
distributed between the intracellular and extracellular com-
partments, K^1 becomes more highly concentrated within the
cell. The intracellular K^1 concentration is 150 mEq/L in the
human body compared to an extracellular concentration of
5 mEq/L (mEq  5 milliequivalents, which is the millimolar
concentration multiplied by the valence of the ion—in this
case, by one).
As a result of the unequal distribution of charges between
the inside and outside of cells, each cell acts as a tiny bat-
tery with the positive pole outside the plasma membrane and
the negative pole inside. The magnitude of this difference
in charge, or potential difference, is measured in voltage.
Although the voltage of this battery is very small (less than
a tenth of a volt), it is of critical importance in such physi-
ological processes as muscle contraction, the regulation of the

Figure 6.24 The effect of fixed anions on the
distribution of cations. Proteins, organic phosphates, and
other organic anions that cannot leave the cell create a fixed
negative charge on the inside of the membrane. This negative
charge attracts positively charged inorganic ions (cations),
which therefore accumulate within the cell at a higher
concentration than is found in the extracellular fluid. The amount
of cations that accumulates within the cell is limited by the
fact that a concentration gradient builds up, which favors the
diffusion of the cations out of the cell.

+ + +

+ + +

+ + + + + +

Concentration gradient

Plasma membrane

Fixed anions

Electrical attraction

Figure 6.25 Potassium equilibrium potential. If K^1

were the only ion able to diffuse through the plasma membrane,

it would distribute itself between the intracellular and extracellular

compartments until an equilibrium was established. At

equilibrium, the K^1 concentration within the cell would be higher

than outside the cell because of the attraction of K^1 for the fixed

anions. The intracellular and extracellular K^1 concentrations are

normal when the inside of the cell is 2 90 millivolts compared to

the outside of the cell. This membrane potential is the equilibrium

potential ( E (^) K ) for potassium.

Diffusion

K+ K+ K+ K+ K+

K+

• 90 mV

Electrical

attraction

Extracellular

Intracellular electrode

electrode

K+

Voltmeter

Fixed anions