Essential Cell Biology 5th edition

Principles of Transmembrane Transport
391
charges on these solutes, and their strong electrical attraction to
water molecules, inhibit their entry into the inner, hydrocarbon
phase of the bilayer. Thus protein-free lipid bilayers are a billion
(10 9 ) times more permeable to water, which is polar but uncharged,
than they are to even small ions such as Na + or K + .
The Ion Concentrations Inside a Cell Are Very Different
from Those Outside
Because lipid bilayers are impermeable to inorganic ions, living cells are
able to maintain internal ion concentrations that are very different from
the concentrations of ions in the medium that surrounds them. These differences
in ion concentration are crucial for a cell’s survival and function.
Among the most important inorganic ions for cells are Na + , K + , Ca 2+ , Cl – ,
and H + (protons). The movement of these ions across cell membranes
plays an essential part in many biological processes, but is perhaps most
striking in the production of ATP by all cells (discussed in Chapter 14) and
in the communication of nerve cells (discussed later in this chapter).
Na + is the most plentiful positively charged ion (cation) outside the cell,
whereas K + is the most abundant inside (Table 12–1). For a cell to avoid
being torn apart by electrical forces, the quantity of positive charge inside
the cell must be balanced by an almost exactly equal quantity of negative
charge, and the same is true for the charge in the surrounding fluid.
The high concentration of Na + outside the cell is electrically balanced
chiefly by extracellular Cl – , whereas the high concentration of K + inside
is balanced by a variety of negatively charged inorganic and organic ions
(anions), including nucleic acids, proteins, and many cell metabolites
(see Table 12–1).
Differences in the Concentration of Inorganic Ions
Across a Cell Membrane Create a Membrane Potential
Although the electrical charges inside and outside the cell are generally
kept in balance, tiny excesses of positive or negative charge, concentrated
in the neighborhood of the plasma membrane, do occur. Such
electrical imbalances generate a voltage difference across the membrane
called the membrane potential.
When a cell is “unstimulated,” the movement of anions and cations
across the membrane will be precisely balanced. In such steady-state
TABLE 12–1 A COMPARISON OF ION CONCENTRATIONS INSIDE AND OUTSIDE A TYPICAL MAMMALIAN CELL
Ion Intracellular Concentration (mM) Extracellular Concentration (mM)
Cations
Na + 5–15 145
K + 140 5
Mg 2+ 0.5* 1–2
Ca 2+ 10 –4 * 1–2
H + 7 × 10 –5 (10 –7.2 M or pH 7.2) 4 × 10 –5 (10 –7.4 M or pH 7.4)
Anions**
Cl – 5–15 110
*The concentrations of Mg 2+ and Ca 2+ given are for the free ions. There is a total of about 20 mM Mg 2+ and 1–2 mM Ca 2+ in cells,
but most of these ions are bound to proteins and other organic molecules and, for Ca 2+ , stored within various organelles.
**In addition to Cl – , a cell contains many other anions not listed in this table. In fact, most cell constituents are negatively charged
(HCO 3 – , PO 4 3– , proteins, nucleic acids, metabolites carrying phosphate and carboxyl groups, and so on).