BioPHYSICAL chemistry

Membrane potentials

The cell membrane is a lipid bilayer. The composition of cell membranes
varies among cell types and growth conditions, but in general the major
component is phospholipids. Phospholipids are amphipathic molecules;
that is, they have both a polar head group and a hydrophobic fatty acid
portion (Chapter 7). Because of the central hydrophobic core of the bilayer,
the cell membrane is largely impermeable to ions and polar molecules.
In contrast, water traverses membranes much more readily. The perme-
ability of small molecules ranges over several orders of magnitude and
is correlated with the solubility of each molecule in nonpolar solvents
relative to water (Figure 18.1). The cell makes use of this ability to con-
trol the relative concentrations of ions to build ion gradients across the
cell membrane for many metabolic processes.
The cell controls the balance of ions through certain proteins present
in the membrane, called transporters and channels. For example, in

Figure 18.1Permeability of a membrane for different types of molecules.

18 Membrane potentials, transporters, and channels

18 Membrane potentials, transporters, and channels

Na

K

Cl Glucose

Tryptophan

Indole H 2 O

Urea

Glycerol

10 ^1410 ^1210 ^1010 ^810 ^610 ^410 ^2

Increasing permeability

P (cm s^1 )