Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 53

membrane from one side to the other. Because the membrane

is not solid—phospholipids and proteins are free to move

laterally—the proteins within the phospholipid “sea” are not

uniformly distributed. Rather, they present a constantly changing

mosaic pattern, an arrangement known as the fluid-mosaic model

of membrane structure ( fig. 3.2 ).

Scientists now recognize that the fluid-mosaic model of the

plasma membrane is somewhat misleading, in that the mem-

brane is not as uniform in structure as implied by figure  3.2.

The proteins in the plasma membrane can be localized accord-

ing to their function, so that their distribution is patchy rather

than uniform. Thus, proteins in some regions are much more

crowded together in the plasma membrane than is indicated in

figure 3.2. This can be extremely important, as when the mem-

brane proteins serve as receptors for neurotransmitter chemicals

released by nerve fibers at the synapse (chapter 7).

The proteins found in the plasma membrane serve a vari-

ety of functions, including structural support, transport of

molecules across the membrane, and enzymatic control of

The plasma membrane (also called the cell membrane ),

and indeed all of the membranes surrounding organelles within

the cell, are composed primarily of phospholipids and proteins.

Phospholipids, described in chapter 2, are polar (and hydro-

philic) in the region that contains the phosphate group and non-

polar (and hydrophobic) throughout the rest of the molecule.

Since the environment on each side of the membrane is aque-

ous, the hydrophobic parts of the molecules “huddle together”

in the center of the membrane, leaving the polar parts exposed

to water on both surfaces. This results in the formation of a

double layer of phospholipids in the cell membrane.

The hydrophobic middle of the membrane restricts the

passage of water and water-soluble molecules and ions. Cer-

tain of these polar compounds, however, do pass through the

membrane. The specialized functions and selective transport

properties of the membrane are primarily due to its protein

content. Membrane proteins are described as peripheral or

integral. Peripheral proteins are only partially embedded in

one face of the membrane, whereas integral proteins span the

Figure 3.2 The fluid-mosaic model of the plasma membrane. The membrane consists of a double layer of phospholipids,

with the polar regions (shown by spheres) oriented outward and the nonpolar hydrocarbons (wavy tails) oriented toward the center.

Proteins may completely or partially span the membrane. Carbohydrates are attached to the outer surface.

Glycoprotein

Glycolipid

Cholesterol

Intracellular side

Extracellular side

Proteins

Phospholipids

Carbohydrate

Polar

end

Nonpolar

end