Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 2

Overview of Cellular Physiology in Medical Physiology 33

transporting ions across the membrane. Other proteins func-
tion as
carriers,
transporting substances down electrochemical

gradients by facilitated diffusion. Still others are

ion channels,

which, when activated, permit the passage of ions into or out of

the cell. The role of the pumps, carriers, and ion channels in

transport across the cell membrane is discussed below. Proteins

in another group function as

receptors

that bind

ligands

or

messenger molecules, initiating physiologic changes inside the

cell. Proteins also function as

enzymes,

catalyzing reactions at

the surfaces of the membrane. Examples from each of these

groups are discussed later in this chapter.

The uncharged, hydrophobic portions of the proteins are

usually located in the interior of the membrane, whereas the

charged, hydrophilic portions are located on the surfaces.

Peripheral proteins are attached to the surfaces of the mem-

brane in various ways. One common way is attachment to gly-

cosylated forms of phosphatidylinositol. Proteins held by

these

glycosylphosphatidylinositol (GPI) anchors

(Figure

2–3) include enzymes such as alkaline phosphatase, various

antigens, a number of cell adhesion molecules, and three pro-

teins that combat cell lysis by complement. Over 45 GPI-

linked cell surface proteins have now been described in

humans. Other proteins are

lipidated,

that is, they have spe-

cific lipids attached to them (Figure 2–3). Proteins may be

myristolated, palmitoylated,

or

prenylated

(ie, attached to

geranylgeranyl or farnesyl groups).

The protein structure—and particularly the enzyme con-

tent—of biologic membranes varies not only from cell to cell,

but also within the same cell. For example, some of the enzymes

embedded in cell membranes are different from those in mito-

chondrial membranes. In epithelial cells, the enzymes in the

cell membrane on the mucosal surface differ from those in the

FIGURE 2–2
Organization of the phospholipid bilayer and
associated proteins in a biological membrane.
The phospholipid
molecules each have two fatty acid chains (wavy lines) attached to a phos-
phate head (open circle). Proteins are shown as irregular colored globules.
Many are integral proteins, which extend into the membrane, but periph-
eral proteins are attached to the inside or outside (not shown) of the
membrane. Specific protein attachments and cholesterol commonly
found in the bilayer are omitted for clarity.
(Reproduced with permission from
Widmaier EP, Raff H, Strang K:
Vander’s Human Physiology: The Mechanisms of Body
Function
, 11th ed. McGraw-Hill, 2008.)

Extracellular fluid

Intracellular fluid

Carbohydrate

portion of

glycoprotein

Intregral

proteins

Polar regions

Nonpolar regions

Peripheral

protein

Transmembraneproteins Phospholipids

Channel

FIGURE 2–3
Protein linkages to membrane lipids.
Some are linked by their amino terminals, others by their carboxyl terminals. Many are at-
tached via glycosylated forms of phosphatidylinositol (GPI anchors).
(Reproduced with permission from Fuller GM, Shields D:
Molecular Basis of Medical Cell Biology.
McGraw-Hill, 1998.)

N

H

O

Lipid membrane Cytoplasmic or external face of membrane

Gly Protein

Protein

Protein

Protein

Protein

N-Myristoyl COOH

S-Cys

O

O

O

CCH 2

CH

C

H 2

OOP Inositol OC

O O

O

C

NH 2

S-Palmitoyl

S-Cys NH 2

Geranylgeranyl

S-Cys NH 2

CC

Farnesyl

GPI anchor

(Glycosylphosphatidylinositol)

Hydrophobic domain Hydrophilic domain