Ganong's Review of Medical Physiology, 23rd Edition

Extracellular fluid
Carbohydrate
portion of
glycoprotein
Channel
Intregral
proteins
Nonpolar regions
Transmembrane
proteins
Polar regions
Peripheral
protein
Intracellular fluid
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 phosphate
head (open circle). Proteins are shown as irregular colored globules.
Many are integral proteins, which extend into the membrane, but peripheral
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.)
transporting ions across the membrane. Other proteins function
as carriers, transporting substances down electrochemical
CHAPTER 2 Overview of Cellular Physiology in Medical Physiology 33
Phospholipids
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 membrane
in various ways. One common way is attachment to glycosylated
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 proteins
that combat cell lysis by complement. Over 45 GPIlinked
cell surface proteins have now been described in
humans. Other proteins are lipidated, that is, they have specific
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 content—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 mitochondrial
membranes. In epithelial cells, the enzymes in the
cell membrane on the mucosal surface differ from those in the
FIGURE 2–3 Protein linkages to membrane lipids. Some are linked by their amino terminals, others by their carboxyl terminals. Many are attached
via glycosylated forms of phosphatidylinositol (GPI anchors). (Reproduced with permission from Fuller GM, Shields D: Molecular Basis of Medical Cell Biology.
McGraw-Hill, 1998.)
Lipid membrane Cytoplasmic or external face of membrane
O
N-Myristoyl
N
H
Gly Protein COOH
S-Palmitoyl
Geranylgeranyl
Farnesyl
GPI anchor
C
C
(Glycosylphosphatidylinositol)
O
S-Cys
Protein
S-Cys Protein NH2 S-Cys Protein NH2 O
C CH 2
C
O
CH
C
H 2
O O
O P O Inositol O C
O
NH 2
Hydrophobic domain Hydrophilic domain
Protein