48
Chapter 3
functions are related to these asymmetries in chemical com-
position between the two surfaces of a membrane.
All membranes have the general structure just described,
which is known as the
uid-mosaic model
because membrane
proteins fl oat in a sea of lipid (
Figure 3–9
). However, the
proteins and, to a lesser extent, the lipids (the distribution of
cholesterol, for example) in the plasma membrane differ from
those in organelle membranes. Thus, the special functions of
membranes, which depend primarily on the membrane pro-
teins, may differ in the various membrane-bound organelles
and in the plasma membranes of different types of cells.
Membrane Junctions
In addition to providing a barrier to the movements of mole-
cules between the intracellular and extracellular fl uids, plasma
membranes are involved in the interactions between cells to
form tissues. Most cells are packaged into tissues and are not
free to move around the body. But even in tissues, there is usu-
ally a space between the plasma membranes of adjacent cells.
This space, fi lled with extracellular (interstitial) fl
uid, provides
a pathway for substances to pass between cells on their way to
and from the blood.
The forces that organize cells into tissues and organs are
poorly understood, but they depend, at least in part, on the
ability of certain transmembrane proteins in the plasma mem-
brane, known as
integrins,
to bind to specifi c proteins in the
extracellular matrix and link them to membrane proteins on
adjacent cells.
Many cells are physically joined at discrete locations along
their membranes by specialized types of junctions, including
desmosomes, tight junctions, and gap junctions.
Desmosomes
(
Figure 3–10a
) consist of a region between two adjacent cells
where the apposed plasma membranes are separated by about
20 nm. Desmosomes are characterized by accumulations of pro-
tein known as dense plaques along the cytoplasmic surface of the
plasma membrane. These proteins serve as anchoring points for
cadherins.
Cadherins
are proteins that extend from the cell into
the extracellular space, where they link up and bind with cad-
herins from an adjacent cell. In this way, two adjacent cells can
be fi rmly attached to each other. The presence of numerous des-
mosomes between cells helps to provide the structural integrity
of tissues in the body. In addition, other proteins such as kera-
tin fi laments anchor the cytoplasmic surface of desmosomes to
interior structures of the cell. It is believed that this helps secure
the desmosome in place and also provides structural support for
the cell. Desmosomes hold adjacent cells fi rmly together in areas
that are subject to considerable stretching, such as the skin. The
specialized area of the membrane in the region of a desmosome
is usually disk-shaped; these membrane junctions could be lik-
ened to rivets or spot-welds.
A second type of membrane junction, the
tight junc-
tion
(
Figure 3–10b
), forms when the extracellular surfaces
of two adjacent plasma membranes join together so that no
extracellular space remains between them. Unlike the desmo-
some, which is limited to a disk-shaped area of the membrane,
the tight junction occurs in a band around the entire circum-
ference of the cell.
Most epithelial cells are joined by tight junctions. For
example, epithelial cells cover the inner surface of the intes-
tinal tract, where they come in contact with the digestion
products in the cavity (lumen) of the tract. During absorp-
tion, the products of digestion move across the epithelium
and enter the blood. This movement could theoretically take
place either through the extracellular space between the epi-
thelial cells or through the epithelial cells themselves. For
many substances, however, movement through the extra-
cellular space is blocked by the tight junctions; this forces
organic nutrients to pass through the cells, rather than
between them. In this way, the selective barrier properties of
the plasma membrane can control the types and amounts of
substances absorbed. The ability of tight junctions to impede
molecular movement between cells is not absolute. Ions
and water can move through these junctions with varying
degrees of ease in different epithelia.
Figure 3–10c
shows
both a tight junction and a desmosome near the luminal
border between two epithelial cells.
A third type of junction, the
gap junction,
consists
of protein channels linking the cytosols of adjacent cells
(
Figure 3–10d
). In the region of the gap junction, the two
opposing plasma membranes come within 2 to 4 nm of each
other, which allows specifi c proteins (called
connexins
) from
the two membranes to join, forming small, protein-lined
channels linking the two cells. The small diameter of these
channels (about 1.5 nm) limits what can pass between the
cytosols of the connected cells to small molecules and ions,
such as sodium and potassium, and excludes the exchange of
large proteins. A variety of cell types possess gap junctions,
including the muscle cells of the heart, where they play a
very important role in the transmission of electrical activity
between the cells.
Cell Organelles
The contents of cells can be released by grinding a tissue against
rotating glass or metal surfaces (homogenization) or using var-
ious chemical methods to break the plasma membrane. The
cell organelles thus released can then be isolated by subjecting
Phospholipid
bilayer
Proteins
Figure 3–9
Fluid-mosaic model of cell membrane structure. Only proteins and
phospholipids are shown; other membrane components are omitted
for clarity. The proteins may move within the bilayer.
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