Essential Cell Biology 5th edition

Membrane Proteins
383
apical plasma
membrane
lateral plasma
membrane
basal plasma
membrane
protein A
tight
junction
protein B
Figure 11–32 Membrane proteins are
restricted to particular domains of the
plasma membrane of epithelial cells in the
gut. Protein A (green) and protein B (red)
can diffuse laterally in their own membrane
domains but are prevented from entering the
other domain by a specialized cell junction
called a tight junction. The basal lamina
(yellow ) is a mat of extracellular matrix that
supports all epithelial sheets (discussed in
Chapter 20).
basal lamina
the carbohydrate on the glycoproteins, proteoglycans, and glycolipids is
located on the outside of the plasma
ECB5 e11.32/11.32
membrane, where it forms a sugar
coating called the carbohydrate layer or glycocalyx (Figure 11–33).
This layer of carbohydrate helps protect the cell surface from mechanical
damage. And because the oligosaccharides and polysaccharides attract
water molecules, they also give the cell a slimy surface, which helps
motile cells such as white blood cells squeeze through narrow spaces
and prevents blood cells from sticking to one another or to the walls of
blood vessels.
Cell-surface carbohydrates do more than just protect and lubricate the
cell, however. They have an important role in cell–cell recognition and
adhesion. Transmembrane proteins called lectins are specialized to bind
to particular oligosaccharide side chains. The oligosaccharide side chains
of glycoproteins and glycolipids, although short (typically fewer than 15
sugar units), are enormously diverse. Unlike proteins, in which the amino
acids are all joined together in a linear chain by identical peptide bonds,
sugars can be joined together in many different arrangements, often
forming elaborate branched structures (see Panel 2–4, pp. 72–73). Using
a variety of covalent linkages, even three different sugars can form hundreds
of different trisaccharides.
The carbohydrate layer on the surface of cells in a multicellular organism
serves as a kind of distinctive clothing, like a police officer’s uniform. It is
characteristic of each cell type and is recognized by other cell types that
= sugar unit
transmembrane
glycoprotein
adsorbed
glycoprotein
transmembrane
proteoglycan
carbohydraterich
layer
lipid
bilayer
glycolipid
EXTRA-
CELLULAR
SPACE
CYTOSOL
Figure 11–33 Eukaryotic cells are coated
with sugars. This carbohydrate-rich layer
is made of the oligosaccharide side chains
attached to membrane glycolipids and
glycoproteins, and of the polysaccharide
chains on membrane proteoglycans. As
shown, glycoproteins that have been
secreted by the cell and then adsorbed
back onto its surface can also contribute.
Note that all the carbohydrate is on the
external (noncytosolic) surface of the
plasma membrane.