Page 21 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 3

Membrane proteins purification. These proteins are bound to fatty acids or
VetBooks.ir The proteins of the cell membrane are incorporated into integral membrane proteins.
the lipid bilayer, where they perform membrane-specific
functions (Figures 1–1 and 1–2). Membrane proteins Membrane polysaccharides
include transport proteins, enzymes and specific receptor The outer surface of the plasmalemma is associated with a
proteins. These are globular and, like the membrane lipids, sugar residue coat of varying thickness (glycocalyx; cell coat,
amphipathic. Membrane proteins similarly exhibit hydro- surface coat). The glycocalyx consists of oligosaccharide
phobic inwardly oriented regions and hydrophilic moieties chains that are joined by covalent bonds with membrane
that project from one side or, more commonly, both sides proteins (forming glycoproteins) or, to a lesser extent, with
of the membrane. lipids (forming glycolipids) (Figure 1.1). All membrane pro-
Several types of membrane proteins are recognised. teins, and about 10% of lipids, are associated with sugar
These are named according to their functions: residues. The accumulation of carbohydrates on the outer
surface contributes to the asymmetry of the plasmalemma.
2+
· pumps: e.g. for amino acids, sugars, Na , Ca , The structure of the oligosaccharide side chains is
+
· channel proteins: passive diffusion of ions or mol- extremely complex. It comprises relatively few sugars (includ-
ecules through the membrane, ing glucose, fucose, mannose, glucosamine, galactose, sialic
· receptor proteins: cell recognition and binding of acids) that are interconnected in various ways. Sialic acid
ligands (e.g. hormones, antibody reactions), (N-acetyl neuraminic acid) imparts a negative charge to the
· linking proteins: binding with the intracellular cyto- cell surface. The glycocalyx, together with the plasmalemma,
skeleton (e.g. actin filaments) or the extracellular is constantly renewed by the cell. This process involves the
matrix (e.g. fibronectins), delivery of sugars to the cell membrane by transport vesi-
· enzymes: e.g. ATPase associated with the inner cles produced in the Golgi apparatus. In this way, the cell
membrane of mitochondria, disaccharidases and membrane contributes to the specificity of the cell surface.
dipeptidases involved in digestion and The glycocalyx performs various functions. It forms
· structural proteins: establish connections to the a large number of receptors that receive information
plasmalemma of neighbouring cells (e.g. between directly from other cells, or indirectly through messenger
epithelial cells). molecules (chemical signals such as hormones) and trans-
mit the information to the cell. The glycocalyx contributes
According to their location with respect to the lipid bilayer, to recognition of endogenous or foreign cells and to the
a distinction is made between: initiation of immune responses. Blood groups are deter-
mined by glycolipids in the glycocalyx of erythrocytes. The
· integral membrane proteins and majority of processes occurring during cellular differentia-
· peripheral membrane proteins. tion are also mediated by the glycocalyx.

Plasmalemma
INTEGRAL MEMBRANE PROTEINS The plasmalemma carries receptors for chemical signals
Integral proteins span the entire membrane. They are dif- from the extracellular compartment (e.g. hormones, neu-
ficult to extract, and project beyond the cell membrane rotransmitters) and serves as a selective filter for regulation
on both sides (transmembrane proteins, intramembra- of intra- and extracellular electrolyte concentrations. In
nous proteins and channel proteins). Their hydrophobic addition, the plasmalemma contributes to the uptake of
segments are bound to one or more fatty acid chains, thus nutrients and release of cellular products.
anchoring them within the lipid bilayer. Through the expression of specific molecules, the
Integral membrane proteins can move laterally within plasmalemma mediates a large number of temporary
the membrane (membrane fluidity). Lateral movement associations among cells, contributes to adhesion between
of membrane proteins can be inhibited by certain struc- similar and unlike cells and participates in cell recognition.
tures including cytoskeletal components (actin filaments, Direct connections between cells play an important role
intermediate filaments, microtubules), physical connec- in growth and differentiation events, both during embryo-
tions between cells (gap junctions, tight junctions) and genesis and in the adult animal. Cell adhesion molecules
lipid rafts. (CAM), which first appear on the plasmalemma during
embryonic development, are fundamental in the forma-
PERIPHERAL MEMBRANE PROTEINS tion of specialised intercellular junctions. Cell recognition
Peripheral proteins penetrate the inner or outer lamina is the critical first step in all immune responses; the surface
of the cell membrane to a variable extent and are eas- molecules that develop for this purpose are components of
ier to extract from the aqueous fraction during protein the major histocompatibility complex (MHC).

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