Page 23 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

The cell (cellula) 5

are incorporated into cellular metabolic processes. Oxygen (e.g. in neurons), neurotransmitters (e.g. acetylcholine
VetBooks.ir also passes into the cell. receptors on muscle cells) or mechanical forces (e.g. in the
These mechanisms are subject to hormonal and neu-
inner ear).
ral control. The breakdown of substrates within the cell
is associated with generation of mechanical, thermal, Vesicular transport
electrical or chemical energy. Eventually, the breakdown Numerous macromolecules pass into or out of the cell
products are used to synthesise new structural elements without compromising the integrity of the cell membrane.
and fuels. Cell-specific substances destined for secretion This occurs through the formation of membrane-bound
by the cell are also produced. Metabolic waste products vesicles that are pinched off from the plasmalemma
are eliminated by excretion. (vesicular transport). Within the cell, the vesicles are able
to transfer their contents to other cellular compartments.
Mechanisms of substrate uptake The formation of vesicles can be demonstrated using
Substances that enter or leave the cell must pass through electron microscopy. Two types of vesicular transport are
the boundary created by the plasmalemma. This occurs recognised:
through membrane transport or vesicular transport.
· endocytosis (uptake of macromolecules) and
Membrane transport · exocytosis (release of macromolecules).
The structure of the cell membrane permits the uptake of
low molecular weight and polar substances. This can occur
via (Figure 1.3): ENDOCYTOSIS
In the process of endocytosis, the plasmalemma surrounds
· simple diffusion or the molecule before invaginating and budding off as a ves-
· membrane protein transport, through binding with: icle (Figures 1.4 and 1.5). The uptake of vesicular contents
− carrier proteins or is often induced by an increase in the density of the plas-
− channel proteins. malemma in the vicinity of certain actin filament proteins
(particularly clathrin, molecular weight 180,000 Da) (see
Carrier proteins permit the transport of small, water- below). These proteins subsequently coat the exterior sur-
soluble molecules. These are usually specific to certain face of the vesicle (coated vesicles).
types of molecules. Upon coupling with a molecule, There are three mechanisms of endocytosis:
the protein undergoes conformational changes that
result in carriage of the molecule across the membrane. · pinocytosis,
This may involve active transport against a concentra- · receptor-mediated endocytosis and
tion gradient (e.g. Na /K pumps) or passive transport · phagocytosis.
+
+
(e.g. glucose).
Channel proteins also transport small water-soluble
molecules. They are regulated by membrane potentials

1.3 Transmembrane molecular transport (schematic).

Vet Histology.indb 5 16/07/2019 14:52