Page 62 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 47

Membrane proteins may also act as secretory products are enclosed within a
membrane vesicle for temporary storage in
VetBooks.ir receptor sites for attachment of disease‐ the cell or for transport to the plasma
causing agents, such as viruses. Some
membrane, where exocytosis releases the
viruses bind to appropriate membrane
receptors on susceptible cells, which products into the extracellular fluid as a
then endocytose the virus receptor com- form of secretion. Mucopolysaccharides
plex. This provides a means by which the may form in the Golgi apparatus, and glyco­
virus can infect the susceptible cell. proteins are terminally synthesized there
as combinations of carbohydrates and
proteins that have been transported to the
Cytoplasm and Cytoplasmic Golgi apparatus by the smooth and rough
Organelles endoplasmic reticulum.

Cytoplasm The Endoplasmic Reticulum
and Ribosomes
Cytoplasm is the material filling the
inside of the cell and containing the intra­ The endoplasmic reticulum is a mem­
cellular organelles and nucleus (Fig. 2‐7). branous network found throughout the
Organelles are intracellular structures cytoplasm of the cell (Fig. 2‐7). It was first
organized for a particular function within described in the endoplasm (the cytoplasm
the cell. These include the Golgi apparatus, deepest in the cell), giving rise to the
endoplasmic reticulum (smooth and rough), name endoplasmic reticulum. Although still
mitochondria, centrioles, free ribosomes, called the endoplasmic reticulum, it has
lysosomes, peroxisomes, and a variety of been observed in all parts of the cytoplasm
crystals, granules, and droplets (formerly and may be continuous with the outer
called inclusions, collectively). The rela­ nuclear membrane. The endoplasmic
tively liquid component of cytoplasm is reticulum is in the form of tubules and
cytosol; the organelles are arranged within sheets, with occasional enlarged sacs or
the cytosol by a complex system of intra­ vesicles called cisternae.
cellular filaments and microtubules called In some sites, the endoplasmic reticulum
the cytoskeleton. is associated with ribosomes, which appear
Dissolved or suspended within the fluid like beads along the membrane. Granular or
cytosol are a variety of proteins, sugars, rough endoplasmic reticulum (Figs. 1‐2
and salts. Many of the proteins are and 2‐7) is endoplasmic reticulum associated
enzymes that function in the metabolic with ribosomes and is involved in protein
activities of the cell. Within some cells are synthesis. Agranular, or smooth endoplas-
proteins that function as cytosolic recep­ mic reticulum, is devoid of ribosomes and
tors binding ligands that have gained is associated with lipid synthesis, including
access to the cytosol after passing through cholesterol and phospholipids. The endo­
the cell membrane. plasmic reticulum can also serve as intracel­
lular storage site for calcium. The second
The Golgi Apparatus messenger IP binds to IP receptors,
3
3
releasing calcium into the cytosol.
The Golgi apparatus varies in size and
location in cells of different tissues but gen­ Mitochondria
erally appears as a stack of flattened mem­
branous sacs (lamellae) near the nucleus Mitochondria are ovoid organelles about
(Fig. 2‐7). It functions as the site of the final 10 μm long. The double membrane of the
stages of synthesis and packaging of secre­ mitochondrion, with the cristae projecting
tory products of the cell. Within the Golgi, into the interior, provides a large surface