Cellular Structure, Proteins, and Metabolism
51
the rough endoplasmic reticulum pass into the lumen of the
reticulum and are then transferred to yet another organelle,
the Golgi apparatus. They are ultimately secreted from the cell
or distributed to other organelles.
Endoplasmic Reticulum
The most extensive cytoplasmic organelle is the network of mem-
branes that forms the
endoplasmic reticulum
(
Figure 3–12
).
These membranes enclose a space that is continuous through-
out the network.
Two forms of endoplasmic reticulum can be distinguished:
rough, or granular, and smooth, or agranular. The rough endo-
plasmic reticulum has ribosomes bound to its cytosolic sur-
face, and it has a fl attened-sac appearance. Rough endoplasmic
reticulum is involved in packaging proteins that, after process-
ing in the Golgi apparatus, are secreted by the cell or distrib-
uted to other cell organelles.
The smooth endoplasmic reticulum has no ribosomal
particles on its surface and has a branched, tubular structure.
It is the site at which certain lipid molecules are synthesized, it
plays a role in detoxifi cation of certain hydrophobic molecules,
and it also stores and releases calcium ions involved in control-
ling various cell activities.
Golgi Apparatus
The
Golgi apparatus
is a series of closely apposed, fl attened
membranous sacs that are slightly curved, forming a cup-shaped
structure (
Figure 3–13
). Associated with this organelle, par-
ticularly near its concave surface, are a number of roughly
spherical, membrane-enclosed vesicles.
Proteins arriving at the Golgi apparatus from the rough
endoplasmic reticulum undergo a series of modifi cations as they
pass from one Golgi compartment to the next. For example,
carbohydrates are linked to proteins to form glycoproteins,
and the length of the protein is often shortened by remov-
ing a terminal portion of the polypeptide chain. The Golgi
apparatus sorts the modifi
ed proteins into discrete classes
of transport vesicles that will travel to various cell organelles
Figure 3–12
Endoplasmic reticulum.
Electron micrograph from D. W. Fawcett,
The Cell, An Atlas of Fine Structure,
W. B. Saunders Company, Philadelphia, 1966.
Rough
endoplasmic
reticulum
Structure:
Extensive
membranous network of
flattened sacs. Encloses
a space that is continuous
throughout the organelle
and with the space between
the two nuclear-envelope
membranes.
Has ribosomal particles
attached to its cytosolic
surface.
Function:
Proteins synthesized
on the attached ribosomes
enter the lumen of the reticulum
from which they are ultimately
distributed to other organelles
or secreted from the cell.
Smooth
endoplasmic
reticulum
Structure:
Highly branched
tubular network that does not
have attached ribosomes but
may be continuous with the
rough endoplasmic reticulum.
Function:
Contains enzymes
for fatty acid and steroid
synthesis. Stores and releases
calcium, which controls various
cell activities.
Rough endoplasmic reticulum
Smooth endoplasmic reticulum
Ribosomes
Lumen
Lysosome
Rough endoplasmic
reticulum
Mitochondria
Smooth endoplasmic
reticulum
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