18
UNIT 1
Basic Principles of Cell Structure and Function
Golgi
complex
Cisterna of
rough
endoplasmic
reticulum
Outer
nuclear
membrane
Nuclear
pore
Condensing
(secretory)
vesicle
A
Smooth
endoplasmic
reticulum
Intermediate
filaments
Mitochondrion
Plasmalemma
Nucleus
B
Figure 2-5A.
Cytoplasmic organ-
elles: Rough endoplasmic reticulum
and the Golgi complex.
EM,
3
49,000
This view includes only a small area
at the edge of the
nucleus
of a cell
that is actively synthesizing proteins
for secretion. Both euchromatin and
heterochromatin can be seen, but the
nucleolus, although present in the
cell, is not in view here. The
nuclear
pore
is the gateway for materials
leaving the nucleus, for example,
mRNA, tRNA (transfer RNA), and
preribosomal particles. Entering
the nucleus through nuclear pores
are histones and other proteins of
chromatin, DNA polymerases and
RNA polymerases, and ribosomal
proteins. The
outer nuclear mem-
brane
is studded with ribosomes, an
indication that the nuclear envelope
is continuous with the
RER
, which
is abundant in this cell. A part of the
Golgi complex
is identiF able as a
stack of fl attened membranous sacs
with smooth surfaces. The small
ves-
icles
associated with the Golgi com-
plex include transport vesicles that
convey polypeptides from the RER.
Figure 2-5B.
Cytoplasmic orga-
nelles: Smooth endoplasmic reticu-
lum.
EM,
3
33,000
Smooth endoplasmic reticulum
is
considerably less conspicuous in
appearance than RER with its broad,
fl attened cisternae and arrays of
attached ribosomes. The usual con-
F guration of SER is a labyrinth of
branching tubules with swellings.
Thus, it presents in sections as pro-
F les of smooth-surfaced circular or
oval membranes, much the same as
slices through spherical vesicles. The
true distinctive structure of the SER
is revealed by the occasional proF les
that have a Y-shaped or branching
lumen, as can be seen in this image.
The cytoplasm in this view also con-
tains some
mitochondria
and numer-
ous
intermediate ± laments
coursing
through the cytosol. Some of the F la-
ments near the
plasmalemma
of the
cell are more likely actin F laments,
which are concentrated in the
cortex
(outer layer) of many cells.
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