CHAPTER 2
Cell Structure and Function
15
All of the
ultrastructural features
of cells can be viewed with electron microscopy under optimum conditions of specimen
preparation,
orientation, and magniF cation. The
unit
membrane
that delimits the cell and many of its major components is seen as a thin,
electron-dense line in transmission electron micrographs. Segments of membrane that are oriented vertically to the plane of section
are most sharply deF ned, whereas membrane segments that are oriented at or nearly parallel to the plane of section cannot be readily
discerned. IdentiF
cation of the major organelles relies, to a great extent, on the characteristic arrangements of the membranes that
deF ne them. Two major structures, the
nucleus
and the
mitochondria
, are bounded by
double
membranes
. IdentiF
cation of mito-
chondria is aided by the infoldings of the
inner mitochondrial membrane
that form shelfl ike or tubular cristae, extending through the
interior of the organelle. The
inner nuclear membrane
typically has heterochromatin apposed to it, and the
outer nuclear membrane
is often studded with ribosomes. The membrane that forms the
RER
encloses a cisternal space in the shape of either disks or tubules,
and identiF cation is conF rmed by the presence of polyribosomes on the outer surface of the membrane.
SER
typically has a branch-
ing tubular form, so that, in sections, patches of circular, ovoid, and Y-shaped proF
les of membrane are seen.
Plasmalemma
Rough
endoplasmic
reticulum
Nuclear
envelope
Smooth
endoplasmic
reticulum
Secondary
lysosome
Mitochondria
Figure 2-2.
Membranes de± ne the major components and compartments of the cell.
EM,
3
19,000
Cell Ultrastructure
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