CYTOPLASMIC ORGANELLES
Figure 3­8
Downloaded
by
[Rutgers
University
130.219.8.11]
at
[04/08/15].
Copyright
©
McGraw-Hill
Global
Education
Holdings,
LLC.
Most eukaryotic cells are divided into two major compartments, the nucleus (N) and the cytoplasm. Transcription of
DNA occurs in the nucleus, and protein synthesis takes place in the cytoplasm and mitochondria. Cells can synthes-
Not
ize proteins that are exported from the cell, and also proteins that remain within the nucleus and that part of the
to
cytoplasm termed the cytosol.
be
redistributed
Proteins transported out of the cell are synthesized on the rough endoplasmic reticulum (rough ER), whereas those
remaining within the cell are synthesized on polysomes in the cytosol. Thus, the cytosol is the site of synthesis of
proteins of the cytosol itself, peroxisomes, the cytoskeleton, the nucleus, and some mitochondrial proteins. Mito-
or
chondria can synthesize some of their own proteins because they have a small complement of DNA that is tran-
modified
scribed within the organelles.
Mitochondria (arrows), which can appear as spheres or rods in electron micrographs, are important in the respiratory
in
any
and energy-producing functions of the cell.
way
This micrograph also illustrates a small area of rough ER (arrowhead). This organelle consists of a system of mem-
without
branous sacs, or cisternae, which are continuous with the outer membrane of the nuclear envelope. Ribosomes are
located on the cytosolic region of the ER. In addition, there are regions of smooth ER in the vicinity of the mitochon-
permission.
dria. We shall describe the endoplasmic reticulum in later images in this section.
82

---