Page 79 - Atlas of Histology with Functional Correlations

high-energy needs contain numerous mitochondria, whereas cells with low-

energy needs have few mitochondria. Also, in these high-energy cells, the
mitochondria exhibit large numbers of closely packed cristae, whereas in
cells with low-energy metabolism, the cristae are less extensively developed.
Surrounding the cristae is an amorphous mitochondrial matrix, which
contains enzymes, ribosomes, and, unlike other cytoplasmic organelles, a

small, circular DNA molecule called mitochondrial DNA. New
mitochondria arise from preexisting mitochondria by growth and division.

Rough and Smooth Endoplasmic Reticulum

The endoplasmic reticulum in the cytoplasm is an extensive network of sacs,
vesicles, and interconnected flat tubules called cisternae. Endoplasmic reticulum
may be either rough or smooth. Their predominance and distribution in a given
cell depends on cell function.

Rough endoplasmic reticulum (RER) is characterized by numerous
flattened, interconnected cisternae, whose cytoplasmic surfaces are covered or
studded with dark-staining granules called ribosomes. The presence of
ribosomes distinguishes the rough endoplasmic reticulum, which extends from

the outer membrane of the nuclear envelope to sites throughout the cytoplasm. In
contrast, smooth endoplasmic reticulum (SER) is devoid of ribosomes, and it
consists primarily of anastomosing or connecting tubules. In most cells, smooth
endoplasmic reticulum, which is less abundant than the rough endoplasmic

reticulum, is also continuous with rough endoplasmic reticulum.

FUNCTIONAL CORRELATIONS 2.2 ■ Rough and

Smooth Endoplasmic Reticulum

Cells that synthesize large amounts of protein for export, such as pancreatic
acinar cells or salivary gland cells, exhibit a highly developed and extensive

rough endoplasmic reticulum (RER) with numerous stacks of flattened
cisternae. Thus, the main function of RER is protein synthesis. Proteins that
will be transported or exported either to the outside of the cell or packaged

in organelles such as lysosomes are synthesized by the ribosomes attached to
the surface of the RER. In addition, integral membrane proteins and
phospholipid molecules are synthesized by the RER that become part of the
cell membrane. In contrast, proteins for the cytoplasm, nucleus, and

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