Fundamentals of Anatomy and Physiology

Cell Structure

membrane into amino acids, fatty acids, glycerol, and

so on and material that can be recycled to build new

protein and phospholipids.^

3. Lysosomes also act as suicide agents in old and
   weakened cells. This process is known as autolysis
   (aw-TAHL-ih-sis). The lysosome will expel all of its
   enzymes directly into the cytoplasm of the cell to
   destroy the cell and its organelles.

Endoplasmic Reticulum

The endoplasmic reticulum (en-doh-PLAZ-mik re-
TIK-you-lum), or ER, is a complex system of membranes
that forms a collection of membrane-bound cavities. These
often interconnect into a membrane-bound system of
channels within the cytoplasm. The shape and size of these
cavities vary with the type of cell. When the cavi-ties are
sac-like or channel-like, they are called cisternae - (sis-
TER-nee) and are used to store and transport ma-terials
made by the cell. The ER is attached to the outer membrane
of the nuclear membrane or envelope and ultimately
connects with the cell membrane (Figure- 3-9). With the
use of the electron microscope, it was discov-ered that there
are two types of ER: a rough ER and a smooth ER.

The Rough or Granular ER
All cells will have a rough or granular ER. It is called
rough or granular because it has ribosomes attached to it.
These are the granules on the ER. Because of the attached
ribosomes, the rough ER is a site of protein synthesis.
Proteins that will be secreted by the cell are synthesized
there. The cavities and vesicles of the rough ER serve in the
segregation and transport of these pro-teins in preparation
for further discharge and process-ing. The rough ER may
also be involved in the collection of digestive enzymes to
form lysosomes.

The Agranular or Smooth ER
Occasionally, a smooth or agranular ER will be
attached to a granular ER (see Figure 3-9). Structurally, the
agran-ular form differs from the rough form. It does not
have attached ribosomes. It also differs in function. Only
cer-tain cells have the agranular or smooth ER. It is found
in the cells of the gonads in which sex hormones are being
synthesized. One function appears to be sex hormone
synthesis. It is also found in the cells of the lacteals of the
villi of the small intestine. Thus, it is also believed to be
involved in the transportation of fats. Lacteals will be dis-
cussed in Chapters 15 and 16.

47

Secretory products

(polysaccharides,

proteins, etc.)

Secretory

vesicles

Cisternae^

Golgi^

Proteins

synthesized

for use

in cell

Proteins

synthesized Smooth ER for export

Rough ER

Ribosomes

Nucleus

(^)
®^
Nuclear pore
Learning^
Cengage^
©^
Figure 3- 9 The structure and nature of the
endoplasmic reticulum and the Golgi body.
The Golgi Apparatus
The Golgi (GOHL-jee) apparatus is also called the
Golgi body. It consists of an assembly of flat sac-like
cister-nae that resemble a stack of saucers or pancakes (see
Figure- 3-9). Golgi bodies can differ in both size and
compactness. They function as the points within the cell
where compounds to be secreted by the cell are col-lected
and concentrated. They may be seen attached to the ER.
When the cell’s secretions are a combination of both
proteins and carbohydrates, the -carbohydrates will be
synthesized in the Golgi apparatus and the com-plexes of
carbohydrates and proteins are assembled there. In the
pancreas, enzymes synthesized by the ribosomes- are
collected in the membranes of the Golgi apparatus and then
are secreted. Lysosomes may also form at the Golgi body
when digestive enzymes are collected there.