84
UNIT 1
Organization of the Body
3
Membrane-bound ribosomes
are attached to membranes,
forming a complex called the
rough endoplasmic reticulum
(Figure
3.18)
. Tey synthesize proteins destined either for
incorporation into cell membranes or lysosomes, or for ex-
port from the cell.
Ribosome subunits can switch back and forth between these
two functions, attaching to and detaching from the membranes
of the endoplasmic reticulum, according to the type of protein
they are making at a given time.
Endoplasmic Reticulum (ER)
Te
endoplasmic reticulum (ER)
(en
0
do-plaz
9
mik rĕ-tik
9
u-
lum; “network within the cytoplasm”) is an extensive system of
interconnected tubes and parallel membranes enclosing fluid-
filled cavities, or
cisterns
(sis-ternz) as shown in Figure 3.18.
Coiling and twisting through the cytosol, the ER is continu-
ous with the outer nuclear membrane and accounts for about
half of the cell’s membranes. Tere are two distinct varieties:
rough ER and smooth ER.
Rough Endoplasmic Reticulum
Te external surface of
the
rough ER
is studded with ribosomes, hence the name
“rough” (see Figures 3.2 and 3.18a, b). Proteins assembled on
these ribosomes thread their way into the fluid-filled interior
of the ER cisterns (as described on pp. 105 and 108). When
complete, the newly made proteins are enclosed in vesicles
for their journey to the Golgi apparatus where they undergo
further processing.
Te rough ER has several functions. Its ribosomes manu-
facture all proteins secreted from cells. For this reason, the
rough ER is particularly abundant and well developed in most
secretory cells, antibody-producing plasma cells, and liver cells,
which produce most blood proteins. It is also the cell’s “mem-
brane factory” where integral proteins and phospholipids that
form part of all cellular membranes are manufactured. Te en-
zymes needed to catalyze lipid synthesis have their active sites
on the external (cytosolic) face of the ER membrane, where the
needed substrates are readily available.
Smooth Endoplasmic Reticulum
Te
smooth ER
(see Fig-
ures 3.2 and 3.18) is continuous with the rough ER and con-
sists of tubules arranged in a looping network. Its enzymes (all
integral proteins forming part of its membranes) play no role
in protein synthesis. Instead, the enzymes catalyze reactions
involved with the following tasks:
Metabolize lipids, synthesize cholesterol, and synthesize the
lipid components of lipoproteins (in liver cells)
Synthesize steroid-based hormones such as sex hormones
(testosterone-synthesizing cells of the testes are full of
smooth ER)
Absorb, synthesize, and transport fats (in intestinal cells)
Detoxify drugs, certain pesticides, and cancer-causing chem-
icals (in liver and kidneys)
Break down stored glycogen to form free glucose (in liver
cells especially)
Nuclear
envelope
Ribosomes
Rough ER
(a) Diagrammatic view of smooth and rough ER
Smooth ER
(b) Electron micrograph of smooth and rough ER (25,000
m
)
Nucleus
Figure 3.18
The endoplasmic reticulum.
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