Cells: The Living Units
that bud from the concave
(the “shipping” side) of the
Vesicles containing proteins destined for export pinch oﬀ
from the trans face as
migrate to the plasma membrane and discharge their con-
tents from the cell by exocytosis (
, pathway A).
Specialized secretory cells, such as the enzyme-producing
cells of the pancreas, have a prominent Golgi apparatus.
Te Golgi apparatus pinches oﬀ other vesicles containing
lipids and transmembrane proteins destined for the plasma
membrane (Figure 3.20, pathway B) or for other membra-
Te Golgi apparatus also packages digestive enzymes into
membranous lysosomes that remain in the cell (Figure 3.20,
pathway C), as discussed shortly.
Additionally, skeletal and cardiac muscle cells have an elabo-
rate smooth ER (called the sarcoplasmic reticulum) that plays
an important role in storing and releasing calcium ions during
muscle contraction. Except for the examples given above, most
body cells contain relatively little, if any, smooth ER.
je) consists of stacked and ﬂattened
membranous sacs, shaped like hollow dinner plates, associated
with swarms of tiny membranous vesicles
Golgi apparatus is the principal “traﬃc director” for cellular
proteins. Its major function is to modify, concentrate, and pack-
age the proteins and lipids made at the rough ER and destined
for export from the cell.
Te Golgi’s odd shape is a side eﬀect of its job. A protein com-
plex pulls membranous sacs containing newly synthesized pro-
teins oﬀ the Golgi and in the process, the membranes are ﬂattened
like rubber bands. ±ransport vesicles that bud oﬀ from the rough
ER move to and fuse with the membranes at the convex
the “receiving” side, of the Golgi apparatus. Inside the apparatus,
the proteins are modiﬁed: Some sugar groups are trimmed while
others are added, and in some cases, phosphate groups are added.
Te various proteins are “tagged” for delivery to a speciﬁc
address, sorted, and packaged in at least three types of vesicles
“receiving” side of
(a) Many vesicles in the process of pinching off from the Golgi
(b) Electron micrograph of the Golgi apparatus (90,000
Transport vesicle at
the trans face
“shipping” side of
New vesicles forming
from rough ER
Note: In (a), the vesicles shown in the process of pinching off
from the membranous Golgi apparatus would have a protein coating on their external surfaces.
The diagram omits these proteins for simplicity.
ĭ-sōmz; “peroxide bodies”) are spherical
membranous sacs containing a variety of powerful enzymes, the
most important of which are oxidases and catalases.
Oxidases use molecular oxygen (O
) to detoxify harmful sub-
stances, including alcohol and formaldehyde. Teir most im-
portant function is to neutralize
, highly reactive