Chapter 3
Cells: The Living Units
85
3
that bud from the concave
trans face
(the “shipping” side) of the
Golgi stack:
Vesicles containing proteins destined for export pinch off
from the trans face as
secretory vesicles
,
or granules
, which
migrate to the plasma membrane and discharge their con-
tents from the cell by exocytosis (
Figure 3.20
, 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-
nous organelles.
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.
Golgi Apparatus
Te
Golgi apparatus
(gol
9
je) consists of stacked and flattened
membranous sacs, shaped like hollow dinner plates, associated
with swarms of tiny membranous vesicles
(Figure 3.19)
. Te
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
cis face,
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
Cis face—
“receiving” side of
Golgi apparatus
Secretory vesicle
(a) Many vesicles in the process of pinching off from the Golgi
apparatus.
(b) Electron micrograph of the Golgi apparatus (90,000
m
)
Transport vesicle at
the trans face
Transport
vesicle
from
trans face
Trans face—
“shipping” side of
Golgi apparatus
New vesicles
forming
New vesicles forming
Cisterns
Transport vesicle
from rough ER
Golgi apparatus
Figure 3.19
Golgi apparatus.
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.
Peroxisomes
Peroxisomes
(pĕ-roks
9
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
2
) to detoxify harmful sub-
stances, including alcohol and formaldehyde. Teir most im-
portant function is to neutralize
free radicals
, highly reactive
previous page 119 Human Anatomy and Physiology (9th ed ) 2012 read online next page 121 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off