Cisterna
Transfer vesicle
Golgi complex •
Figure 3.7
The Golgi complex acts as a relay station
where newly made proteins are processed and
sorted to various destinations. •
Functions of the Golgi Complex
1. Modifies, sorts, packages, and transports proteins received
from the rough ER.
2. Forms secretory vesicles that discharge processed proteins
via exocytosis into extracellular fluid; forms membrane vesicles
that ferry new molecules to the plasma membrane; forms
transport vesicles that carry molecules to other organelles,
such as lysosomes.
Smooth ER (SER)—SER is a portion of the ER that
extends from the RER outward and ends as a series
of microtubules. Fatty acids and steroids are made in
the SER. SER also detoxifies harmful substances and
serves as a storage site for ionized calcium, which
gets released as part of an intracellular signal for
muscle contraction and the actions of some chemical
messengers called
hormones.
SER in skeletal muscle cells
and heart muscle cells has a special name, reflecting
its
specialized
function:
sarcoplasmic
reticulum
(
sar'-ko-PLAZ-mik
). You will learn more about the role
of the sarcoplasmic reticulum in Chapter 6.
Golgi complex
The Golgi complex (
GOL-je
) is an
array of 3 to 20 flat membrane sacs or cisternal (
Fig-
ure 3.7
). Most cells have several Golgi complexes with-
in them. The Golgi complex is like a train yard, where
trains arrive and are sorted onto the appropriate tracks
for their destinations. In cells, the Golgi complex pro-
cesses newly made proteins and sorts them to one of two
destinations within the cell: the plasma membrane or
other organelles. Transfer vesicles containing the pro-
teins bud off the ends of the cisterns and proceed to their
destinations.
Lysosomes
Lysosomes
(
Ll-so-somz
) are membrane-
enclosed vesicles that contain digestive enzymes (
Figure
3.8
). Their job is to break down material ingested by the
cell from the extracellular environment, such as proteins
and bacteria, as well as worn-out organelles from inside
the cell. Lysosomes fuse with the vesicle or organelle to be
digested. Once the material has been digested, the lyso-
some releases it into the cytosol, and it gets recycled.
Besides lysosomes, there are two other digestive or-
ganelles, peroxisomes and proteasomes (see Figure 3.1).
The peroxisome (
per-OK-si-som
) contains enzymes called
oxidases that remove hydrogen atoms from various mol-
ecules, such as amino acids and fatty acids. Peroxisomes in
liver cells detoxify alcohol. Hydrogen peroxide (H2O2) is a
byproduct of the chemical reactions within peroxisomes.
Left to itself, H2O2
would be harmful to the cells, so per-
oxisomes also contain an enzyme called catalase, which
breaks down H2O2
into water and oxygen.
Proteasomes (
PRO-te-a-somz
) are tiny, barrel-shaped
organelles. Each cell contains thousands of proteasomes
in the cytosol and nucleus. Proteasomes contain enzymes
called proteases, which break down faulty, damaged, and
unneeded proteins into smaller peptides. Other enzymes
then break the peptides into amino acids, which are re-
cycled (that is, reused within the cell).
Lysosome • Figure 3.8
Lysosomes have a number of digestive enzymes that break down ingested material and
worn-out organelles and release their components into the cytosol.
Digestive
enzymes
Lysosome
Functions of the Lysosome
1. Digest substances that enter
the cell.
2. Digest worn-out organelles.
3. Digest entire cells (autolysis).
4. Carry out extracellular
digestion.
Cells Have Distinct Parts
55
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