86
UNIT 1
Organization of the Body
3
Lysosomes
Born as endosomes which contain inactive enzymes,
lysosomes
(“disintegrator bodies”) are spherical membranous organelles
containing activated digestive enzymes
(Figure 3.21)
. As
you might guess, lysosomes are large and abundant in phago-
cytes, the cells that dispose of invading bacteria and cell debris.
Lysosomal enzymes can digest almost all kinds of biological
molecules. Tey work best in acidic conditions and so are called
acid hydrolases
.
Te lysosomal membrane is adapted to serve lysosomal
functions in two important ways. First, it contains H
1
(proton)
Protein-containing
vesicles pinch off rough
ER and migrate to fuse
with membranes of
Golgi apparatus.
Proteins are
modified within the
Golgi compartments.
Proteins are then
packaged within
different vesicle types,
depending on their
ultimate destination.
Plasma
membrane
Secretion by exocytosis
Vesicle becomes
lysosome
Golgi
apparatus
Rough ER
ER membrane
Phagosome
Proteins in cisterns
Pathway B:
Vesicle membrane
to be incorporated
into plasma
membrane
Pathway A:
Vesicle contents
destined for
exocytosis
Extracellular fluid
Secretory
vesicle
Pathway C:
Lysosome
containing acid
hydrolase
enzymes
1
3
2
Lysosomes
Light green areas are
regions where materials
are being digested.
Figure 3.21
Electron micrograph of lysosomes (20,000
3
).
Figure 3.20
The sequence of events from protein synthesis on the rough ER to the final
distribution of those proteins.
The protein coats on the transport vesicles are not illustrated.
chemicals with unpaired electrons that can scramble the struc-
ture of biological molecules. Oxidases convert free radicals to hy-
drogen peroxide, which is also reactive and dangerous but which
the catalases quickly convert to water. Free radicals and hydrogen
peroxide are normal by-products of cellular metabolism, but they
have devastating effects on cells if allowed to accumulate.
Peroxisomes are especially numerous in liver and kidney
cells, which are very active in detoxification. Tey also play a role
in energy metabolism by breaking down and synthesizing fatty
acids. Peroxisomes look like small lysosomes (see Figure 3.2),
and for many years it was thought that they were self-replicating
organelles formed when existing peroxisomes simply pinch in
half. Recent evidence, however, suggests that most new peroxi-
somes form by budding off of the endoplasmic reticulum via
a special ER machinery that differs from that used for vesicles
destined for modification in the Golgi apparatus.
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