Organization of the Body
Born as endosomes which contain inactive enzymes,
(“disintegrator bodies”) are spherical membranous organelles
containing activated digestive enzymes
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
Te lysosomal membrane is adapted to serve lysosomal
functions in two important ways. First, it contains H
vesicles pinch off rough
ER and migrate to fuse
with membranes of
modiﬁed within the
Proteins are then
different vesicle types,
depending on their
Secretion by exocytosis
Proteins in cisterns
to be incorporated
Light green areas are
regions where materials
are being digested.
Electron micrograph of lysosomes (20,000
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.