86
UNIT ONE
8.
Centrosome.
A centrosome (sen
tro-so
ˉm) (central
body) is a structure located in the cytoplasm near the
nucleus. It is nonmembranous and consists of two
cylinders, called
centrioles,
built of tubelike structures
called microtubules organized as nine groups of three.
The centrioles usually lie at right angles to each other.
carbohydrates, and nucleic acids, including foreign
particles. Certain white blood cells, for example,
engulf infecting bacteria, which lysosomal enzymes
then digest.
Lysosomes also destroy worn cellular parts.
Lysosomes in certain scavenger cells may engulf and
digest entire body cells that have been damaged. How
the lysosomal membrane is able to withstand being
digested is not well understood, but this organelle
sequesters enzymes that can function only under very
acidic conditions, preventing them from destroying
the cellular contents around them. Human lysosomes
contain more than forty different types of enzymes. An
abnormality in just one type of lysosomal enzyme can
be devastating to health (Clinical Application 3.2).
7.
Peroxisomes
(pe
˘-roks
ı˘-so
ˉmz). Peroxisomes are
membranous sacs that resemble lysosomes in size
and shape. Although present in all human cells,
peroxisomes are most abundant in cells of the liver
and kidneys. Peroxisomes contain enzymes, called
peroxidases, that catalyze metabolic reactions that
release hydrogen peroxide (H
2
O
2
), which is toxic to
cells. Peroxisomes also contain an enzyme called
catalase, which decomposes hydrogen peroxide.
The outer membrane of a peroxisome contains
some forty types of enzymes, which catalyze a variety of
biochemical reactions, including
• synthesis of bile acids, used in fat digestion
• breakdown of lipids called very long chain fatty acids
• degradation of rare biochemicals
• detoxiF
cation of alcohol
Abnormal peroxisomal enzymes can drastically affect health
(see Clinical Application 3.2).
Inner membrane
Outer membrane
Cristae
(a)
(b)
Lysosomes
FIGURE 3.12
A mitochondrion is the site of many energy reactions.
(
a
) A transmission electron micrograph of a mitochondrion (28,000×).
(
b
) Cristae partition this saclike organelle.
FIGURE 3.13
Lysosomes are membranous sacs that contain
enzymes that dismantle debris. In this falsely colored transmission
electron micrograph, lysosomes appear as dark circular structures
(14,100×).
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