58 Chapter 3
Peroxisomes
Peroxisomes are membrane-enclosed organelles containing
several specific enzymes that promote oxidative reactions.
Although peroxisomes are present in most cells, they are par-
ticularly large and active in the liver.
Peroxisomes contain enzymes that remove hydrogen
from particular organic molecules. Removal of hydrogen
oxidizes the molecules and the enzymes that promote these
reactions are called oxidases (chapter 4, section 4.3). The
hydrogen is transferred to molecular oxygen (0 2 ), forming
hydrogen peroxide ( H 2 0 2 ). Peroxisomes are important in the
metabolism of amino acids and lipids and the production of
bile acids. Peroxisomes also oxidize toxic molecules, such as
formaldehyde and alcohol. For example, much of the ethanol
(alcohol) ingested in drinks is oxidized to acetaldehyde by
liver peroxisomes.
The enzyme catalase within the peroxisomes prevents the
excessive accumulation of hydrogen peroxide by catalyzing
the reaction 2H 2 O 2 → 2H 2 O 1 O 2. Catalase is one of the fast-
est acting enzymes known (see chapter 4), and it is this reac-
tion that produces the characteristic fizzing when hydrogen
peroxide is poured on a wound.in the liver, striated muscles, and some other tissues; mela-
nin granules in the melanocytes of the skin; and triglycerides
within adipose cells.
Lysosomes
After a phagocytic cell has engulfed the proteins, polysac-
charides, and lipids present in a particle of “food” (such as
a bacterium), these molecules are still kept isolated from the
cytoplasm by the membranes surrounding the food vacuole.
The large molecules of proteins, polysaccharides, and lipids
must first be digested into their smaller subunits (including
amino acids, monosaccharides, and fatty acids) before they can
cross the vacuole membrane and enter the cytoplasm.
The digestive enzymes of a cell are isolated from the
cytoplasm and concentrated within membrane-bound organ-
elles called lysosomes, which contain more than 60 different
enzymes. A primary lysosome is one that contains only diges-
tive enzymes within an environment that is more acidic than
the surrounding cytoplasm.
A primary lysosome may fuse with a food vacuole to form
a secondary lysosome that now contains the engulfed extra-
cellular material. The digestion of structures and molecules
within a vacuole by the enzymes within lysosomes is a process
known as autophagy. Extracellular material digested by this
process includes potentially disease-causing bacteria. Unique
double-membrane vacuoles can also form intracellularly to
contain structures within the cell, including viruses. A vacuole
of this kind, called an autophagosome, can fuse with a lyso-
some so that the lysosomal enzymes degrade the viruses. In
these and other ways, autophagy contributes to immunity.
In addition, autophagosomes can engulf parts of the cyto-
plasm and various organelles before fusing with lysosomes.
This type of autophagy is stimulated in a nonspecific way
when a cell is deprived of nutrients, so that digestion within
lysosomes can release molecules needed by the cell. However,
autophagy also occurs when a cell is not deprived of nutrients.
Under these conditions, autophagy selectively eliminates dam-
aged organelles, such as peroxisomes and mitochondria (dis-
cussed shortly) that are toxic to the cell.
In a similar manner, potentially toxic aggregations of pro-
teins within the cytoplasm may be contained within autopha-
gosomes and digested by lysosomes. Additionally, digestion
by lysosomal enzymes is needed for the proper turnover of
glycogen and certain lipids; lack of a particular enzyme can
result in the undue accumulation of these molecules in the cell
(described in the next Clinical Applications box).
Lysosomes have also been called “suicide bags” because
a break in their membranes would release their digestive
enzymes and thus destroy the cell. This happens normally
in programmed cell death (or apoptosis ), described in
section 3.5. An example is the loss of tissues that must
accompany embryonic development, when earlier struc-
tures (such as gill pouches) are remodeled or replaced as the
embryo matures.
CLINICAL APPLICATION
Autophagosomes engulf parts of the cell before fus-
ing with lysosomes when a cell is deprived of nutrients,
releasing molecules needed by the cell. Autophagy also
occurs when a cell selectively eliminates such damaged
organelles as peroxisomes and mitochondria that are
toxic to the cell. Autophagy and digestion by lysosomal
enzymes produces a normal turnover of glycogen and
certain complex lipids stored in cells. When a lysosomal
enzyme is missing or defective, due to the inheritance of
a specific defective gene, particular molecules can accu-
mulate and destroy tissues. Examples of the approxi-
mately 50 different lysosomal storage diseases include
Gaucher’s disease, Tay-Sach’s disease, and Pompe
disease.Clinical Investigation CLUES
George had a grandparent on each side that had
Gaucher’s disease, the most common lysosomal storage
disease. The doctor ordered tests to see if George inher-
ited the condition.- What are lysosomes, and what are their functions?
- What causes lysosomal storage diseases?