microorganisms, or immune complexes. Compartmentalization of the digestive enzymes within the lysosome allows
the cell to efficiently digest matter without harming the cytoplasmic components of the cell.
- Name the components of the endomembrane system and describe the function of each component.
Peroxisomes
Christian de Duve is also credited with the discovery ofperoxisomes, membrane-bound organelles that are not part
of the endomembrane system (Figure 3.44). Peroxisomes form independently in the cytoplasm from the synthesis
of peroxin proteins by free ribosomes and the incorporation of these peroxin proteins into existing peroxisomes.
Growing peroxisomes then divide by a process similar to binary fission.
Peroxisomes were first named for their ability to produce hydrogen peroxide, a highly reactive molecule that helps to
break down molecules such as uric acid, amino acids, and fatty acids. Peroxisomes also possess the enzyme catalase,
which can degrade hydrogen peroxide. Along with the SER, peroxisomes also play a role in lipid biosynthesis.
Like lysosomes, the compartmentalization of these degradative molecules within an organelle helps protect the
cytoplasmic contents from unwanted damage.
The peroxisomes of certain organisms are specialized to meet their particular functional needs. For example,
glyoxysomes are modified peroxisomes of yeasts and plant cells that perform several metabolic functions, including
the production of sugar molecules. Similarly, glycosomes are modified peroxisomes made by certain trypanosomes,
the pathogenic protozoans that cause Chagas disease and African sleeping sickness.
Figure 3.44 A transmission electron micrograph (left) of a cell containing a peroxisome. The illustration (right) shows
the location of peroxisomes in a cell. These eukaryotic structures play a role in lipid biosynthesis and breaking down
various molecules. They may also have other specialized functions depending on the cell type.
Cytoskeleton
Eukaryotic cells have an internal cytoskeleton made ofmicrofilaments,intermediate filaments, andmicrotubules.
This matrix of fibers and tubes provides structural support as well as a network over which materials can be
transported within the cell and on which organelles can be anchored (Figure 3.45). For example, the process of
exocytosis involves the movement of a vesicle via the cytoskeletal network to the plasma membrane, where it can
release its contents.
Chapter 3 | The Cell 121