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Despite the importance of membrane-bound organelles,
as illustrated in the preceding chapters, the
eukaryotic cell requires more than a set of defined
compartments to function. Eukaryotic cells organize
their components spatially, fixing some at defined
locations in the cell, moving others to attain
optimal positions. The contents of eukaryotic
cells (and in some cases, the cells themselves)
are mobile. Poking a cell with a fine microneedle
stimulates its contents to move vigorously. In
the first half of this century, these directed
movements were considered the unmistakable indicator
of life. Amusingly, the term coined to describe
this essential cellular property was “irritability.”
Spatial organization
within the eukaryotic cell and directed movements
of the cell or its contents are mediated by the
cytoskeleton, a network of filamentous protein
polymers that permeates the cytosol. The cytoskeleton
comprises three major families of proteins: intermediate
filaments, actin, and tubulin. In this chapter,
each family and its principal functions will be
described. Mitosis and cytokinesis will also be
discussed, given the paramount role of the cytoskeleton
in these processes. The cytoskeleton evolved before
plants diverged from animals, and the main features
of the cytoskeleton have been conserved in both.
Thus, much of the information presented applies
to both animals and plants. However, the plant
cytoskeleton has evolved unique functions that
differ from those in animals, as will also be
highlighted in this chapter.
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