Microbiology, 2021

3.4 • Unique Characteristics of Eukaryotic Cells 115
of a eukaryotic cell. In these fluorescently labeled animal cells, the microtubules are green, the actin microfilaments are red, the nucleus is
blue, and keratin (a type of intermediate filament) is yellow.
Microfilaments are composed of two intertwined strands of actin, each composed of actin monomers forming
filamentous cables 6 nm in diameter 25 (Figure 3.48). The actin filaments work together with motor proteins,
like myosin, to effect muscle contraction in animals or the amoeboid movement of some eukaryotic microbes.
In ameboid organisms, actin can be found in two forms: a stiffer, polymerized, gel form and a more fluid,
unpolymerized soluble form. Actin in the gel form creates stability in the ectoplasm, the gel-like area of
cytoplasm just inside the plasma membrane of ameboid protozoans.
Temporary extensions of the cytoplasmic membrane called pseudopodia (meaning “false feet”) are produced
through the forward flow of soluble actin filaments into the pseudopodia, followed by the gel-sol cycling of the
actin filaments, resulting in cell motility. Once the cytoplasm extends outward, forming a pseudopodium, the
remaining cytoplasm flows up to join the leading edge, thereby creating forward locomotion. Beyond
amoeboid movement, microfilaments are also involved in a variety of other processes in eukaryotic cells,
including cytoplasmic streaming (the movement or circulation of cytoplasm within the cell), cleavage furrow
formation during cell division, and muscle movement in animals (Figure 3.48). These functions are the result
of the dynamic nature of microfilaments, which can polymerize and depolymerize relatively easily in response
to cellular signals, and their interactions with molecular motors in different types of eukaryotic cells.
Figure 3.48 (a) A microfilament is composed of a pair of actin filaments. (b) Each actin filament is a string of polymerized actin monomers.
(c) The dynamic nature of actin, due to its polymerization and depolymerization and its association with myosin, allows microfilaments to
be involved in a variety of cellular processes, including ameboid movement, cytoplasmic streaming, contractile ring formation during cell
25 Fuchs E, Cleveland DW. “A Structural Scaffolding of Intermediate Filaments in Health and Disease.” Science 279 no. 5350
(1998):514–519.