Control Systems in the Living World - Building the Most Complex Structure on Earth - page 34

the particles, but they also organize these particles and the content of particles, as
seems to be the case with the transport of neurotransmitter vesicles along axons in
the synapse ( Glade et al., 2004 ). They determine the arrangement and movement
of organelles (including the movement and separation of chromosomes during
mitosis/meiosis) in the cell; they transport nuclei to the surface of the embryo during
the blastoderm stage, thus determining the position of the nucleus within the cell.
Actin (Micro-) Filaments
These polymers of actin molecules are the thinnest (7 nm) ( Figure 1.19 ) of all
three types of cytoskeleton filaments. Actin polymerization generates a push force,
whereas binding with myosin causes the actin cytoskeleton to contract. Actin fila-
ments respond to the binding of a number of proteins with changes in the organi-
zation of the actin cytoskeleton. The actin cytoskeleton responds to a number of
external signals (hormones, growth factors, etc.), and the mediators of these external
influences are approximately 20 Rho guanosine triphosphatase (GTPase) enzymes
( Sit and Manser, 2011 ).
Intermediate Filaments
As their name indicates, these filaments are intermediate in average diameter (12 nm)
between microtubules and actin filaments ( Figure 1.20 ). They are a family of fibrous
proteins encoded by about 70 genes. They exist in monomer and dimmer forms and
contribute to maintaining cell shape but do not participate in cell motility ( Lodish
et al., 2000 ).
Figure 1.20 Structural organization of microtubule, microfilament, and intermediate
filament (from http://www.tutorvista.com/content/biology/biology-iii/cell-organization/
nonmembranous-cell-organelles.php ) .