Control Systems in the Living World - Building the Most Complex Structure on Earth - page 28

Peptidergic
vesicle

Golgi

F-actin

a

d

b

c

e

Microtubules

Regulated secretory pathway

f

Constitutive secretory pathway

Stimulated
secretion

Figure 1.15 Steps for post-Golgi regulated secretory pathway (RSP) vesicle transport to the
release site. Multiple steps are involved in transporting hormone-containing vesicles from the
site of biogenesis at the trans-Golgi network (TGN) to the release site in the RSP. (a) Vesicle
budding; (b) microtubule-based transport; (c) actin-based transport; (d) vesicle tethering; (e)
docking; and (f) fusion with the plasma membrane. These steps share some commonality with
the trafficking of constitutive secretory vesicles, but there are differences as well ( Park and
Loh, 2008 ).

spot in the middle of the body ( Kateriya et al., 2004 ). This very simple photosensi-
tive organelle, later observed in other unicellular organisms, is known as an eyespot.
Chlamydomonas reinhardtii is a unicellular green alga with a single eyespot, which
enables it to perceive the light and determine the direction to move in. This allows it
to find places of optimal light intensity necessary for starch photosynthesis, its main
source of energy. To reach this photosynthetically optimal place, the unicellular must
adapt to the patterns of beating of the flagella. The phototactic response requires a
particular asymmetric positioning of the eyespot ( Mittelmeier et al., 2011 ) which,
like the positioning of most of cell organelles, is determined by the centrioles or the
MTOCs via the dynamic regulation of microtubule acetylation of the daughter four-
membered (D4) microtubule rootlet ( Boyd et al., 2011b ) ( Figure 1.16 ). The length of
the D4 rootlet is the major determinant of the eyespot positioning along the anterior-
posterior axis ( Boyd et al., 2011a ) and the equator ( Boyd et al., 2011b ).
Excitation of eye photosensitive pigments by light is followed by an electrical
response and then by the adaptive change in the beating of the flagella. Finding the
optimal intensity of light from a light source may require complex computation of
the direction in which the unicellular has to move in and out of the possible adjust-
ments it has to perform on the way to the intended destination.

Cytoskeleton Controls Locomotion in Unicellulars
Most unicellulars use cytoskeletal structures such as cilia and flagella for locomo-
tion. Protozoans display a number of behaviors, such as adaptive responses to touch,
light, and chemicals, and they can even learn from experience. These facts indicate