Control Systems in the Living World - Building the Most Complex Structure on Earth - page 29

(A)

Establishment

of asymmetry
Mother side Daughter side

MLT1

ChR traffcking
form golgi

A

ChR transport
on D4 rootlet

(B)

Eyespot assembly

Chloroplast

B

stroma

EYE2

ChRs

M4 D2

M2 D4

Pigment
granules

Connecting
protein

D4
rootlet

PM

CE

Figure 1.16 Eyespot placement and assembly in Chlamydomonas. Left: Diagram showing
the cytoarchitecture of a Chlamydomonas cell. The basal bodies (small blue circles) nucleate
the flagella and sets of two- and four-membered microtubule rootlets. The M2 and M4 rootlets
are inherited from the mother cell, whereas the D2 and D4 rootlets are newly formed in the
daughter cell. The asymmetrically placed eyespot (large orange ellipse) is associated with the
D4 rootlet and positioned near the cell equator. (A) Simplified working model of asymmetric
photoreceptor localization. Channelrhodopsin (ChR) photoreceptors traffic on endomembrane
vesicles from the Golgi to the plasma membrane. MLT1 directs ChRs to the daughter side,
where photoreceptors are transported by motor proteins (dark violet ellipses) along the D4
rootlet. (B) Simplified working model of eyespot assembly. ChRs form a patch in the plasma
membrane. Rootlet or ChR-associated cues guide formation of a patch of EYE2 in either the
inner or outer chloroplast envelope membrane, whose nucleates form eyespot pigment granule
arrays. Specialized proteins in the eyespot establish and maintain connections between the
chloroplast envelope and plasma membrane. CE, chloroplast envelope; PM, plasma membrane
( Boyd et al., 2011b ).

that they receive, process, and store information from the environment. No evidence
suggests that the genome is involved in these processes. The fact that the beating
of cilias is perfectly coordinated (i.e., they beat successively in the same direction)
indicates that electric impulses are generated and are sequentially transmitted to
cilia throughout the cell. Ciliates resemble chemosensory neurons in their responses
to stimuli ( Hufnagel, 2008 ). Unicellular prokaryotes, such as bacteria, also display
chemotaxis toward nutrients and are even capable of assessing levels of nutrients
in the environment along with their stress level and that of other bacteria ( Norris
et al., 2011 ).