Page 112 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 95

We need only one protein to build a device for information transfer and control, that is, a protein
with two conformational, alternate states such as intermediate filament proteins, which can form
lattice-like structures. The propagation of conformational changes along these proteins can be
used to transport or transduce sensory information. Each protein can be considered as a dipole
in one of the two possible states. We can imagine that the conformational change is transmitted
by one dipole to the neighbor proteins as a wave. Therefore, a current flow through these lattice-like
structures could be generated by the mobile electrons of the proteins that interact with their immedi-
ate neighbors via dipole–dipole forces.
The a-helical coiled-coils structural motif in the rod filament is well suited for electron propa-
gation (Figure 2.37b). A current might be propagated distally via PFR 1 and PFR 2 protein–protein
charge transfers in the lattice-like rod filaments. The current flows through these lattice-like struc-
tures and the sole constraint is that each lattice site should possess a dipole moment proportional to
the magnitude of the mobile charge unit and the distance over which it hops, that is, about 1 or 2 nm.
This wave produces a contraction in the PFR and a varying internal resistance that modulates the
flagellar beats. The contraction occurs by displacement of the goblet appendages of the PFR along
the axonemal microtubules, which reduces the distance between the coiled filaments hence gener-
ating longitudinal waves of contraction along the paraxial rod. The stiffening should swing the
flagellum sideways, damping out some undulatory waves of the axoneme.

PHOTORECEPTOR APPARATA

Life is essentially about information, how information is perceived, how it is stored, passed on and
used by organisms as they live and reproduce. In the world of photosynthetic microorganisms,
where virtually all life depends on solar energy, light becomes also a source of information,
used to orient microorganisms spatially and to guide their movements or growth. Responses
using light as a sensory stimulus for orientation towards areas that best match their individual
irradiation requirements are thus a virtually universal behavior among algae. The full exploitation
of light information necessitates proper perceiving devices, able to change the small signal rep-
resented by the light falling upon them in a larger signal and response of an entirely different phys-
ical nature, that is, these devices, termed photoreceptors, must perform perception, transduction,
amplification, and transmission.
The processing of a photic stimulus and its transformation into an oriented movement can be
considered the “vision” phenomenon of motile algae. True vision involves production of a
focused image of the external world, and the optical requirements for an eye probably cannot be
satisfied by algae, requiring true multicellularity with cell specialization and division of labor.
Still, algal “eyes” have many similarities with the complex vision systems of higher organisms,
because they do possess optics, photoreceptors, and signal transduction chain components.
The essential elements of these basic visual systems are the shading device(s), for example, the
eyespot and the detector, that is, the true photoreceptor(s). When the eyespot is absent its function is
performed by the whole algal body.
The eyespot is a sort of roundish shield, inwardly or outwardly concave, made up of one or
more layers of lipidic globules closely packed. These globules contain mainly carotenoids that
can play the shading role due to their strong absorbance in the 400–500 nm range. The most
common type of photoreceptor consists of extensive two-dimensional patches of photosensitive
proteins, present in the plasma membrane in close association with the eyespot. Very often the
photoreceptor cannot be identified by optical microscopy, while the eyespot can be seen easily
because of its size and color, usually orange-red. This is one of the main reasons for the plethora
of data present in the literature on the morphology, composition, and ultrastructure of algal eyespots
with respect to the few data available on their photoreceptors.
Ultrastructural studies over the past 40 years have shown that photosensory systems in algae
have certain characteristics in common, and distinct types of these apparatus can be identified.