Thesis-PDF - IAP/TU Wien

Figure 4.9: Measured monoclinic unit cell of the photoreceptor crystal.
a = 8.9 nm, b = 7.7 nm, c = 8.3 nm, β = 110 ◦ .
dimensional structure, especially how the chromophore pockets are distributed
within the stacked layers (the scheme in Fig. 4.10 shows a possible arrangement).
The TEM image in Fig. 4.6 gives an indication of the spacing and the number of
these layers and counts to the best data available to date.
Signal transduction
Although the interplay of the photoreceptor and the stigma constitutes the most
vital part of the visual system of Euglena, the detection part, the system would
be useless without a corresponding signal transmission mechanism.
The flagellum allegedly plays the key role in signal transduction and finally
receives locomotory commands depending on light stimulus. When a light stimulus
promotes chromophore proteins of the photoreceptor from the ground state to the
excited state, a chain of events is triggered.
The piezoelectric theory is one of the theories how the information of a photon
detection is passed on. The core of this theory is the assumption that the
photoreceptor due to its crystalline nature acts as a capacitor, relaying electrons
to the paraxial 11 rod (PAR) to which it is attached. The photon flux acting on
the pigment mass would induce a flux of electrically charged particles that in turn
generate a signal in the form of an electrical potential. As the PAR contains fibers
that potentially can contract and act upon a flagellar response to the stimulus.
The present microtubuli and ATPase activity could provide further response to
this initial stimulus and it was suggested that they even do a rudimentary form of
information processing (see below). ([82], [99])
11 The PAR is a thin rod between the main core of the flagellum and the photoreceptor swelling.
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