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