Thesis-PDF - IAP/TU Wien
Thesis-PDF - IAP/TU Wien
Thesis-PDF - IAP/TU Wien
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Figure 4.6: A photoreceptor cross section as imaged by TEM. Note the<br />
horizontal striations attributed to the regular layered structure of the photoreceptor<br />
- it consists of about one hundred layers stacked on top of each<br />
other. Image adapted from [87]. Scale bar is 0.1 µm.<br />
triggered almost exclusively by light. In the dark it occurs only about once in a<br />
thousand years.<br />
The isomerization and possible conformational changes of the protein follow a<br />
photocycle and are therefore repeatable. The photocycle leads through a series of<br />
conformational changes from the initial state to an excited state and back again.<br />
Usually also a number of intermediates can be identified. The photocycle of the<br />
chromophore in the photoreceptor of Euglena shows such a cycle including a ground<br />
and an excited state, (the so-called optical bistability is visualized in Fig. 4.7).<br />
As different conformational states possess different fluorescence characteristics, a<br />
photoreceptor whose chromophore proteins are mostly excited differs in emission<br />
from a photoreceptor containing chromophore protein mostly in the ground state<br />
(see Fig. 4.8).<br />
When the excited state is reached, the signal transmission is triggered and the<br />
cellular signal transduction pathway is responsible for amplification of the signal,<br />
signal processing and delivery.<br />
The time it takes for the whole photocycle to complete is only in the order<br />
of microseconds or less. The isomerization alone is one of the fastest processes<br />
ocurring in nature, completing in only about 200 femtoseconds. ([96])<br />
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