Thesis-PDF - IAP/TU Wien

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