Thesis-PDF - IAP/TU Wien

Figure 4.7: Scheme of the photoreceptor photocycle (intermediate states
omitted). Upon absorption of light with a wavelength of around 365 nm the
rhodopsin-like protein changes into the excited state, then upon a second
absorption at a wavelength of around 436 nm it falls back again into the
ground state. This last step from the excited state to the ground state is
almost exclusively triggered by light. It occurs only once in a thousand years
through thermal deactivation at ambient temperature.
Sensitivity
For detecting the direction of the light of a specific range a photoreceptor demands
a high packing density of chromophore molecules organized in a lattice
structure, with high absorption cross section (high absorption probability) of the
chromophore and very low dark noise 10 . For transmitting the detected signal a
photoreceptor must generate an electrical potential difference, or an electrical current.
This is exactly what Euglena’s photoreceptor is designed for.
One of the most investigated photoreception systems is that of Chlamydomonas.
It consist of a patch of rhodopsin-like proteins in the plasma membrane.
The packing density of these molecules appears to be about 20 − 30, 000/µ 2 m of
membrane, with a molar absorption coefficient of 40 − 60, 000 M −1 ∗ cm −1 and a
dark noise of almost zero. The number of embedded molecules per µ 2 m of membrane,
the absorption, the absorption cross section, and the dark noise are at the
best of theoretical limits. Nevertheless, the fraction of photons absorbed from a
single layer of these molecules is less than 0.05%.
An estimate of how many photons this simplest but real photoreceptive system
can absorb is effectuated here. On a sunny day about 10 18 photons ∗m −2 ∗ s −1 per
nanometer wavelength are incident. On a cloudy day this lowers to 10 17 photons
∗m −2 ∗ s −1 per nanometer wavelength. Through absorption of the water column
this number is lowered to 10 17 and 10 16 respectively. Thus a photoreceptor of a
cross section of about 1µm 2 can catch at most 10 5 photons, 10 7 in its 100 nm
absorption band. Since only about 0.05% of these are effectively catched by the
protein monolayer, the maximum number lowers to 5 ∗ 10 3 . Now during revolving
motion of the algae it is illuminated for about 400 ms and shaded by the stigma
10 Dark noise is inherent to such a receptor and independent of light level. It arises from the
random thermal motion of the molecules.
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