Cross photoreceptors

Equation (1) predicts that at low light intensities mutants of type (i) or (ii) have a raised threshold, since either reduction of a or Ptot can be compensated by raising I (Fig. 16). At high light intensities unbleached photoreceptor becomes limiting and one has to take into consideration the ratio Pbi/Ptot this ratio depends on the intensity I and the cross section a ... 

At high intensities this ratio becomes unity (all photoreceptor is bleached) and therefore no phototropism can occur. Equation (2) shows that the bleaching of the photoreceptor does not depend on the absolute concentration of photoreceptor. Therefore mutants with less receptor pigment will be inhibited at the same light intensity as the wild type. However, in mutants with a reduced cross section the ratio of Pbi/Ptot reach unity at higher intensities. Hence they are phototropic active at intensities where the wild type is already inhibited. The third type of mutant (iii) which has a slower regeneration as the wild type should have a normal sensitivity at low intensities but should be phototropical insensitive at higher intensities at which the unbleached... 

Fig. 16. Phototropic threshold of wild type and hypothetical photoreceptor mutants. Solid line = wild type large dashes = mutant with reduced number of photoreceptor dotted line = mutant with reduced absorption cross-section small dashes = mutant with slow regeneration. The changes of threshold of the hypothetical mutants were chosen arbitrarily. The figure was adapted to Fig. 6 and the solid line of the wild type represent data of Foster and Lipson (1973)...

Fig. 1. Schematic diagram of a horizontal cross-section of the eye and retina (A) and of the photoreceptor cells (B).

Figure 3 Cross section of (a) mono- and (b) dual-layer photoreceptors.

Figure 4 A cross-section schematic of the single-layer photoreceptor configuration. Thicknesses of single-layer photoreceptors are typically 10 to 15 pm.

Figure 5 A cross-section schematic of the dual-layer photoreceptor configuration. Transport layer thicknesses are typically 15 to 30 pm. Generation layer thicknesses are usually between 0.5 to 5.0 pm.

Enokida et al. (1991) reported that ultraviolet-induced fatigue of a photoreceptor using poly(methylphenylsilylene) (PMPS) as the transport layer could be explained by photodecomposition of the transport layer. The PMPS was both decomposed and cross-linked by the ultraviolet exposures. 

Figure 2. Schematic representations of a prototypic cilium and the photoreceptor cilium in comparison. (A) Scheme of a prototypic cilium, in longitudinal extension and cross sections through subciliary compartments axoneme (9x2 + 2 microtubule arrangement), transition zone (9x2 + 0 microtubule arrangement) and centriole (9x3 + 0 microtubule arrangement) of the basal body. (B) Scheme of the ciliary part of a rod photoreceptor cell. Axonemal microtubules (MX) project into the outer segment (OS). The OS is linked via the connecting cilium (CC) to the inner segment (IS). The CC corresponds to the transition zone of a prototypic cilium. The basal body complex (BB) is localized in the apical region of the IS. The calycal process (CP) of the IS is linked by extracellular fibers with the membrane of the CC.

Spira, A. W. and Milman, G.E. (1979) The stmcture and distribution of the cross-striated fibril and associated membranes in guinea pig photoreceptors. Am. J. Anat. 155, 319-337. 

Figure 2. A schematic microscopic cross-section of a dual-layer photoreceptor. (Reprinted with permission from Ref. [3o].)...

Figure 5.3C shows a cross-section of the retina, but this tissue came from a donor who had lost useful vision due to a retinal degeneration. There is a complete loss of photoreceptor outer segments, the almost complete absence of cells in the ONL (i.e., photoreceptor inner segments). However, the inner retinal cells are preserved in substantial numbers. 

The second line of evidence relates to more recent metabolic innovations such as photosynthesis. LUCA, it seems, could not photosynthesize. No form of photosynthesis based on chlorophyll is found in any archaea. A completely different form of photosynthesis, based on a pigment called bacteriorhodopsin, similar to the photoreceptor pigments in our eyes, is practised by the so-called halobacteria, archaea that live in high-salt conditions. This mode of photosynthesis is not found in any bacteria. These disparate forms of photosynthesis presumably evolved indepen-dently in bacterial and archaeal lineages some time after the age of LUCA, and subsequently remained tied to their respective domains. If a metabolic innovation as important as photosynthesis did not cross from one domain to another, there is no reason to think that other forms of respiration would have done so. We should certainly be wary of postulating that respiratory genes crossed domains unless we have evidence that they did so and the evidence from evolutionary trees suggests that they did not. 

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