Biochemistry - The Chemical Reactions of Living Cells, Volume 3 - D. Metzler 1980

Light in Biology
Vision
Structure of the Rod Outer Segment

The photoreceptors of the eye perform Functions that are entirely different from those of METABOLISM/14.html">Chloroplasts. Visual receptors are designed to initiate nerve impulses; consequently, their primary characteristic is high sensitivity, with certain receptors capable of capturing virtually every photon striking them [133, 133a]. This function is served by multilayered membranes densely packed with highly absorbing molecules.

The human retina contains over 10⁸ tightly packed receptor Cells of two types: rods and cones. Rods are exceptionally sensitive cells capable of responding to as few as five quanta of light. Specialized for dim-light Vision, they provide a "black-and-white image" and are concentrated in the peripheral Regions of the retina. The less sensitive cones are located primarily in the central retina and are divided into three types distinguished by their spectral sensitivity, thereby mediating Color Vision.

Rods (Fig. 13-28), which have been studied in much greater detail than other retinal receptors, exhibit a remarkably high rate of metabolism. The rods of the human eye can function for a hundred years [134]. They undergo a remarkable renewal process in which older membrane discs at the tip of the rod are shed [135] and replaced by new ones generated near the nuclear region. The outer segment is bounded by a Cell/33.html">Plasma Membrane. Within this membrane (apparently without direct attachment to it) lie ~500 parallel stacked discs with a diameter of ~2 µm and a center-to-center spacing of ~32 nm. Each disc is formed by a pair of membranes ~7 nm thick, separated by a very narrow space. In electron micrographs, the intra-disc space appears sealed at the edges. Adjacent discs are separated by a somewhat wider interspace.

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FIG. 13-28. A. Diagram of The Structure of a vertebrate retinal rod [135]. OS — outer segment; CC — connecting cilium; M — densely packed Mitochondria; N — Nucleus; ST — synaptic terminal. B. Electron micrograph of a longitudinal section through a rod outer segment in the rat retina (courtesy of J. Dowling [133b]).

FIG. 13-29. Absorption spectrum of bovine rhodopsin dispersed in Water in the presence of a non-ionic detergent. [Shich H. et al., JBC, 244, 529—536 (1969).]

Disc membranes are composed of 60% protein and 40% lipid (Table 5-1). Approximately 80% of the protein fraction consists of rhodopsin (visual purple), a lipoprotein that is insoluble in water but soluble in detergent solutions. Digitonin is widely used to disperse rhodopsin molecules without altering their optical properties. Mammalian rhodopsin has a Molecular Weight of ~28,000–35,000; each molecule contains a single chromophore characterized by a λ_mах of 500 nm (Fig. 13-29). Because rhodopsin accounts for the bulk of the membrane protein, its molecules must be packed closely together. Freeze-fracture replicas of rod cross-sections reveal globular particles ~4–5 nm in diameter, which are too large to represent isolated rhodopsin molecules (unless each molecule is surrounded by a specialized lipid shell). Fluorescence energy transfer experiments by Wu and Stryer (Section B, 2) led to the Conclusion that the rhodopsin molecule is elongated (with a maximum dimension of 7.2 nm) and is capable of spanning the membrane. It has been proposed that the molecule possesses a large, nearly spherical headpiece located on the inner side of the membrane and a slender tail that traverses The Lipid Bilayer [136]. The chromophore is oriented such that the transition dipole moment vector is parallel to the plane of the discs (i.e., perpendicular to the direction of incoming photons).