EFFECTS OF IONIZING RADIATION ON VISION 535 



if the rhodopsin is in the presence of water and heat (Wald et al., 1950). 

 Presumably, the water and heat permit ions and molecules to move more 

 rapidly to their reaction site, the freed bond sites on the retinene and opsin, 

 and even furnish some of the energy needed for the reactions. It is sug- 

 gested that these reactions catalyze other reactions and so make large 

 numbers of reacting particles available for further reactions. 



Rhodopsin is found only in the outer segment of the rod cell, where it 

 forms part of a submicroscopic lattice. The outer segment has an outer 

 membrane consisting of a layer of aqueous protein molecules next to a 

 layer of lipoprotein molecules with the rhodopsin molecules lying in between 

 (Wolken, 1958). This membrane is stacked up in folds for the entire length 

 of the outer segment and continues into tubules (like those found within 

 cilia), which extend into the rod's inner segment (De Robertis, 1956; Sjo- 

 strand, 1960). It can be estimated from data on rods taken from cattle that 

 the human rod's outer segment contains several million rhodopsin mole- 

 cules (Hubbard, 1954). The remarkable thing is that light absorption by 

 any one of these molecules, whatever its position, can cause activity to spread 

 from that rod to adjoining neurons and on through the visual system. 



The spread of activity, even just along the rod's outer segment, is too 

 fast to be attributed to diffusion, so another mechanism must be found. 

 There is no doubt that the peculiar structure of the outer segment membrane 

 is concerned. In vertebrate and arthropod eyes and in chloroplasts of 

 plants, photopigments have always been found arranged in similar repeat- 

 edly invaginated continuous membranes (Wolken, 1958). My guess is that 

 the isomerized rhodopsin molecule, which in every case lies at the boundary 

 between the inside and outside of the outer segment, catalyzes the formation 

 of free electrons or electron holes. These may then be rapidly transferred 

 in crystal fashion along the membrane lattice to the inner segment. In 

 support of this suggested mechanism is the finding that illumination of 

 chloroplasts causes the formation of electron spin resonances (Tollin et al., 

 1958). Whatever the actual mechanism, it is apparent that the submicro- 

 scopic organization of the cell has made possible the rapid transfer of energy 

 along large portions of the cell. 



At the cellular level little is known about the activity transfer mechanisms 

 of the vertebrate. The mechanisms of the somewhat simpler compound eye 

 of the horseshoe crab, Limulus polyphemus, probably acts as follows (Lipetz, 

 1960b). When the photoreceptor is illuminated, it releases a substance that 

 changes the surface membrane's permeability on the adjoining dendrite of 

 a neuron. This changed permeability permits positive ions to flow into the 

 dendrite from the higher concentration outside the neuron, and this in turn 

 causes a net outward flow of similarly charged ions through the cell mem- 

 brane at the neuron's cell body and axon. This ion flow through the axon 



