116 Inside the Living Cell 



have shown that at the limit of detection, not more than a very few 

 quanta are actually absorbed in the rods in order to produce a 

 detectable sensation of light. Because one nerve serves an appreciable 

 area, this kind of vision is not very sharp. 



The cones which sometimes have an optic nerve to themselves and 

 sometimes share an optic nerve in groups of two or three, provide 

 for a sharply discriminating kind of vision under conditions of good 

 illumination. They are also sensitive to different kinds of light, as 

 they are responsible for colour vision. 



What does light do when it reaches the retina? It is absorbed by 

 a pigment called visual purple or rhodopsin, which can easily be 

 extracted from the eyes of animals and has been much studied in 

 recent years by R. A. Morton in Liverpool and by G. Wald in 

 Harvard. It has been found to be a protein combined with a special 

 light absorbing molecule. The latter is a near relative of Vitamin A — 

 thus explaining why deficiency of Vitamin A leads to night blindness. 

 Different animals make use of slightly different derivatives of Vitamin 

 A; beef rhodopsin uses Vitamin Ai and fish rhodopsin the shghtly 

 different Vitamin Ao. These molecules all have a row of alternating 

 single and double bonds, which give them their light absorbing 

 property. Molecules of this kind have one electron for each carbon 

 atom, called a 7r-electron, which is in a special state. Instead of being 

 anchored to one carbon atom as the others are, it is free to wander 

 up and down the carbon chain. The effect of the absorption of light 

 is to activate the --electrons, so that they take up the energy of the 

 light absorbed and acquire a state of high energy in which they are 

 able to escape from the light absorbing molecule into the protein 

 it is attached to. This results in the bleaching of the dye. Dr Wald 

 has described this change as follows : 'When rhodopsin is exposed to 

 light in the retina, two major changes occur, the carotinoid (the dye 

 molecule) is cleaved from the protein and is degraded through orange 

 intermediates, first to orange retinene 1, and then to colourless 



Rhodopsin (or visual purple) 



slowly ''ght^^-^^ dark 



in dark 



Vitamin Ai + protein — &— — Retinene + protein 



Retinene is the 'aldehyde' of Vitamin A 



