224 A. ARVANITAKI AND N. CHALAZONITIS 



pigments in the photoreceptors' outer segment) is high enough to justify an 

 approach to excitation and to photo-physical processes from the standpoint 

 of phenomena in the sohd state physics, as first suggested by Szent- 

 Gyorgyi (1941), and worked out in the case of dye molecules aggregates 

 and of chlorophyll molecules in the chloroplast (Bradley and Calvin, 1955; 

 Calvin, 1955, 1959; Mac Rae and Kasha, 1958; Rabinowitch, 1958; and others; 

 among earlier studies, Kautsky and Hirsch, 1931). 



The absorption of a light quantum by a chlorophyll molecule raises it to 

 its excited state; the excited state "wanders around as an exciton" until it finds 

 apoint of ionization leading to the creation of a "conduction" electron leaving 

 a "hole" behind. The pair, excited electron and "hole", might move and be 

 found in a site possibly far removed from the original locus of the excitation. 

 This migration theory or energy transfer as proposed by Calvin requires the 

 presence of a relevant cytostructural, highly organized apparatus such as 

 that of the lamellae and grana in the chloroplast where chlorophyll and caro- 

 tenoids are highly concentrated and incorporated in orderly arrays. Such 

 migration of energy might permit, as has been seen in solutions of certain 

 dyes, transfers at distances of nearly one m.icron from the site of excitation. 



The binding energy of the electron-"hole" pair being small, under the 

 mere influence of thermal motion, or the pull of an applied electric field, 

 electron and "hole" might be thrown into separate traps, where they could 

 survive for relatively long periods of time. The whole picture is consistent 

 with the semi-conducting properties found in various dried, organic sub- 

 stances including haemoglobin (Inokuchi, 1951; Eley, Parfitt, Perry and 

 Taysum, 1953; Cardew and Eley, 1959), with the photoconductivity of dried 

 chloroplasts and moreover with the demonstration of the separation of 

 electric charges on illumination of the junction between chlorophyll and 

 carotene. The potential difference (chlorophyll negative with respect to caro- 

 tene) may then be as high as 600-1-300 mV (Arnold and Maclay, 1959). 

 The trapped "holes" can be carried away by some electron donor molecule 

 to form chemical radicals; the trapped electrons are picked up by suitable 

 carriers and transferred to acceptors such as pyridine nucleotides or others. 

 Such space separation of micro-oxidant from micro-reducing loci would be 

 the point at which photoenzymatic processes enter the picture. 



Applying the above to the illuminated pigmented nerve cells, cardiac 

 cells, stained axon and even to the retinal photo-receptor, we might envisage 

 the primary photophysical process as the creation of myriads of micro- 

 photobatteries in the cellular sites where photons are absorbed. Proceeding 

 from the creation of micro-oxidant and micro-reducing loci, transitions might 

 be induced into the nearby respiratory chain. In fact data are available indicat- 

 ing a high acceleration of electron transfer in the respiratory chain, in illu- 

 minated cells (Chalazonitis, 1954): 



