THE PHOTOCHEMICAL NATURE OF THE PHOTOSENSORY 



PROCESS. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton University, 



Omaha.) 



(Received for publication, November 21, 1919.) 

 I. 



Many animals which are sensitive to light respond by an invariable, 

 characteristic reflex. Typical of such organisms are the ascidian, 

 Ciona intestinalis, and the common, North Atlantic, long-neck clam, 

 Mya arenaria (Fig. 1). Both of these animals, when illuminated, re- 

 spond by a vigorous retraction of the siphons. The properties of 

 their sensitivity have been investigated to some extent, and have 

 been described in a series of papers (Hecht, 1918-19, a, b, c, d). As a 

 result, an hypothesis has been suggested which accounts for this type 

 of irritability in terms of an underlying, chemical mechanism. 



In its essentials this hypothesis involves the behavior of two proc- 

 esses: one, a reversible photochemical reaction; the other, an ordinary, 

 simple, chemical reaction. The light acts on a photosensitive sub- 

 stance S and decomposes it into its two precursors P and A . The de- 

 gree of sensitivity of the sense organ depends, not on the quantity of 

 photosensitive substance S, but on the concentration of its precursors 

 P and A . Because of this, the amount of fresh precursors necessary 

 for a response is always a constant fraction of the amount of precur- 

 sors already present in the system. The fresh precursors serve to 

 catalyze the simple, chemical conversion of an inactive substance L 

 into one T which then initiates the nervous impulse. This eventually 

 leads to a contraction of the siphon musculature. The reaction sys- 

 tem as a whole may be expressed in the following equations: 



S^P + A; L\\P + A\\-^T 



in which || P -|- -4 I| means catalysis by P or ^, or both. 



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