RESPONSES OF ORGANISMS TO LIGHT 



It would be out of place to enter fully into the mechanism of photosynthesis 

 by chlorophyll here ; for a recent summary the reader is referred to the mono- 

 graph by Hill and Whittingham.i The chlorophyll group of pigments are tetra- 

 pyrrolic compounds in which magnesium is present in non-ionic form ; they are 

 related to hgemin which, however, contains a central iron atom. The completed 

 process whereby carbohydrates are synthesized has long been known and may 

 be represented by the equation : 



.rCOj + .rHaO + radiant energy -^ Ca;H2j;0a; + .rOj + stored energy. 

 The intimate mechanism, however, has only recently begun to be analysed, an 

 advance largely due to the use of radio-active carbon (i*C) as a " tracer ". 

 Although many of the details are still obscure, particularly the way in which 

 chlorophyll absorbs radiant energy and directs it into chemical processes, the 

 basic reactions are known and can indeed be carried out in the test-tube. The 

 essential process is the photolysis of water. Chlorophyll induces the energy 

 derived from light to break the hydrogen-oxygen bonds in the molecule of water ; 

 the hydrogen therefrom is used to convert the single carbon atoms of CO 2 into 

 long-chained carbohydrates through the medium of phosphoglyceric acid and 

 the oxygen is liberated as a free gas ; meantime a store of chemical energy is 

 provided by the photosynthesis of energy-rich compounds such as adenosine 

 triphosphate, the break-down of which by simple hydrolysis releases large 

 amounts of energy to drive the process. It is probable that these and the many 

 other compounds fovind in jjlants are formed by enzyme-reactions from one or 

 more of the constituents of the photosynthetic cycle at either the C3 or Cg level. ^ 



Apart from this basic activity which characterizes the vegetable 

 world, light produces photochemical reactions of great variety in 

 living organisms. The energy thus liberated produces in the most 

 primitive creatures the only response available — a change of general 

 activity, frequently of motion, just as do other stimuli, mechanical, 

 gravitational, thermal, chemical or electrical ; in the higher forms a 

 multitude of activities may be initiated or influenced. 



These responses we will review under four main headings. In the 

 first place, the response may take the form of a change in general 

 metabolic activity, usually, but not invariably, an increase of activity 

 under the influence of light. As a natural extension of this, the diurnal 

 cycle of light and darkness has in the course of evolution so impressed 

 itself upon a number of the fundamental activities of many organisms 

 (including man) that these show a corresponding rhythm which has 

 eventually become innate and endogenous (photoperiodism). In the 

 second place, the response may be expressed as a variation in movement. 

 In its simplest form this is also merely a change in general activity 

 wherein movements are random in nature and undirected (photo - 

 kinesis) ; as an evolutionary extension of this the movements initiated 

 by light come under the directional control of the stimulus so that the 

 organism is orientated by light in a definite way ; such movements 



1 Photosynt}(e.sis. London, 1955. See also Proc. roy. Soc. B, 157, 291 (1963). 



2 For reviews, see Arnon (An?i. Rev. plant Physiol.. 7, 325, 1956, Nature (Lond.), 

 184, 10, 1959), Rosenberg (Ibid., 8, 1957). 



