818 LIGHT AND LIFE 



bly lived at the expense of organic compounds formed through the 

 action of ultraviolet radiation on a mixture of ammonia, methane, 

 water, hydrogen, and carbon monoxide, in the "primeval soup." 



To trap the energy of the light for useful purposes — to conduct 

 photosynthesis — is of course the primary activity in which life de- 

 pends upon light. The endeavor to understand how the absorption 

 of a quantum of light leads through excited states of atoms to some 

 chemical reaction and how a flow of electrons may be used in a living 

 system to produce some reduced pyridine nucleotide and some adeno- 

 sine triphosphate, which together make possible the reduction of 

 carbon dioxide to carbohydrate, forms the essence of the modern ex- 

 ploration of this basic life process. 



Photosynthesis makes important the orientation of the organism 

 in the environment. The orientation of the photosynthetic organism, 

 or its parts, to the light (phototropism and photoperiodism) is 

 matched by the orientation manifested by heterotrophs that seek out 

 their food through sensitivity to light (phototaxis) . Predation, and 

 the avoidance of predation, led in the course of evolution to vision; 

 and once organisms could see, there emerged now and again some 

 biological use for the primitive, and usually wasteful, capacity of or- 

 ganisms to emit light (bioluminescence) . Vision, refined to a dis- 

 crimination of colors, made it possible through natural selection for 

 pigments to be utilized in a new way, as invitations to visitors, or 

 warnings to predators; and pattern enhanced by color became the 

 basis of myriad mutual relationships between flowering plants and 

 insect or bird pollinators, between predators and prey, between trees 

 and tree-dwellers, etc. Recognition of one's own species, and of sex, 

 became instant. Social groupings sprang up, knit by visual as well 

 as chemical signals. Eventually man stood upright, looked about, and 

 wove his conceptions into ever more elaborate patterns. From being a 

 light-worshipper, he became in time a seeker after light, an explorer of 

 the nature of light and its chemical effects, of the way in which 

 chlorophyll traps the energy of light and turns it to such use that 

 thereby the world is fed and nourished. This Symposium presents 

 some of the latest advances and newest insights into the biochemistry 

 and biophysics — the biology — of light. 



Molecular Structure and Excited States 



The effect of light on living systems involves the absorption of 

 (juanta of radiation of apjirojiriate wave-lengths and the promotion 

 of photochemical reactions. Requisite to an understanding of these 



