coordinated metal compounds, particularly metalloporphy- 

 rins, as represented by heme and chlorophyll. He began a 

 study on the electronic behavior of such compounds at that 

 time. When Professor Calvin joined the Chemistry Depart- 

 ment at Berkeley, these studies were encouraged by Professor 

 Gilbert N, Lewis, and they have been continued to the present 

 time. In time they will contribute to our understanding of the 

 precise way in which chlorophyll and its relatives accomplish 

 the primary quantum conversion into chemical potential, 

 which is then used to drive the carbohydrate synthesis. 



It has long been known that the reduction of carbon di- 

 oxide to carbohydrate is probably a dark reaction, separate 

 from the primary quantum conversion act. This knowledge 

 stemmed from the early work of F. F. Blackman on the dark 

 reactions of photosynthesis and its interpretation by Otto 

 Warburg, and particularly from the comparative biochemical 

 studies of Cornelius van Niel. Finally, Robert Hill separated 

 the photo-induced production of molecular oxygen chemi- 

 cally and physically from the reduction of carbon dioxide 

 when he demonstrated oxygen evolution by illuminated 

 chloroplasts, using ferric ion as an oxidant in place of carbon 

 dioxide. 



We can summarize the over-all conversion of light energy 

 into chemical energy in the form of carbohydrate and 

 oxygen by several steps. First, the light energy absorbed by 

 chlorophyll and related pigments is converted into the high 

 chemical potential energy of some compounds. Second, these 

 compounds react with water and produce oxygen and good 

 reducing agents as well as other cofactors containing high 

 chemical f)otential energy. Finally, these reducing and ener- 

 getic cofactors react with carbon dioxide and other inorganic 

 compounds to produce organic compounds. 



One of the principal difficulties in studying the synthetic 

 pathway is that the machinery which converts carbon dioxide 

 and minerals to organic compounds is itself composed of 

 organic compounds made up of the same elements. Ordinary 

 analytical methods do not allow us to distinguish easily be- 



VI 



