108 



ribulosc diphosphate (RuDP) as the 

 precursor. 



Curioush', RuDP is a more formid- 

 able structure than PGA. It is nearly 

 twice as large, and you may wonder 

 where a primar)' building block of such 

 size comes from. "From the plant," an- 

 swers Dr. Cahin. "We start with a 

 functioning green plant, and its cells 

 are alread}' stocked with all the ingredi- 

 ents of living matter." These include 

 first of all the indispensable enzymes, 

 those specialized structures which assist 

 or chaperone the reactions of other 

 molecules. Also present are various pro- 

 teins, acids, alkalis, salts, and sugars, 

 and among the sugars is this RuDP. 

 Thus, to make sugar, the plant must 

 already have some sugar as well as the 

 necessary enzymes and other essential 

 substances. 



As the sequence starts, the five- 

 carbon RuDP joins with the one-car- 

 bon CO2 to form a six-carbon mole- 

 cule. This initial product comes under 

 the chaperonage of an enzyme and 

 promptly breaks in half to form two 

 similar three-carbon structures— two 

 molecules of PGA. Here, then, is the 

 origin of the PGA. The hydrogen— 

 which chlorophyll split off from water 

 in the light phase — now enters the cy- 

 cle. Accompanied by an input of chem- 

 ical energ)', the atom of hydrogen joins 

 the three-carbon PGA to form a three- 

 carbon sugar known as triose. Thus the 

 first sugar product is achieved, and 

 from then on the plant proceeds to 

 fabricate glucose, sucrose, and the other 

 forms of carbohydrate by using triose 

 molecules or parts of them as building 

 blocks. 



This radioactive tracking down of 

 the path of carbon in photosynthesis 

 is a triumph of chemical detection. Al- 

 though at first Calvin's results were 

 challenged and in some quarters con- 

 tradicted, they are now universally ac- 

 cepted, and he has turned his attention 

 to the light phase of photosj-nthesis. 



PHYSIOLOGY 

 SECRET OF THE GREEN BODIES 



Economists predict that within a 

 decade or two the demand for basic 

 food for man and domestic animals will 

 be at twice the world's present produc- 

 tion of agricultural products. The chron- 

 ically hungry in Asia, Africa, and parts 

 of Europe and the Americas number 

 around a billion people, and how to 

 feed more and more billions is a prob- 

 lem that cannot much longer be left 

 to the chance conjunction of whatever 

 quanta happen to fall on whatever 

 molecules of chlorophyll that may be 

 exposed in farm or orchard. 



"All agriculture seems to be a prim- 

 itive, medieval, if not archaic process, 

 even if we use tractors," declared bio- 

 chemist Albert Szent-Gyorgyi in a re- 

 cent preview of the shape of things to 

 come. "To wait until plants grow and 

 develop their chlorophyll and accumu- 

 late the energy seems ridiculously prim- 

 itive and slow at our present rate of 

 scientific potential. Why cannot we 

 rather construct a 'chlorophyll bomb' 

 to blow up need and poverty?" 



Wliat Dr. Szent-Gyorg]i'i is propos- 

 ing, I take it, is the transfer of the 

 photosynthetic process from the green 

 plant's living cell to the chemist's non- 

 living test tube. This is a dream that 

 harks back through seventy-five \'ears 

 of experimentation; and failure after 

 failure of these efforts confirmed the 

 conclusion that photos\nthesis is a 

 business of living matter that cannot 

 be separated from the intact cell. It 

 was so written in the textbooks. Never- 

 theless, chemists continued to break up 

 plant tissue, to recover the green stuff, 

 and to test the behavior of these frag- 

 ments in light. 



When a leaf cell is examined micro- 

 scopically, the eye sees at once that its 

 green color is not diffused throughout 

 the protoplasm but is concentrated in 

 small floating bodies. These are called 

 chloroplasts. Further dissection reveals 



