Nutrition of Green Plant Cells - 165 



the light had acted for as little as one minute, 

 radiocarbon could be detected in a variety of 

 compounds — not only in sugars, but also in 

 amino acids and other substances. However, 

 when the illumination was cut to 5 seconds, 

 or less, almost 80 percent of the fixed radio- 

 carbon was found in just one compound, 

 namely phosphoglyceric acid (Fig. 9-6). Ap- 

 parently this 3-carbon compound is among 

 the first to be formed during photosynthesis. 



Naturally, the identification of phospho- 

 glyceric acid (PGA) as an early product of 

 CO;, fixation led to the assumption that this 

 compound must be a precursor of glucose 

 (see Fig. 8-5). This assumption has proved 

 correct. Essentially, the build-up of glucose 

 and other products represents a reversal of 

 the steps by which these compounds are 

 broken down in cells generally. Energy and 

 matter are required, however, for the climb- 

 ing of this uphill path and these are provided 

 by ATP and TPNH 2 , previously generated 

 by light. First the PGA molecule is energized 

 by phosphorylation, forming diphosphogly- 

 ceric acid, at the expense of a molecule of 

 ATP. This energized molecule undergoes 

 reduction by TPNH 2 , forming triose phos- 

 phate and releasing inorganic phosphate to 

 the medium. The further steps, leading 

 finally to starch, also follow the familiar 

 pathway of cellular metabolism, as is shown 

 in Figures 8-5 and 9-6. Moreover, the addi- 

 tional requirements of energy and of elec- 

 tron-hydrogen couplets are provided by ATP 

 and TPN ■ H 2 , respectively. 



It is important to realize that carbohy- 

 drates are not the only products formed 

 during the dark period after a green plant 

 cell has been illuminated — as is indicated in 

 Figure 9-6. Radiocarbon soon finds its way 

 into various amino acids and also into fatty 

 acids. PGA appears to be an intermediary in 

 these reactions. Moreover, the reducing po- 

 tential of light-generated TPN-H 2 is essen- 

 tial for the conversion of the oxidized 

 (nitrate) form of nitrogen ( — N0 3 ), which is 

 absorbed by the plant, into the reduced 

 (amino) form ( — NH 2 ), which is found in the 



amino acids. In short, the energy stored as 

 ATP and TPNH 2 by the photochemical 

 mechanisms of the green plant, although 

 mainly utilized for carbohydrate synthesis, is 

 also employed for other syntheses. 



The Calvin cycle of carbon dioxide assimi- 

 lation is not very simple, however. It is so 

 complex, in fact, that only a few highlights 

 can be given in this brief account. 



PGA proved not to be the first compound 

 formed when carbon dioxide undergoes re- 

 duction 0=C=0 -f e~ -f H+ -» — C— OH 



II 

 O 



which must occur when C0 2 is incorporated 

 into an organic molecule. The C0 2 acceptor 

 proved to be a doubly phosphorylated 5- 

 carbon sugar, ribulose diphosphate (Fig. 

 9-6). When the green algae previously satu- 

 rated with light and C0 2 were suddenly de- 

 prived of C0 2 , there was an accumulation of 

 ribulose diphosphate, but not of phospho- 

 glyceric acid — as was first shown by Alex 

 Wilson in the Calvin Laboratory. This result 

 led to the proposal that the first step in C0 2 

 fixation is by the carboxylation of the very 

 active compound, ribulose diphosphate (Fig. 

 9-6). As a result of carboxylation, apparently, 

 a 6-carbon diphosphate intermediary is 

 formed. This diphosphate intermediary ap- 

 pears to be exceedingly unstable, however, 

 and it has not been isolated. It immediately 

 breaks down, yielding two molecules of 

 PGA. PGA, therefore, is not the first, but 

 the second, product formed, subsequent to 

 the primary fixation of C0 2 . 



Other phases of the complex Calvin cycle 

 can only be mentioned here, although they 

 have been worked out in considerable detail. 

 Replenishment of the supply of ribulose di- 

 phosphate is achieved from part of the PGA, 

 as this intermediary continues to be pro- 

 duced. In other words, not all of the PGA is 

 utilized for the synthesis of glucose and other 

 organic substances. A considerable part is 

 returned to ribulose monophosphate, which 

 then, at the expense of ATP, is converted to 

 the active C0 2 fixer, ribulose diphosphate. 



