757 



SUMMARY 



a hexose molecule prior to the production of PGA. If so, 

 this in turn reopens the question as to whether TPNH is the 

 reductant in this phase of carbon fixation. The data of 

 Krogmann indicate at best one TPN or two DPN molecules per 

 photosynthetic unit in chloroplasts made by non-aqueous pro- 

 cedures, and this represents almost all of the TPN and half 

 the DPN of the leaf. These figures suggest either that pyri- 

 dine nucleotides are fully engaged in carbon fixation, or 

 that they are simply bypassed. The question will not be 

 fully answered, however, until the details of the initial 

 carboxylation and reduction steps are elucidated (Bassham, 

 Gibbs). At the present time, it appears highly suggestive 

 that some sulfhydryl inhibitors (lipoic acid, arsenite) 

 affect carbon fixation in the whole system although they are 

 not knovjn to inhibit any of the enzymatic steps established 

 for the Calvin cycle in vitro, 



A new direction in carbon metabolism has been clearly 

 established for the two carbon pathway, starting from phos- 

 phogly colic acid (presumably arising from pentose phosphate) 

 (Tolbert, ''hittingham) . Under the usual natural conditions-- 

 high light intensities, ZQ% or njore oxygen, low GO2 concentra- 

 tions — 50% or more of the carbon flowing through photosynthe- 

 sis can be shown to be running through this previously little 

 explored two carbon pathway. A major exception to the 

 generally accepted viev;s on the pathway of carbon and indeed 

 most other areas of photosynthesis, remains in the closely 

 woven concepts of Warburg, lucidly described by Vennesland. 



iVhile this survey has so far considered analyses of path- 

 ways, further advances depend entirely upon a more complete 

 knowledge of the participating constituents. For instance, 

 plastoquinone A, implicated in electron transport by experi- 

 ments involving extraction with hexane and re-addition, 

 already appears to have two components active at two sites 

 (Trebst). The occurrence of as many as eight structurally 

 different quinones, most of them showing activity in recon- 

 stitution experiments suggests a more complex set of path- 

 ways than so far envisioned (Dilley). »'ork is proceeding 

 on isolated cytochromes (Forti, Perini), and iron has been 

 shown to be the electron carrier of ferredoxin (Fry). 

 Properties and possible functions of plastocyanin are under 

 investigation (Katoh). The nature of electron transport in 

 the chloroplast surely depends on the existence of both 

 aqueous and non-aqueous phases; hence, the fundamental 

 importance of knowing the nature and function of the abundant 



