214 PLANT BIOCHEMISTRY 



Three clockwise trips around this cycle are needed to fix three 

 molecules of carbon dioxide and yield a net gain of one molecule of 

 3-phosphoglyceraldehyde. Actually, of course, none of these inter- 

 mediates accumulates extensively during photosynthesis. Instead the 

 extra material is rapidly converted to other carbohydrates, especially 

 sucrose and starch. Since reactions of 3-phosphoglyceraldehyde lead 

 in so many different directions, the carbon dioxide fixed is rapidly 

 distributed among several different intermediates. 



Carbon dioxide is also fixed by reversal of some of the normal 

 equilibrium reactions of respiration. The extent to which these 

 processes operate in living cells is difficult to evaluate because ribu- 

 lose-],5-diphosphate fixes carbon dioxide even in the dark. This latter 

 contribution is limited unless TPNH is available for the reduction; 

 however, it does complicate the interpretation of tracer experiments, 

 making the extent of fixation by other mechanisms uncertain. These 

 secondary systems are indicated as reversible processes in the appro- 

 priate metabolic diagrams; for example, see pages 168 to 171. 



CARBOHYDRATE METABOLISM 



The carbohydrates of plants are all synthesized by the plants them- 

 selves and depend on reduction of carbon dioxide for the carbon 

 needed. Light absorbed during photosynthesis serves by means of 

 energy conversion and coupling mechanisms as the driving force in 

 the synthesis of carbohydrates. Many of the individual reactions are 

 markedly endergonic (requiring free energy). The energy needed 

 comes primarily from the high-energy nucleotides as ATP, DPNH, 

 and TPNH formed during the early stages of photosynthesis. 



Plants put their carbohydrates to many uses, often as components of 

 special substances like nucleic acids and glycosides. Other carbo- 

 hydrates are structural materials. Probably little if any carbohydrate 

 from these categories is ever reused by the plant under normal condi- 

 tions. However, plants do respire in the dark and to a smaller extent 

 in the winter during dormancy. This respiration may be the automatic 

 result of the presence of enzymatic equipment and serve no useful 

 purpose. More probably, respiration supplies energy needed for tissue 

 repair and replacement of compounds like proteins that might other- 

 wise break down spontaneously. Furthermore, plants definitely grow 

 at night during the appropriate seasons and must depend on respira- 

 tion for energy. This energy is stored chemically, of course, in the 

 form of reserve materials, principally carbohydrates and lipides. 



To a great extent the utilization of reserve carbohydrates is a 



