PLANT METABOLISM 393 



A carboxylation involves no reduction and requires relatively little energy. 

 JNIuch energy is needed for reduction of the carboxylated compound to 

 the level of a carbohydrate, a reaction which can be pictured as: 



RCCOOH + 2H -^ RCHOHCOOH 



II 

 



RCHOHCOOH + 2H -^ RCHOHCHO + HoO 



It is evident that 4 hydrogens are required to reduce one carbon dioxide 

 molecule to the oxidation-reduction level of carbohydrate and that a 

 molecule of water is produced. 



Shortly after the turn of the twentieth century, it was realized that 

 photosynthesis could be divided into distinct light and dark reactions, 

 i.e., reactions which do, or do not, require light in order to proceed. 

 There is no evidence that the initial fixation of carbon dioxide, and its 

 subsequent reduction, requires light directly. In fact, all evidence shows 

 that the reactions of carbon dioxide in photosynthesis are dark reactions. 

 Some of the most convincing experiments are based upon the use of 

 specific inhibitors at varying light intensities. If an inhibitor docs not 

 decrease the rate of photosynthesis at low light intensities, but does at 

 high intensities- (where the dark reactions are the ones which limit the 

 rate of the overall process) , it is an inhibitor for a dark reaction. Cyanide, 

 for example, acts in this fashion, and since it inhibits carboxylation, the 

 conclusion is drawn that carboxylation does not require light. 



The rate of photosynthesis is dependent on the partial pressure of 

 carbon dioxide, and though it is difficult to generalize for many plants 

 at different light intensities, it may be said that raising the concentra- 

 tion of carbon dioxide from its usual level of 0.03 per cent to 0.1 per 

 cent will usually double the rate of photosynthesis. Most plants are 

 not inhibited by carbon dioxide up to 10 per cent, but their growth is 

 reduced by concentrations greater than this. 



Partial reactions 



As pointed out, photosynthesis is a complex of reactions rather than 

 a single reactit)n. The study of such an involved process is obviously 

 much easier if it can be first separated into its individual steps. One 

 of the most notable achievements of recent years has been the recon- 

 struction of partial reactions in cell-free preparations; that is, certain 

 ground-up plant tissues have been found to carry out parts of the photo- 

 synthetic process. This has permitted the study of individual reactions 

 or reaction series in a system much simpler than that encountered in 

 intact cells. 



In 1937, Hill found tliat a suspension of chloroplasts plus leaf extract 



