104 PHOTOSYNTHESIS 



is virtually irreversible. It has, however, been proved that in 

 the case of the glucose dehydrogenase the initial product is 

 gluconolactone. Korkes and Strecker were able to show that 

 the reaction is reversible under acid conditions where hydro- 

 lysis of the lactone is diminished. We may also conclude 

 from the work of O. Cori and Lipmann that the oxidation 

 product of glucose 6-phosphate is also the lactone. Thus, as 

 in the case of the triose phosphate system, the formation of 

 the carboxylic acid from the aldose can be regarded as a 

 two-step process. The hexonolactones, however, have not 

 been shown to take part in any group transfer reaction, and 

 would appear to hydrolyse spontaneously. 



REDUCTION OF CARBONYL AND ALCOHOL GROUPS 



When related to the production of oxygen from water the 

 reduction of aldehydes and ketones to alcohols involves less 

 energy than the previously described reductions; in the 

 presence of reduced coenzyme I the reduction can proceed, 

 resulting in a fairly complete oxidation of the coenzyme 

 when equilibrium is reached. The ethanol-acetaldehyde sys- 

 tem is of this type. When ethanol is formed by dismutation 

 of acetaldehyde with the production of acetate the reaction 

 is to be regarded as virtually irreversible. Again the further 

 reduction of the alcohol to the hydrocarbon level also is by 

 no means the largest energy step in CO2 reduction. This 

 may be seen from the fact that methane is produced by 

 bacteria from CO2 plus a carbon compound such as formate, 

 the process actually yielding energy, which is utilized for 

 growth. Oxidations and reductions which involve CHg, 

 CHOH, and CO groups are fundamental in fermentation 

 and glycolysis, and thus are represented in all living cells. 



THE PRODUCTS OF PHOTOSYNTHESIS 



During growth the immediate products of photosynthesis 

 are converted by a series of metabolic reactions into a 

 variety of compounds including carbohydrates, fats, and 

 proteins. In short, the final product of photosynthesis is the 

 plant itself. In the first chapter we described the determina- 

 tion of photosynthesis by the appearance of starch. Smith 



