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fermentation processes, particularly those that occur in the absence of 

 air, as for example, alcoholic fermentation, in which a part of the 

 sugar molecule is reduced to alcohol and another part is oxidized to 

 carbon dioxide. In all these processes, virtually all the intermediate 

 steps are spontaneous, biologically controlled, exothermic reactions « 



Mth respect to the gaseous exchange process, photosynthesis 

 proved to be the reverse of respiration, because it results in the 

 assimilation of carbon dioxide and the evolution of oxygeno With respect 

 to the nutrition of plants, photosynthesis is also the reverse of respi- 

 ration, for it restores the cellular material and the energy that are 

 depleted by respirationo But with respect to mechanism, photosynthesis 

 incorporates a single powerful driving force that is absent in respira- 

 tiono This driving force is sunlight, which, as has been noted above, 

 is effective only in the green parts of plants »^^o 



CHLOROPLAST PIGMENTS AND PHOTOSYNTHESIS 



Function of the Chloroplast Pigment s 



Hhen photosynthesis was foiind to result from the energy of ab- 

 sorbed sunlight, many attempts were made to elucidate the energy ab- 

 sorption mechanismo The principles of the new field of photochemistry 

 indicated that the green pigment of plants (chlorophyll - the green of 

 leaves) must be an active photochemical intermediate, because to be 

 effective in photosynthesis, sxinlight must be absorbed, and only colored 

 substances absorb this visible light. 



With the development of the microscope, the green pigment of 

 plants was seen to be localized in microscopic bodies or plastids within 

 the plant cells o These green plastids were called ohloroplasts, and 



