Environment fixation: Other photosynthetic assimilation pathways

Photosynthesis encompasses more than carbon dioxide fixation and carbohydrate synthesis.

- TAPAN KUMAR MAITRA

Photosynthesis encompasses more than carbon dioxide fixation and carbohydrate synthesis. In plants and algae, the ATP and NADPH generated by photosynthetic energy transduction reactions are consumed by a variety of other anabolic pathways found in chloroplasts. Carbohydrate synthesis is only one example of carbon metabolism; the synthesis of fatty acids, chlorophyll, and carotenoids also occurs in chloroplasts. Moving beyond carbon metabolism, several key steps of nitrogen and sulfur assimilation are localized in chloroplasts. The reduction of nitrite (NO2) to ammonia (NH3), for example, is catalyzed by a reductase enzyme in the chloroplast stroma, with reduced ferredoxin serving as an electron donor. The ammonia is then channeled into amino acid and nucleotide synthesis, portions of which also occur in chloroplasts. Furthermore, much of the reduction of sulfate (SO42-) to sulfide (S2-) is catalyzed by enzymes in the chloroplast stroma.

In this case, both ATP and reduced ferredoxin provide energy and reduce power. The sulfide, like ammonia, may then be used for amino acid synthesis.