RESUMO
Light-dependent reactions of photosynthesis takes place in the thylakoids of chloroplasts where light energy harvested from the sun drives the synthesis of ATP and NADPH. The major pathway of photosynthetic chain is the linear electron transport (LET), in which both photosystems (PSI and PSII) are involved, and ATP and NADPH are produced. However, ratio in production of those components is insufficient to cover the Calvin cycle energy requirements, depending on the metabolism of the cell. Moreover, disturbance in metabolism homeostasis, caused by environmental stress conditions, increases ATP demand, which cannot be covered by LET. Thus, in photosynthetic apparatus must exist alternative electron transport pathways, these include: cyclic electron transport (CET) mediated by NDH complex or PGR5/PGRL1 proteins, water-water cycle and PTOX enzyme. Activity of alternative pathways can optimize ratio in production of ATP/NADPH, appropriately to requirements, which allows to achieve redox balance and ATP contents.
