Figure 2.

Metabolic pathways for ATP and NAD(P) rebalancing and dissipation in an illuminated leaf. Linear photosynthetic electron flow in the thylakoid (A) generates a fixed stoichiometry of NADPH and ATP that does not match the requirement of the cell’s anabolic metabolism (shown in the middle). Anabolic demands will vary, for example by assimilating ammonium rather than nitrate or depending on the growth phase (cell division or cell expansion). The ratio of NADPH and ATP produced can be adjusted by utilizing cyclic electron flow around PSI, which reduces the production of NADPH relative to ATP (B). Alternatively, the conversion of photorespiratory NADH to ATP by the mitochondrial electron transport chain (C) can be adjusted by the engagement of uncoupling pathways such as the alternative NADH dehydrogenases (ND), the alternative oxidase (AOX), and uncoupling protein (UCP). At high light, there is an excess of energy and both ATP and NADPH must be dissipated. NADPH can be dissipated by the water-water cycle (D) and by pigment cycles such as the xanthophyll cycle (E), while ATP can be dissipated by ATP-consuming futile cycles (F). Flows of electrons and protons are indicated by yellow and cyan lines, respectively. CBB, Calvin-Benson-Bassham; Fd, ferredoxin; FNR, ferredoxin-NADPH oxidoreductase; FQR, ferredoxin-quinone oxidoreductase; LHC, light-harvesting complex; NDH, chloroplast NAD(P)H dehydrogenase; PC, plastocyanin; PQ, plastoquinone; PS, photosystem.