A, The ATP/NADPH ratio required to satisfy CO₂ assimilation and photorespiration as a function of CO₂ partial pressure at Rubisco. The ratio depends on the value assumed for Γ_* (μbar), the CO₂ photocompensation point, which increases with temperature. The ratios produced by linear electron flow are shown assuming three, four, and 4.67 H⁺/ATP. B, The proportion of light absorbed by PSI and PSII necessary to satisfy the linear electron flow and cyclic electron flow required to provide the ATP/NADPH for different values of Γ_* and likely H⁺/ATP ratios. A constant value is frequently used to calculate linear electron flow from chlorophyll fluorescence.

Details of the linear (orange arrows) and cyclic (black arrows) electron flow pathways that produce NADPH and ATP. Proton movement is denoted by blue arrows. Four different cyclic pathways have been proposed and may operate in parallel: (1) NDH, PQ is reduced by NAD(P)H:PQ oxidoreductase; (2) FQR/PGR5, electrons are transferred from Fd to PQ via the FQR; (3) Nda2, a type 2 NAD(P)H:PQ oxidoreductase; (4) the Q_i (PQ reductase) site of the cytochrome b₆f complex is used to reduce PQ and may involve the Fd:NADP⁺ oxidoreductase (FNR). Other thylakoid components in the diagram include the ATP synthase, PSI, PSII, PQ (yellow hexagon), plastocyanin (PC), and light-harvesting complexes associated with PSII (LHCII).

The coupling between light capture, electron flow, and photophosphorylation, which produce NADPH and ATP with their consumption by CO₂ assimilation (photosynthetic carbon reduction [PCR]) and photorespiration (photosynthetic carbon oxidation [PCO]) in a cell. Three of the cell organelles are shown: C, chloroplast; P, peroxisome; M, mitochondrion. The photosynthetic electron transport chain is represented in a thylakoid membrane, with electron flow from water through PSII, the cytochrome b/f complex (f), and PSI to NADPH, and proton flow into the lumen and out through the ATPase (CF) to generate ATP. The dominant path for electron flow is linear electron flow (LEF), while two alternative paths are shown: (1) cyclic electron flow around PSI (CEF); and (2) electrons can leave from PQH₂ via the plastoquinol oxidase (PTOX), which oxidizes plastoquinol and reduces oxygen to water. Some electrons from LEF can return to oxygen via the WWC or can be exported from the chloroplast via the malate valve to generate ATP in mitochondria (Mal, malate; OAA, oxaloacetate). Equal photon capture by PSII and PSI is required for LEF, and additional photon absorption by PSI is required for cyclic electron flow. The proportion of photons delivered to PSII reflects the relative amounts of chlorophyll associated with the two photosystems. State transitions can dissociate chlorophyll protein complexes from PSII, which may contribute to PSI, thereby enabling greater cyclic electron flow. The q_E quenching in the light-harvesting chlorophyll protein complexes associated with the xanthophyll cycle, which is activated by low pH in the lumen, reduces the effective efficiency of the antenna. The conductivity of the ATPase can be varied to alter the luminal pH relative to the rate of ATP synthesis, thus providing feedback via the q_E mechanism.