Contents lists available at ScienceDirect Journal of Theoretical Biology journal homepage: www.elsevier.com/locate/yjtbi Modeling the light-induced electric potential difference (ΔΨ), the pH difference (ΔpH) and the proton motive force across the thylakoid membrane in C 3 leaves Hui Lyu, Dušan Lazár ⁎ Department of Biophysics, Centre of the Region Haná for Biotechnological and Agricultural Research, Faculty of Science, Palacký University, Šlechtitelů 27, 78371 Olomouc, Czech Republic ARTICLE INFO Keywords: Photosynthesis Ion movements Electrochromic shift ABSTRACT A model was constructed which includes electron transport (linear and cyclic and Mehler type reaction) coupled to proton translocation, counter ion movement, ATP synthesis, and Calvin-Benson cycle. The focus is on modeling of the light-induced total electric potential difference (ΔΨ) which in this model originates from the bulk phase electric potential difference (ΔΨ b ), the localized electric potential difference (ΔΨ c ), as well as the surface electric potential difference (ΔΨ s ). The measured dual wavelength transmittance signal (ΔA515– 560 nm, electrochromic shift) was used as a proxy for experimental ΔΨ. The predictions for theoretical ΔΨ vary with assumed contribution of ΔΨ s , which might imply that the measured ΔA515–560 nm trace on a long time scale reflects the interplay of the ΔΨ components. Simulations also show that partitioning of proton motive force (pmf) to ΔΨ b and ΔpH components is sensitive to the stoichiometric ratio of H + /ATP, energy barrier for ATP synthesis, ionic strength, buffer capacity and light intensity. Our model shows that high buffer capacity promotes the establishment of ΔΨ b , while the formation of pH i minimum is not ‘dissipated’ but ‘postponed’ until it reaches the same level as that for low buffer capacity. Under physiologically optimal conditions, the output of the model shows that at steady state in light, the ΔpH component is the main contributor to pmf to drive ATP synthesis while a low ΔΨ b persists energizing the membrane. Our model predicts 11 mV as the resting electric potential difference across the thylakoid membrane in dark. We suggest that the model presented in this work can be integrated as a module into a more comprehensive model of oxygenic photosynthesis. 1. Introduction In higher plants and algae, the light-dependent photosynthetic reactions are facilitated by pigment-protein complexes located in the thylakoid membrane of chloroplasts (for general reviews see, e.g., Govindjee, 1982; Eaton-Rye et al., 2012; Blankenship, 2014). Light is captured by light harvesting complexes (LHCs), which funnel excitons to photochemical reaction centers of photosystem I (PSI) and photo- system II (PSII). Special subsets of chlorophyll molecules in PSI (P700) and PSII (P680) are excited by the excitation energy transfer; this then leads to charge separation in P700 and P680. The electron separated from P700 is transported through PSI and then to ferredoxin (Fd), which, in turn, reduces NADP + to NADPH using ferredoxin-NADP + - oxidoreductase (FNR). The electron separated from P680 is trans- ported through PSII and then via the plastoquinone (PQ) pool, the cytochrome b 6 f (cytb 6 f) complex and plastocyanin (PC) to the oxidized http://dx.doi.org/10.1016/j.jtbi.2016.10.017 Received 19 April 2016; Received in revised form 7 October 2016; Accepted 28 October 2016 ⁎ Corresponding author. E-mail address: lazard@seznam.cz (D. Lazár). Abbreviations: ATP, adenosine triphosphate; BC, buffer capacity; b H ,b L , the high and low potential hemes b of the cytochrome b 6 f complex; CBC, Calvin-Benson cycle; CD, charge difference; CET, cyclic electron transport; CO sp , coefficient which relates ΔΨ s to ΔpH; CS, current summing; cytb 6 f, cytochrome b 6 f; ΔA515-560, electrochromic shift measured by dual- wavelength technique; ΔpH, pH difference across the membrane; ΔΨ, (total) electric potential difference, i.e., voltage, across the membrane; ΔΨb, delocalized bulk phase electric potential difference across the membrane; ΔΨc, localized (“Coulombic”) electric potential difference across the membrane; ΔΨs, surface electric potential difference across the membrane; E C , electric capacity; ECS, P515, electrochromic shift; Fd, ferredoxin; FNR, ferredoxin-NADP + -oxidoreductase; ΔG atp , energy barrier for ATP synthesis; G Δ atp 0 , standard Gibbs free energy change of ATP formation; GHK, Goldman–Hodgkin–Katz; O atp , fraction of “open” (active) ATPsynthases; LET, linear electron transport; LHCs, light harvesting complexes; NADPH, reduced nicotinamide adenine dinucleotide phosphate; NPQ, non-photochemical quenching of chlorophyll fluorescecnce; OEC, oxygen evolving complex; OP, open probability; P680, P700, electron donor in photosystem II and in photosystem I – reaction center chlorophylls with absorption peaks at 680 nm and at 700 nm; PFD, photon flux density; PC, plastocyanin; pmf, proton motive force; PQ, plastoquinone; PSI, PSII, photosystem I, photosystem II; Q A ,Q B , the first and the second plastoquinone electron acceptors in photosystem II; ST, single turnover Journal of Theoretical Biology 413 (2017) 11–23 0022-5193/ © 2016 Elsevier Ltd. 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