Modeling The Light-Induced Electric Potential Difference (Delta Psi), The Ph Difference (Delta Ph) And The Proton Motive Force Across The Thylakoid Membrane In C-3 Leaves

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 (Delta Psi) which in this model originates from the bulk phase electric potential difference (Delta Psi(b)), the localized electric potential difference (Delta Psi(c)), as well as the surface electric potential difference (Delta Psi(s)). The measured dual wavelength transmittance signal (Delta A515-560 nm, electrochromic shift) was used as a proxy for experimental Delta Psi. The predictions for theoretical Delta Psi vary with assumed contribution of Delta Psi(s), which might imply that the measured Delta A515-560 nm trace on a long time scale reflects the interplay of the Delta Psi components. Simulations also show that partitioning of proton motive force (pmf) to Delta Psi(b) and Delta pH 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 Delta Psi(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 Delta pH component is the main contributor to pmf to drive ATP synthesis while a low Delta Psi(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.