Fig. 5.

Scheme of the two-electron gate (TEG) model and related reactions. The two-electron gate mechanism, by which electrons are transferred from Q_A to Q_B, is represented by the reactions on line II. The rate constants are: k_L, overall rate constant of Q_A reduction; k_AB1 and k_AB2, rate constants of ET from the reduced Q_A to Q_B and Q_B^‒, respectively; k_BA1 and k_BA2, rate constants of backward ET from Q_B^‒ and Q_B^2‒ to Q_A, respectively. The reactions above and below line II describe the reversible exchange of doubly reduced Q_B (after its double protonation, which is implicitly assumed) with a PQ molecule from the PQ pool (rate constants k_(B/PQ)exch and k_(PQ/B)exch); the reversible oxidation of the plastoquinol (rate constants k_ox and k_red) is implicitly assumed to be the result of chlororespiration and cyclical electron flow around PSI. Modified from Lazár and Schansker (2009).