Mathematical Modelling of Free-Pool and Channelled Electron Transport in Photosynthesis: Evidence for a Functional Supercomplex around Photosystem 1

A mathematical model of photosynthesis (Laisk & Eichelmann (Phil. Trans. R. Soc. Lond. B 323, 369 (1989))) is further developed which aims to reproduce experimental data from parallel measurements of photosynthetic rate and the reduction state of P700 (the donor pigment of photosystem 1 (PS1)) in leaves. In the model, the electron transport chain comprises photosystem 2, plastoquinone, cytochrome b/f, plastocyanin, photosystem 1, ferredoxin and NADPH. In a first version, low-molecular carriers are assumed to be freely diffusible. Carbon metabolism is described in detail. Most of the known regulatory factors are included. Under conditions of upstream (cytochrome b/f) limitation, the computed quantum yield of photosynthetic O₂ evolution is proportional to the fraction of reduced P700, in agreement with experiment. Under downstream limitation (electron accumulation due to low ATP:NADPH ratio), photosynthesis does not decline until P700 becomes almost completely reduced. This is in contradiction to experiments. Introduction of the kinetics of channelled electron transport in a second version of the model causes photosynthetic rate to decline proportionally with excess reduction of P700 under downstream limitation, in agreement with experimental data. Such a result supports the proposed existance of functional supercomplexes of cytochrome b/f, plastocyanin, photosystem 1, ferredoxin and ferredoxin-NADP reductase which can carry out channelled electron transport during photosynthesis.