The Z-Scheme Diagram of Photosynthesis

Rajni Govindjee

The Electron Transport Pathway from Water (H2O) to NADP+ (the Nicotinamide Adenine Dinucleotide Phosphate, oxidized form).
Many versions of the Z-scheme are available in the literature.This particular diagram was developed by Wilbert Veit and Govindjee, 2000, and can be also found at Abbreviations used are (from left to the right of the diagram): Mn for a manganese complex containing 4 Mn atoms, bound to Photosystem II (PSII) reaction center; Tyr for a particular tyrosine in PSII; O2 for oxygen; H+ for protons; P680 for the reaction center chlorophyll (Chl) in PSII: it is the primary electron donor of PSII; Excited (Chl) P680 for P680* that has the energy of the photon of light; Pheo for pheophytin molecule (the primary electron acceptor of PSII; it is like a chlorophyll a molecule where magnesium (in its center) has been replaced by two "H"s); QA for a plastoquinone molecule tightly bound to PSII; QB for another plastoquinone molecule that is loosely bound to PSII; FeS for Rieske Iron Sulfur protein; Cyt. f for Cytochrome f; Cytb6 (L and H) for Cytochrome b6 (of Low and High Energy); PC for copper protein plastocyanin; P700 for the reaction center chlorophyll(Chl;actually a dimer, i.e., two molecules together) of PSI; it is the primary electron donor of PSI; Excited (Chl) P700 for P700* that has the energy of the photon of light; Ao for a special chlorophyll a molecule (primary electron acceptor of PSI); A1 for a phylloquinone (Vitamin K) molecule; FX, FA, and FB are three separate Iron Sulfur Centers; FD for ferredoxin; and FNR for Ferredoxin NADP oxido Reductase (FNR). Three major protein complexes are involved in running the "Z" scheme: (1) Photosystem II; (2) Cytochrome bf complex (containing Cytb6; FeS; and Cytf ) and (3) Photosystem I. The diagram does not show where and how ATP is made.

For a description of how the Z Scheme operates, click here.