Absorption spectra primary electron donor

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

Fig. 9. (A) Absorption spectrum of Rb. sphaeroides used as a reference to show the Qx and Qy bands of the primary donor (P), BChl [B] and bacteriopheophytin [BO] (B) Femtosecond absorption changes at 920 (a), 785 (b) and 545 nm (c) vs. the delay time of the monitoring pulse measured at room temperature, and (C) absorption changes at 920 (a) and 794 nm (b) measured at 25 K. Figure source (A) see Fig. 7 (B) Holzapfel, Finkele, Kaiser, Oesterheldt, Scheer, Stilz and Zinth (1990) Initial electron transferin the reaction center from Rhodobacter sphaeroides. Proc Nat Acad Sci, USA 87 5170 (C) Zinth and Kaiser (1993) Time-resolved spectroscopy of the primary electron transfer in reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis. I n JR Norris and J Deisenhofer (eds) The Photosynthetic Reaction Center, Voi il, p 82. Acad Press.

---