Introduction

Photosystem I (PSI), like Photosystem II (PSII) considered in the previous chapter, is an integral part of the chloroplast electron transport system. Although PSI may act independently of PSII in cyclic flow, both are required for the continuous maintenance of electron flow from water to NADP+ for CO2 incorporation. Whereas with inhibitors of PSII (Chapter 2) interaction with a protein component resulted in the indirect cessation of electron flow, herbicides considered in connection with PSI intercept electrons directly.

In the generation of PSI, electrons expelled from P700 are raised to a negative potential (possibly –900 mV) to electron acceptors A0 and A1, that are forms of chlorophyll a. The donor and acceptors are associated with 70 kD pigment proteins that span the thylakoid membrane lipid bilayer. The subsequent electron acceptor is the hypothetical X, possibly an iron–sulphur centre with a potential of around –700 mV, and then two iron–sulphur centres A and B (Fig. 1) with potentials of around –590 mV and –530 mV. These are associated with two subunits of 18 and 16 kD and are probably identical to the former acceptor P430–. These two centres function either in series or in parallel, and are almost certainly bound ferredoxin, and link to soluble ferredoxin and ferredoxin-NADP+ reductase and hence NADP+ (Fig. 1) (Haehnel 1984).