The ability of long-chain unsaturated fatty acids to serve as models for the action of the protein fraction of Ricinus leaf extract on chloroplasts (and algae) has been determined experimentally for several parameters. These include steady-state fluorescence emission and excitation (measured at −196 °), fluorescence induction, light-induced absorption changes of chlorophylls a II and a I, Hill activity, and chloroplast ultrastructure. In addition, the sequential inhibition of system II- and system I-associated electron flow has been demonstrated as a function of increasing concentration of exogenous fatty acid.