Changing concepts about the distribution of Photosystems I and II between grana-appressed and stroma-exposed thylakoid membranes.

Thylakoid membranes of higher plants and some green algae, which house the light-harvesting and energy transducing functions of the chloroplast, are structurally unique. The concept of the photosynthetic unit of the 1930s (Robert Emerson, William Arnold and Hans Gaffron), needing one reaction center per hundreds of antenna molecules, was modified by the discovery of the Enhancement effect in oxygen evolution in two different wavelengths of light (Robert Emerson and his coworkers) in the late 1950s, followed by the 1960 Z scheme of Robin Hill and Fay Bendall. It was realized that two light reactions and two pigment systems were needed for oxygenic photosynthesis. Changing ideas about the distribution of Photosystem II (PS II) and PS I between the green-appressed and stroma-exposed thylakoid membrane domains, which led to the concept of lateral heterogeneity, are discussed.