• Electron transport;
  • light-harvesting;
  • photophosphorylation;
  • photosynthesis;
  • quantum yield;
  • thylakoid structure and function

Plant thylakoid membranes contain three structurally distinct domains: the planar appressed membranes of the grana; the planar non-appressed stroma thylakoids; and the highly curved, non-appressed margins of the grana. Evidence is presented to suggest that the grana margins form a significant structural domain, which has hitherto been neglected. If indeed the grana margins contain some of the cytochrome b/f complex, photosystem (PS) I complex and ATP synthase, they form a third functional domain of the laterally heterogeneous continuous thylakoid membrane network. The consequences of grana margins containing complexes are explored with respect to linear electron transport under light-saturating and light-limiting conditions, non-cyclic vs cyclic photophorylation, and the regulation of light energy distribution to both PS I and PS II.