The role of lipids in the global organization of thylakoid membranes of higher plants

Oxygenic photosynthesis is a process which is highly relevant for human kind. In higher plants the first steps of photosynthesis (capturing of light and the conversion of solar energy into chemical energy) occur in the internal membranes of chloroplasts – the thylakoid membranes. These constitute a complex ensemble of pigments, proteins and lipid molecules, forming a continuous 3D membrane network that encloses a single interior aqueous phase (lumen). It has long been recognized that in higher plants the thylakoid is highly folded and forms membrane domains with strikingly different protein composition (see below) - grana stacks and unstacked (stromal) lamellae (Fig. 1, cf [1,2]).

According to the most recent 3D models the stroma membranes are arranged in a circular fashion around the granum and ensure the connections between neighboring grana stacks [3] or alternatively the granum layers are formed by bifurcations of stromal sheets that fuse within the granum body, the adjacent granum layers being connected via membrane bridges [4].

Grana formation leads to lateral segregation of the main photosynthetic complexes - photosystem II (PSII) and photosystem I (PSI). The function of grana has long been discussed (see for example [5,6]). It appears that grana are not indispensable for plant survival but are needed for better adaptation to land conditions via regulation of light harvesting and controlling the balance of energy flow between PSII and PSI [6]. The spatial separation of PSII and PSI in the stacked and unstacked membrane regions, respectively, as well as the macromolecular organization of PSII in stacked grana thylakoids is a selforganizing process, which most likely is an important feature for the maintenance of the functional integrity of the photosynthetic machinery [7].