MEMBRANE IN CHLOROPLASTS - A TOPOLOGICAL APPROACH TO GRANA AND FRETS

The membrane system of chloroplasts, where photosynthesis takes place, presents a remarkable organization of flat thykaloids in grana and frets which contributes to the efficiency of the photochemical and biochemical steps of the process. A topological analysis of this organization is proposed, which develops a relationship with screw dislocation walls in lamellar liquid crystals. This analysis makes possible a rather simple geometrical description of the organization depending upon two parameters only: the stacking distances of thykaloids in grana and frets. This suggests that the formation of grana and frets, and their overall structure, might be viewed as a separation between two lamellar stackings of thykaloids whose different spacings are associated with a distribution along the membrane of the molecules intervening in the photochemical process.