Fig. 1.

Models of the plant thylakoid membrane. (A) A 10-nm-thick STEM tomographic slice of lettuce chloroplast. The grana stacks (G) are interconnected by unstacked stroma thylakoids (SL). Both membrane domains are immersed in an aqueous matrix, called stroma, which in turn is bordered by the chloroplast envelope (black arrowhead). (Scale bar, 200 nm.) (B) The helical model. The stroma lamellae wrap around the grana as right-handed helices, connecting to the thylakoids within the stack through slit-like apertures located at the rim of the stacks. A single file of grana is shown, with its oppositely sloped long edges marked in green and blue. Reprinted with permission from ref. 21. (Inset) A model of a granum–stroma assembly, configured akin to the helical model, composed of a granum core surrounded by multiple helical frets. The arrow marks a slit at the edge of the granum. Adapted by permission from ref. 75, Springer Nature: Plant Molecular Biology, copyright 2011. (C) The bifurcation model. The grana stacks are formed by bifurcations of the stroma lamellae (black arrowhead). Neighboring discs in the stacks are additionally connected by internal membrane bridges located near the bifurcation sites at the grana periphery (arrow).