Fig. 6.

The effect of the density of the left-handed helical junctions on network energy. (A–D) Surface energy minimization of idealized thylakoid network architectures, with each grana column surrounded by 4 distinct, intertwined right-handed helices. The grana are interconnected by interspacing left-handed helical junctions, whose pitch and diameter are about one-quarter of those of the right-handed helices. The system was modeled with varying densities of left-handed helical junctions: 1 (A), 2 (B), and 4 (C) per granum. (D) A top view of the model in C is shown with junctions highlighted in purple. The densities are labeled in A–C, Insets, which show a portion of each model from the top view with arrows pointing to left-handed helical junctions. (A–C) The edge of one layer is highlighted in white to show its tilt in relation to the grana columns; with a ratio of 4:1 (C), the layers are on average orthogonal to the grana columns. The ratio of helical junctions to grana was calculated for the bulk of the assembly, with grana located at the edges of the model making fractional contributions (1/2 for grana at the edges and 1/4 for those at the corners). (E) Top view schematic of the arrangement corresponding to the minimum surface energy of the network (modeled in C and D), with 4 helical junctions associated on average with each granum (depicted here as 16 helical connectors per 4 grana). Grana and stroma lamellae are colored in yellow and green, the right-handed helices in light blue, and the left-handed helical junctions are highlighted in purple (with a hole representing the elongated channel—see Fig. 4B). (F) A section of a 3D reconstructed model providing a similar view of the corresponding elements (shown with and without left-handed helical junctions segmented in purple). The average number of helical junctions per granum is ∼4, as was generally the case for the networks analyzed in this work.