Fig. 2.

Models of dark- (Left) and light-adapted (Right) granal thylakoids. The structures of PSII and plastocyanin (PC, small globule inside the lumen) are drawn to scale. In the dark, the measured luminal thickness does not allow a face-to-face arrangement of OECs residing on opposite membranes (Lower), indicating that they assume a staggered arrangement (Upper Left). Although expansion of the lumen in the light (Upper Right) may allow for OECs to arrange face to face, adopting such an arrangement upon illumination is unlikely (see text).