1.The filamentous alga Mougeotia twists and rotates its flat chloroplast in response to changes in light intensity and direction, using the actin cytoskeleton to apply turning forces. The three light micrographs show cells in Mougeotia filaments, with the flat, plate-like chloroplast in face view (left) or edge on (right) or with one half rotated with respect to the other at the hinge region (horizontal arrow). The confocal stereo images (right) illustrate different kinds of chloroplast rotation - rotation at the "hinge" (left) or in response to very localized light intensity gradients, giving rise to twisting within one half of one chloroplast. The dark (less fluorescent) areas in the stereo images are pyrenoids They also show up in the light micrographs, especially if the cells have been active in photosynthesis and there is a sheath of starch grains around each pyrenoid.
N = nucleus, P = pyrenoid.

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