RECONSTRUCTIONS OF THE GRANA-FRETWORK SYSTEM OF A CHLOROPLAST

The grana-fretwork system was reconstructed from serial sections representing 0.5 μm slice, in profile view, from the midregion of a chloroplast of Nicotiana tabacum. Reconstructions show grana in multiple strata. The fretwork integrates the grana in all three dimensions. Large grana are visualized as complex structures consisting of two or more eccentrically stacked solids that vary in height and diameter. The close spacing and overlapping positions of grana in the plastid explain why phase microscopy cannot be used to demonstrate adequately the numerous strata of grana within a chloroplast. The grana are not far enough apart to allow for successful optical sectioning of a plastid in face view. In profile view no distinct grana can be resolved because the grana-fretwork system has the aspect of a “honeycomb.” Thus, observations with a light microscope are not adequate to determine the arrangement of grana in a chloroplast, and the recently-proposed model showing all grana of a plastid to be arranged in a single spiral ribbon must be rejected as incompatible with properties of real plastids.