Fig. 3.

Thin section electron micrograph of a chemically fixed chloroplast in a young tobacco leaf. The chloroplast lies flat against the plasma membrane and the cell wall (CW) and presents a more or less elliptical outline. The stacked grana thylakoids (GT) are interconnected by non-stacked stroma thylakoids (ST). Stroma (S) surrounds the membranes, and the lightly stained regions of the stroma indicates the presence of DNA. Because this chloroplast was still growing, when it was fixed for TEM analysis, the grana stacks vary in height and have irregular margins. A few plastoglobules (PG) lie adjacent to stroma thylakoids. Two envelope membranes (EM) form the boundary layer of the chloroplast. The arrowheads point to contact sites between thylakoid membranes and the inner envelope membrane. Such sites are frequently seen in growing chloroplasts and most likely represent sites of galactolipid transfer from the lipid-synthesizing inner envelope membrane to the growing thylakoid membranes. A small vesicle (V) is seen close to the inner envelope membrane. They are also seen infrequently. From Staehelin (1986); Bar 0.5 mm