Structure of Chloroplast Membranes as Revealed by Natural and Experimental Fixation with Tannic Acid: Particles in and on the Thylakoid Membrane

Abstract

The ultrastructure of thylakoid membranes in chloroplasts after fixation of leaf tissue with tannic acid is described and compared with the ultrastructure of cells with a native content of tanniferous substances. Generally the presence of tannic acid during fixation from either an intracellular source or by addition to the fixative produces a pronounced negative contrast in thylakoid membranes and other cytomembranes. The thylakoid membranes appear as a layer of electron-transparent particles demarcated by very electron-dense material on both sides (i.e., in the thylakoid lumen and the chloroplast stroma). The intramembranous particles are of two different sizes (average diameter 35—40 Å and 55—60 Å), the larger of which in oblique sections are descernible as aggregations of smaller elements. The thylakoid lumen contains particles often continuous with the larger intramembranous particles. Evidence from disrupted chloroplasts suggests the presence of inner and outer electron-dense leaflets bordering the particle layer in the thylakoid membranes, and these findings are discussed in relation to fluid mosaic membrane models. Apart from the luminal particles and intramembranous particles fixation with tannic acid reveals 100 Å particles associated with the outer surface of unstacked thylakoid membranes. These elements possibly represent chloroplast coupling factor particles. The intramembranous particles recognizable in thin sections are compared to the particles observed by freeze-fracturing of the membranes.