Structure of the Photosynthetic Apparatus in Protein-embedded Chloroplasts

The photosynthetic apparatus in higher plants has been found to be located in complex lipoprotein membrane systems in the chloroplasts called grana. Electron microscopy has provided invaluable information in determining the fine structure of grana (6-9, 11, 21, 28-30). However, due to the harsh methods employed in fixation (usually by osmium tetroxide or potassium permanganate) and dehydration in lipid solvents (usually alcohol or acetone), the preservation of native structure has been questioned (11, 12, 16, 20, 22). Utilization of the freeze-etch technique for electron microscopy (1, 3, 23-26) and low angle X-ray scattering (13, 14, 18, 19) have successfully avoided some potential artifacts, but the resolution of these techniques has not been sufficient to visualize the crosssectional molecular structure of grana. Recently, Farrant and McLean (5) developed procedures for embedding biological materials in bovine serum albumin (BSA), without the harsh treatment with heavy metal fixatives and lipid solvents. In the present study, isolated spinach chloroplasts were fixed in low concentrations of glutaraldehyde, formaldehyde, or diethylmalonimidate and were embedded in BSA. Chloroplasts in cross-linked BSA sections were examined by transmission electron microscopy.