Abstract
Vegetative cells of Azotobacter vinelandii contain a system of intracytoplasmic membranes in the form of numerous internal vesicles. The three-dimensional morphology of these internal vesicles was established by an examination of stereopair electron micrographs of negatively stained cells. The vesicles assumed a variety of forms ranging from nearly spherical units to short, curved tubules. These structures were found at the periphery of the cytoplasm, subjacent to the cytoplasmic membrane. Large flattened cisternae were also present in some cells. The amount of intracytoplasmic membrane varied widely even among individual cells from the same culture. The total surface area of the intracytoplasmic membranes was greater than that of the cytoplasmic membrane in many cells. To assess the possible association of cytochrome oxidase activity with the intracytoplasmic membranes, enzyme localization experiments were conducted with the cytochemical substrate 3,3'-diaminobenzidine. The results showed that a cyanide-sensitive cytochrome oxidase activity is located at the intracytoplasmic membrane. The quantity of cytochrome oxidase activity present in the internal membranes is probably less than that present in the cytoplasmic membrane.