Ultrastructure of isolated heavy beef heart mitochondria revealed by the freeze-etching technique

A study of rapid mitochondrial structural changes in vitro by spray-freeze-etching

The spray-freeze-etching technique has been used to study energy-linked mitochondrial structural changes in rat liver mitochondria incubated in vitro. The technique involves spraying the suspension of mitochondria into liquid propane at -190 degrees C, and does not require the use of cryoprotectants or chemical fixatives. The results confirmed that freshly isolated mitochondria have a condensed matrix and that this expands at the expense of the outer compartment to give the orthodox configuration when the mitochondria are incubated in a K+ medium in the presence of substrate and phosphate. Addition of adenosine diphosphate (ADP) caused a rapid shrinkage of the matrix compartment, and the time-course and extent of this shrinkage has been measured quantitatively by coupling a rapid sampling device to the spray-freezing apparatus. These data show that for orthodox mitochondria the onset of phosphorylation is accompanied by a reduction of 30% in the matrix volume in 20's, and there is no evidence that the decrease in matrical volume affects the phosphorylation efficiency. These results suggest that natural ionophores in the mitochondrial inner membrane make it permeable enough to permit a rapid readjustment of matrix volume after the addition of ADP, and that the associated ion movement does not cause uncoupling of oxidative phosphorylation.

Ultrastructure of free-living and nitrogen-fixing forms of Rhizobium meliloti as revealed by freeze-etching

Freeze-etching of Rhizobium meliloti provided considerable insight into the ultrastructure of this bacterium and into the changes accompanying the transformation from the free-living rod forms to the nitrogen-fixing bacteroid forms. In the small rods, one cleavage plane was revealed at the level of the cell wall and a second at the level of the plasma membrane. Very little structure was evident at the cell wall level, but distinctly different convex and concave fracture faces were exposed at the cell membrane cleavage plane. During the transformation into the bacteroidal state the wall decreased in thickness, became less rigid, and developed a particulate surface. In addition, changes in particle density were observed in the plasma membrane. The fine structure of the plant membranes, the infection threads, and the arrangement of the bacteroids within the plant cells also were revealed.