The three-dimensional organization of the internal compartments of conventionally fixed and embedded rat-liver mitochondria has been determined by tomographic reconstruction from tilt-series images collected on the Albany high-voltage electron microscope. The results indicate that the inner membranes of these organelles are predominantly tubular in the orthodox (expanded matrix) conformation, as previously suggested by scanning electron microscopy. In the condensed (contracted matrix) conformation, the intracristal space opens up into large irregularly shaped compartments which are connected to each other and to the external (intermembrane) space by tubes with approximately the same diameter (20 nm) as those observed in the orthodox state. These results raise several questions, in particular about the nature of the structural transitions that occur in the cristae during matrix expansion and contraction, and about the influence of inner-membrane shape on the diffusion of ions and metabolites between the intracristal and intermembrane compartments.

No abstract

During oxidative phosphorylation, free energy of substrate oxidation is used to generate electrochemical gradients across the mitochondrial inner membrane. Full understanding of this process requires knowing the pathways for internal diffusion of ions and metabolites inside the organelle. This, in turn, requires detailed information about the organization of the membranes that compartmentalize the mitochondrion and the distribution of transport proteins (proton pumps, ion channels, metabolite carriers) on these membranes.We have undertaken the study of the compartmentation of rat-liver mitochondria (conventionally fixed and plastic-embedded) using electron microscopic tomography. Reconstructions of isolated mitochondria have been computed using projection images collected on the Albany high-voltage electron microscope from sections (0.5-1-μm thick) tilted around one axis over +/− 70° at 2° increments. A problem encountered with this approach is the directional loss of resolution due to the “missing wedge” of information in Fourier space in the direction parallel to the tilt axis. To overcome this problem, we have developed an alignment algorithm that allows projections collected over two or more tilting directions to be used in the same modified back-projection calculation.