Mitochondria are organelles involved in respiration and energy production in eukaryotes. The shape, number, size and even ultra structure of mitochondria vary between cells, tissues and organisms. Progress in the field of 3D imaging of ultra structural refinement by cryo electron tomography, largely using cryo scanning electron microscopy and focused ion beam/SEM procedures, has greatly enhanced our understanding of cellular complexity.
Mitochondria are large organelles (0.5-5.0 μm) in most mammalian cells and show a highly complex ultrastructural architecture. They are surrounded by two membranes, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM), which are separated by the intermembrane space (IMS) and the mitochondrial matrix, respectively. Mitochondria have morphological substructures adapted to the requirements of cellular metabolism that arise from invaginations of the IMM into the matrix space.
The rigid and smooth part of the IMM is termed the inner boundary membrane (IBM), the side bearing the invaginations is termed the cristae membrane, and the parts forming wide vesicles with slight brims at the openings are referred to as the necks. Cristae are the shelf-like folds of the IMM that protrude inwards the matrix. Cristae are barriers for electrical communication between different regions of the matrix. They can increase the surface-to-volume ratio of the mitochondria and are sites of oxidative phosphorylation.
Outer Mitochondrial Membrane:
Technically, the outer membrane is a smooth and relatively thin membrane 70–80 À wide and deficient in general kay colleges. It contains, however, a number of integral proteins of high molecular mass which are best demonstrated by the freeze-fracture technique. Since a considerable proportion of these proteins is of non-mitochondrial origin, the possibility arises that their incorporation into the outer membrane involves a number of intermediary steps involving one or more intracellular organelles including endoplasmic reticulum and Golgi apparatus in eukaryotes. The situation is completely different from that of inner membrane proteins which are almost exclusively synthesized at the level of mitochondria themselves. At present it is very incomplete information on the transition routes of precursors and the mechanisms of incorporation into the outer membrane. It is unknown whether there is a cytosolic factor mediating the transfer of precursors to the organelle, how the translocase of the outer membrane operates and whether Lap is involved in insertion and subsequent folding of the precursors. There are also persistent problems regarding the composition of the lipid milieu in which integral proteins of the outer membrane are solubilized and how the events of outer membrane biogenesis are altered in genetically altered mitochondria producing abnormal precursor polypeptides.
Inner Mitochondrial Membrane
The inner mitochondrial membrane is a ubiquitous feature of mitochondria in all oxygen-respiring cells. Aside from enveloping the matrix with its boundary plan, it has a significant role in energy capture in respiration and photophosphorylation, the conformational state of the mitochondrial coupling machinery, the divalent ion entry and reabsorption reaction, and the regulation of the mitochondrial metabolism through adenine-nucleotide translocation (Chandra, 1962). The inner mitochondrial membrane contains carrier proteins that transport a variety of small molecules and an electrochemical potential between the matrix and the cytosol. However, the most prominent feature of the inner mitochondrial membrane is the invagination to various extents, forming cristae. They sacrificially enlarge the surface area of the inner membrane that accommodates metabolic proteins and use it for the production of chemical energy in cellular respiration. On the basis of the form, cristae are divided into three types: lamellar, tubular, and platelet. The remainder of the inner mitochondrial membrane, excluding the cristae, is portioned as the boundary membrane. The cristae form a continuous, membrane-bound chamber, which is divergent from the vacuolation space of the boundary membrane. Their diameter, which is variable and up to 600 nm, relates to the total amount of oxidative phosphorylation proteins across the inner membrane during mitochondrial remodeling.
The prominent features of the inner mitochondrial membrane are cristae formation and two types of boundary membranes. Cristae formation within the inner membrane is a ubiquitous feature of mitochondria. Tubular cristae (TC), which transform into proximal tubular cristae (PTC) via intermediate tubular cristae (ITC), protrude into a heart-shaped chamber (CF). Tubular cristae (TC) traverse the lumina of parallel invaginations of the inner membrane in mitochondria. But at their tips, they turn into a plate that forms a side by which it is divided from the boundary membrane, giving rise to a phyllocristae structure.
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