Like the bacteria from which they originated, mitochondria have an outer and an inner membrane. The two membranes have distinct functions and properties, and delineate separate compartments within the organelle. The inner membrane, which surrounds the internal mitochondrial matrix compartment (Figure 14–8), is highly folded to form invaginations known as cristae (the singular is crista), which contain in their membranes the proteins of the electron-transport chain. Where the inner membrane runs parallel to the outer membrane, between the cristae, it is known as the inner boundary membrane. The narrow (20–30 nm) gap between the inner boundary membrane and the outer membrane is known as the intermembrane space. The cristae are about 20 nm-wide membrane discs or tubules that protrude deeply into the matrix and enclose the crista space. The crista membrane is continuous with the inner boundary membrane, and where their membranes join, the membrane forms narrow membrane tubes or slits, known as crista junctions.
Like the bacterial outer membrane, the outer mitochondrial membrane is freely permeable to ions and to small molecules as large as 5000 daltons. This
Figure 14–8 Structure of a mitochondrion. (a) tomographic slice through a three-dimensional map of a mouse heart mitochondrion determined by electron-microscope tomography. the outer membrane envelops the inner boundary membrane. the inner membrane is highly folded into tubular or lamellar cristae, which crisscross the matrix. the dense matrix, which contains most of the mitochondrial protein, appears dark in the electron microscope, whereas the intermembrane space and the crista space appear light due to their lower protein content. the inner boundary membrane follows the outer membrane closely at a distance of ≈20 nm. the inner membrane turns sharply at the cristae junctions, where the cristae join the inner boundary membrane. (B) tomographic surface-rendered portion of a yeast mitochondrion, showing how flattened cristae project into the matrix from the inner membrane (Movie 14.2). (C) Schematic drawing of a mitochondrion showing the outer membrane (gray), and the inner membrane (yellow). Note that the inner membrane is compartmentalized into the inner boundary membrane and the crista membrane. there are three distinct spaces: the inner membrane space, the crista space, and the matrix. (a, courtesy of tobias Brandt; B, from K. Davies et al., Proc. Natl Acad. Sci. USA 109:13602–13607, 2012. With permission from the National academy of Sciences.)
Figure 14–9 Biochemical fractionation of purified mitochondria into separate components. Large numbers of mitochondria are isolated from homogenized tissue by centrifugation and then suspended in a medium of low osmotic strength. In such a medium, water flows into mitochondria and greatly expands the matrix space (yellow). While the cristae of the inner membrane unfold to accommodate the swelling, the outer membrane— which has no folds—breaks, releasing structures composed of the inner membrane surrounding the matrix. these techniques have made it possible to study the protein composition of the inner membrane (comprising a mixture of cristae, boundary membranes, and cristae junctions), the outer membrane, and the matrix.
is because it contains many porin molecules, a special class of β-barrel-type membrane protein that creates aqueous pores across the membrane (see Figure 10–23). As a consequence, the intermembrane space between the outer and inner membrane has the same pH and ionic composition as the cytoplasm, and there is no electrochemical gradient across the outer membrane.
If purified mitochondria are gently disrupted and then fractionated (Figure 14–9), the biochemical composition of membranes and mitochondrial compartments can be determined.
the Inner Membrane Cristae Contain the Machinery for Electron transport and atp Synthesis