Mitochondria have a distinctive appearance when viewed by electron microscopy. They often appear as rounded or sausage-shaped structures (Figure 1a, b and Figure 22a, b), measuring about 0.5-1.0 µm in diameter and 2-8 µm in length; although their size and shape vary, and they are often much bigger in plants. For many years all mitochondria were assumed to have this fairly regular shape, but recent research, in which mitochondria have been observed in living cells, has shown that mitochondria are dynamic; they divide and fuse, and the shapes of individual mitochondria change. Mitochondria also move; their positions in the cell are not fixed. In many cells, mitochondria are long and extended and have a network-like structure (Figure 22c). So, rather like the ER, the appearance of mitochondria as round or elongated and flattened spheres in micrographs is often simply due to the plane in which they are sectioned.
Mitochondria have a double membrane; the inner membrane is thrown into many folds known as cristae, which are what gives the organelle its distinctive appearance. The two membranes effectively create two compartments in the mitochondrion. The intermembrane space lies between the two membranes; the much larger mitochondrial matrix lies within the inner membrane, as shown in Figure 22b.
The mitochondrial matrix contains many enzymes, including some of those involved in the breakdown of glucose. It also contains mitochondrial DNA, and the molecules and cellular components needed to transcribe and translate mitochondrial genes including ribosomes. Mitochondrial genes encode most of the proteins of the inner mitochondrial membrane, which plays a fundamental role in the generation of ATP. The majority of mitochondrial proteins, however, are encoded by the cell's nuclear DNA and are synthesised in the cytosol and imported into the mitochondrion. Again, the correct delivery of mitochondrial proteins from the cytosol depends on specific signal or targeting sequences.
Mitochondria produce most of the ATP in eukaryotic cells, and are hence more abundant in cells that have high requirements for energy, such as muscle cells.