Mitochondria

The mitochondria are literally the "power plants" of the cell because they transform organic compounds into en­ergy that is easily accessible to the cell. Mitochondria do not make energy, but they extract it from organic com­pounds. Mitochondria contain the enzymes needed for capturing most of the energy in foodstuffs and converting it into cellular energy. Generally, this multistep process is called cellular respiration because it requires oxygen. Cells store most of this energy as high-energy phosphate bonds found in compounds such as adenosine triphosphate (ATP), which powers the various cellular activities. Energy that is not used or stored is dissipated as heat used to main­tain body temperature.

Mitochondria are found close to the site of energy consumption in the cell (e.g., near the myofibrils in mus­cle cells). The number of mitochondria in a given cell type varies by the type of activity the cell performs and the en­ergy needed to undertake this activity. For example, a dra­matic increase in mitochondria occurs in skeletal muscle repeatedly stimulated to contract.

Mitochondria are composed of two membranes: an outer membrane that encloses the periphery of the mito­chondrion and an inner membrane, which forms shelf-like projections, called cristae (Fig. 4-6). The outer and

RE 4-6 Mitochondrion. The inner membrane forms transverse folds called cristae, where the enzymes needed for the final step in adenosine triphosphate (ATP) production (i.e., oxidative phosphor-ylation) are located.

inner membranes form two spaces. An outer space is found between the two membranes. Deep to the outer space, the inner or matrix space contains an amorphous matrix. The outer mitochondrial membrane is involved in lipid synthesis and fatty acid metabolism, whereas the inner membrane contains the respiratory chain enzymes and transport proteins needed for the synthesis of ATP.

Mitochondria contain their own DNA and ribosomes and are self-replicating. The DNA is found in the mito­chondrial matrix and is distinct from the chromosomal DNA found in the nucleus. Mitochondrial DNA, known as the "other human genome," is a double-stranded, circular molecule that encodes the ribosomal RNA (rRNA) and trans­fer RNA (tRNA) required for intramitochrondial synthesis of proteins needed for the energy-generating function of the mitochondria. Although mitochondrial DNA directs the synthesis of 13 of the proteins required for mitochon­drial function, the DNA of the nucleus encodes the structu­ral proteins of the mitochondria and other proteins needed to carry out cellular respiration.

Mitochondrial DNA is inherited matrilineally (i.e., from the mother), thus providing a basis for familial lineage studies. Mutations have been found in each of the mito­chondrial genes, and an understanding of the role of mito­chondrial DNA in certain diseases is beginning to emerge. Most tissues in the body depend to some extent on oxida­tive metabolism and can therefore be affected by mito­chondrial DNA mutations.