Energy production

The main function of the mitochondrion is the production of energy, in the form of adenosine triphosphate (ATP). The cell uses this energy to perform the specific work necessary for cell survival and function.

The raw materials used to generate ATP are the foods that we eat, or tissues within the body that are broken down in a process called catabolism. The breaking down of food into simpler molecules such as carbohydrates, fats, and protein is called metabolism. These molecules are then transferred into the mitochondria, where further processing occurs. The reactions within the mitochondria produce specific molecules that can have their electrical charges separated within the inner mitochondrial membrane. These charged molecules are processed within the five electron transport chain complexes to finally combine with oxygen to make ATP. The process of the charged substances combining with oxygen is called oxidation, while the chemical reaction making ATP is called phosphorylation. The overall process is called oxidative phosphorylation. The product produced by this process is ATP.

Programmed cell death

Cell death can occur either by injury due to toxic exposure, by mechanical damage, or by an orderly process called programmed cell death or apoptosis. Programmed cell death occurs during development as the organism is pruning away unwanted, excess cells. It also occurs during infections with viruses, cancer therapy, or in the immune response to illness. The process of programmed cell death is another function of mitochondria.

Normally, ATP production is coupled to oxygen consumption. During abnormal states such as fever, cancer, or stroke, or when dysfunction occurs within the mitochondria, more oxygen is consumed or required than is actually used to make ATP. The mitochondria become partially “uncoupled” and produce highly reactive oxygen species called free radicals. When the production of free radicals overwhelms the mitochondria’s ability to “detoxify” them, the excess free radicals damage mitochondrial function by changing the mitochondrial DNA, proteins, and membranes. As this process continues, it can induce the cell to undergo apoptosis. Abnormal cell death due to mitochondrial dysfunction can interfere with organ function.

Cell-specific functions

Other functions of mitochondria are related to the cell type in which they are found. Mitochondria are involved in building, breaking down, and recycling products needed for proper cell functioning. For example, some of the building blocks of DNA and RNA occur within the mitochondria. Mitochondria are also involved in making parts of blood and hormones such as estrogen and testosterone. They are required for cholesterol metabolism, neurotransmitter metabolism, and detoxification of ammonia in the urea cycle. Thus, if mitochondria do not function properly, not only energy production but also cell-specific products needed for normal cell functioning will be affected.

By Russell P. Saneto, D.O., Ph.D., Children’s Hospital and Regional Medical Center/University of Washington School of Medicine, Seattle, WA.

Topic Editor: Russell P. Saneto,D.O.,Ph.D.
Last Reviewed:6/8/04