Mitochondria are organelles found in the cytoplasm of all eukaryotic cells. They vary considerably in shape and size, but are all composed of four compartments: a smooth outer membrane, a convoluted inner membrane that forms recognizable structures called cristae, the intermembrane space, and the matrix. Mitochondria are the "powerhouses" of cells; their function is to convert energy found in nutrient molecules and store it in high-energy phosphate bonds in a molecule called adenosine triphosphate, which is the universal energy-yielding component necessary for the reactions that modulate many fundamental cellular processes. Mitochondrial ATP is produced through the process of oxidative phosphorylation, a process that uses molecular oxygen as the final electron acceptor.
The products of metabolism are carried from the cytoplasm into the mitochondrial matrix, where they go through the citric acid, or Krebs cycle. The Krebs cycle promotes the reduction of the catabolism-generated coenzymes NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) to NADH and FADH2, respectively, which are rich in electron energy. As these molecules are reoxidized, they supply electrons that are carried to final electron acceptor via an elaborate respiratory, or electron, transport chain. The electron transport system is a chain of electron acceptors located in the inner membrane of the mitochondria.
Hydrogens are passed down from NADH to the electron transport chain in a series of redox reactions, where they become dissociated from their electrons and are released as protons. The electrons entering the electron transport system have a relatively high energy content. As they are transferred from one acceptor molecule to the next, they lose much of their energy, some of which is used to pump the protons across the inner mitochondrial membrane. This sets up an electrochemical gradient across the inner mitochondrial membrane, which provides the energy for ATP synthesis. Therefore, the function of this chain is to permit the controlled release of free energy to drive the synthesis of ATP from ADP (adenosine diphosphate, formed from the breakdown of ATP) and inorganic phosphate. This oxidative phosphorylation process engages five respiratory-chain enzyme complexes located within the inner mitochondrial membrane. Four of these complexes—I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome-c reductase), IV (cytochrome-c oxidase)—catalyze the transport of electrons to molecular oxygen. Complex V (ATP synthase) uses the proton motive force to form ATP from ADP and inorganic phosphate. Oxygen is the final electron acceptor in the electron transport system, which is why organisms that respire aerobically require oxygen.
Mitochondria contain their own deoxyribonucleic acid (DNA). Each human cell contains several hundred mitochondria and thousands of copies of the mitochondrial genome (mtDNA). The human mtDNA molecule is a closed circular molecule and is 16,569 base pairs (bp) in length. Out of the thirty-seven mitochondrially encoded genes, thirteen encode polypeptides that are subunits of the respiratory chain enzyme complexes; twenty-two encode transfer RNA and two encode ribosomal RNA. The twenty-four genes that encode RNA are needed for mitochondrial protein synthesis.
Relationship of energy functions to cellular and organismic aging
Although oxygen plays an essential role as the terminal electron acceptor during respiration, oxygen and its metabolites are potentially cytotoxic (toxic to cells). During the course of normal oxidative phosphorylation, between 1 percent to 3 percent of all oxygen reduced by mitochondria escape from the electron transport chain into the mitochondrial inner membrane and are converted into reactive oxygen species (ROS) that have the ability to oxidize macromolecules. These oxidants, produced continuously as by-products of the anaerobic metabolic process, include superoxide (0-2), hydrogen peroxide (H202), and hydroxyl radicals (HO-) and are a continuous threat to cellular macromolecules— ROS attacks result in molecular defects found in proteins, lipids, and DNA. However, the damage to the cells is balanced by the existence of cellular enzymatic defenses, which have evolved to battle reactive oxygen species. Unfortunately, these defenses are not perfect, and, cellular macromolecules can become damaged. The accumulation of damaged macromolecules is thought to contribute significantly to aging.
In 1956, Denham Harman first proposed that free radicals play a major role in the aging process by causing cumulative macromolecule damage. Harman subsequently extended his theory and proposed that mitochondria are the major players in aging, since mitochondria are the major targets of free radicals (Harman, 1981). The free-radical theory of aging has gained a lot of support. Many age-correlated genetic data implicate mitochondrial dysfunction in the process of aging. Because of their vulnerability, the mitochondrial DNA molecules are particularly affected. In contrast to nuclear DNA, which is assembled in nucleosomes and protected by histones and other proteins, mtDNA is "naked," facilitating direct ROS attacks. In addition, mtDNA is attached to the inner mitochondrial membrane, and is therefore accessible by the by-products of respiration and a primer target for damage by ROS. Yakes and Van Houten (1997) have shown that the mtDNA damage is more extensive and persists longer than nuclear DNA damage in human cells following oxidative stress.
Age-related mitochondrial bioenergetic defects have also been reported in the electron transport chain and oxidative phosphorylation. Defective electron transport chains increase the production of mitochondrial free radicals, which in turn cause a further decline in mitochondrial functions, leading ultimately to a decline of the ATP level. Since a sufficient supply of ATP is necessary for life, the accumulation of bioenergetically defective cells is a key factor in the process of aging. Furthermore, during aging the ROS-scavenging enzymes decline, which further increases both free radicals and oxidative stress within the mitochondria.
Three different kinds of studies have been used to show that mitochondrial respiratory functions decline with age. First, histochemical analysis of respiratory enzymes has revealed an age-correlated deficiency in cytochrome-c oxidase (complex IV of the respiratory chain). This was first shown by Josef Müller-Höcker (1989) using cardiomyocytes in the human heart. Cox-deficient cardiomyocytes (heart muscle cells) are regularly present in humans beginning in the sixth decade of life. The second type of study, derives from measurements of enzymatic activities of each respiratory chain complex, as shown first by Yen et al. (1989) and Trounce et al. (1989). The third kind of study involves monitoring the changes of the mitochondrial membrane potential. The development of mitochondrial fluorescent indicators and sophisticated fluorescence microscopy has enabled organellar events to be studied (Smiley et al., 1991). As explained above, the energy released during oxidation reactions in the mitochondrial respiratory chain is stored as an electrochemical gradient consisting of transmembrane electrical potential. Since maintenance of membrane potential is essential for ATP synthesis, the decline seen in mitochondrial membrane potential is a good indicator of mitochondria malfunction.
Potential role of DNA damage and DNA mutations
What happens to the mitochondrial genome as it gets older has now been extensively studied and documented. It is known that mtDNA mutations can compromise the mitochondria function in many ways: they can disrupt both transcription and the translation of encoded proteins; they can produce nonfunctional ribosomal RNA, (RNA), transfer RNA (+RNA) and proteins; and they can impair mtDNA replication. The mitochondrial genome has a great ability to mutate during the life span, producing a heterogeneous array of somatic mutations. The mutation rate for the mitochondrial genome is ten to twenty times larger than for nuclear DNA study by Khrapko et al. (1997) showed a several-hundred-fold higher rate of somatic mutations both in vivo and in vitro in human mtDNA than in nuclear DNA. This increased rate is due to both a high spontaneous mutation rate and the sensitivity of the mitochondrial genome to exogenous environmental mutagens. Five different types of mtDNA mutations have been shown to be age-associated: point mutations, deletions, additions, duplications and rearrangements. One problem that has been raised by some experts concerning studies of the age-related accumulation of specific mutations in human mtDNA is that although the level of a specific mtDNA mutation increases substantially with age, any of these age-associated mutations affects no more than 1 percent of the organelle mtDNA molecules. However, a large number of specific mutations are likely to occur at each of the 16,569 nucleotide positions within the mitochondrial genome during a lifetime, so that even if each mutation is found at a low level, the increasing accumulation of a large number of mutations will eventually reach a critical level, leading to nonfunctional mitochondria. Furthermore, the load of mtDNA mutations is usually underestimated, since most of the mutations are only detectable using the polymerase chain reaction (PCR). This technique, routinely used to estimate the relative proportions of age-associated mutant DNA may give biased results, as it is dependent on the choice of primers and PCR conditions selected by the individual conducting the study.
One of the most reported mtDNA mutations is the so-called common deletion. Initially identified by Cortopassi and Arnheim (1990), the accumulation of mtDNA molecules exhibiting a 4,977 base pairs deletion increases with age. This deletion occurs between two thirteen base pairs sequence repeats, removing almost five kilobase pairs of mtDNA that encodes six essential polypeptides of the respiratory chain as well as five tRNAs. This deletion was subsequently shown by many other investigators to increase with age in many different tissues. Many other age-associated mtDNA mutations have been identified Khrapko et al. (1999) used long PCR techniques in single cell cardiomyocyte from elderly patients to show that multiple mutations coexist in various tissues of aged individuals and that single mutations occur within individual cells. A large age-dependent accumulation of specific mutations in a critical control region for mtDNA replication has been shown in human fibroblasts (Michikawa et al., 1999).
The incidence of mutant mtDNA has been found to correlate with oxidative damage to mtDNA. Adachi et al. (1993) provided the first evidence that ROS is responsible for the occurrence of mtDNA deletions. A large number of DNA base modifications resulting from oxidative stress have been reported—the one that has been the most widely studied is oxidized nucleotide 8-OH-dG (8-hydroxy-deoxyguanosine). This specific product of oxidative damage to DNA has been shown to accumulate with age, and it correlates with an increase of mtDNA 7.4 kilobase pairs deletion (Mecocci et al., 1993).
As mitochondrial respiration and oxidative phosphorylation gradually uncouple from each other, the activity of the mitochondrial respiratory chain gradually declines. The immediate consequence of a decline of respiratory functions is a decline of ATP synthesis, which will further elevate ROS generation. As the production of ROS species in mitochondria increases, the oxidative damage is reflected by an increasing number of mtDNA mutations. Therefore, respiratory enzymes will incorporate the defective mtDNA-encoded subunits and show impaired respiratory function. This vicious circle operates in an age-dependent manner and plays an important role in aging. This scenario can be also amplified by exogenous factors—many types of mtDNA mutations occur more frequently in sun-exposed skin and mtDNA deletions in the human lung are significantly increased by cigarette smoking, suggesting that ROS resulting from environmental factors play a role in promoting mtDNA damage during aging. Although the mitochondrial free-radical theory of aging has gained prominence, it is important to remember that aging is a multifactorial biological process and that many other cellular components are involved.
Ultrastructural changes are also seen in the mitochondria of aged individuals. The mitochondria become larger and less numerous and they exhibit vacuolization, cristae rupture, and accumulations of occlusions.
See also Cellular Aging; DNA Damage and Repair; Theories of Biological Aging: DNA Damage; Theories of Biological Aging: Error Catastrophe.
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Khrapko, K.; Bodyak, N.; Thilly, W. G.; van Orsouw, N. J.; Zhang, X.; Coller, H. A.; Perls, T. T.; Upton, M.; Vijg, J.; and Wei, J. Y. "Cell-By-Cell Scanning of Whole Mitochondrial Genomes in Aged Human Heart Reveals a Significant Fraction of Myocytes with Clonally Expanded Deletions." Nucleic Acids Research 27 (1999): 2434–2441.
Khrapko, K.; Coller, H. A.; Andre, P. C.; Li, X. C.; Hanekamp, J. S.; and Thilly, W. G. "Mitochondrial Mutational Spectra in Human Cells and Tissues." Proceedings of the National Academy of Science, U.S.A. 94 (1997): 13798–13803.
Mecocci, P.; Macgarvey, U.; Kaufman, A. E.; Koontz, D.; Shoffner, J. M.; Wallace, D. C.; and Beal, M. F. "Oxidative Damage to Mitochondrial DNA Shows Marked Age-Dependent Increases in Human Brain." Annals of Neurology 34 (1993): 609–616.
Michikawa, Y.; Mazzucchelli, F.; Bresolin, N.; Scarlato, G.; and Attardi, G. "Aging-Dependent Large Accumulation of Point Mutations in the Human mtDNA Control Region for Replication." Science 286 (1999): 774–779.
MÜller-HÖcker, J. "Cytochrome C Oxidase Deficient Cardiomyocytes in the Human Heart, an Age-Related Phenomenon." American Journal of Pathology 134 (1989): 1167–1173.
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Smiley, S. T.; Reers, M.; Mottola-Hartshorn, C.; Lin, M.; Chen, A.; Smith, T. W.; Steele, G. D. J.; and Chen, L. B. "Intracellular Heterogeneity in Mitochondrial Membrane Potentials Revealed by a J-Aggregate-Forming Lipophilic Cation JC-1." Proceedings of the National Academy of Science, U.S.A. 88 (1991): 3671–3675.
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Yen, T.-C.; Chen, Y.-S.; King, K.-L.; Yeh, S.-H.; and Wei, Y.-H. "Liver Mitochondrial Respiratory Functions Decline With Age." Biochemical and Biophysical Research Communications 165 (1989): 994–1003.
See Status of older people: tribal societies; Theories, social
Mitochondria (singular mitochondrion) are abundant organelles present in nearly all eukaryotic cells . The main function of mitochondria is to produce adenosine triphosphate (ATP), the cellular energy source. Mitochondria are believed to be the evolutionary result of early anaerobic (nonoxygen-using) eukaryotic cells engulfing aerobic (oxygen-utilizing) bacteria, resulting in a symbiotic relationship between the two organisms. The eukaryotic cells received ATP in exchange for supplying nutrients to the engulfed bacteria, and the bacteria provided ATP and allowed the eukaryotic cell to survive in the increasing oxygen atmosphere present early in Earth's history.
Evidence for Bacterial Origin
The bacterial origin of these organelles is evident in the structure of the mitochondrion, its method of reproduction, and its genetics. Mitochondria are generally oval to elongated in shape, like bacteria, and are approximately 0.5 to 1 micron in diameter. Two membranes like those present in many types of bacteria surround the mitochondrion.
Animal mitochondria possess an amazingly simple genetic system. The human mitochondrial genome is a circular deoxyribonucleic acid (DNA) molecule (like a bacterial chromosome ) made up of only 16,569 base pairs of DNA encoding thirty-seven genes. The mitochondrial genome is too small to encode all of the genes necessary for the mitochondrion to function. Instead, most of the genes necessary for mitochondrial functions are contained in the nuclear genome. At some point in evolution, these genes were moved from the mitochondrion to the nucleus and integrated into the nuclear chromosomes. The mechanism by which this transfer occurred is unknown.
Mitochondria have their own ribosomes and transfer ribonucleic acid (tRNA) to make mitochondrial-encoded proteins within the mitochondrial matrix (the fluid enclosed by the membrane). The bacterial origins of the ribosome are most evident in the unique sequences of the ribonucleic acid (RNA) and proteins that comprise it. Mitochondrial ribosomes are more like bacterial ribosomes than they are like the cytoplasmic ribosomes made in the nucleus. The mitochondrial machineries that make proteins in the mitochondrial matrix and replicate the mitochondrial DNA are also sensitive to several antibiotics that inhibit bacterial growth. The other cellular systems for protein synthesis and DNA replication in the nucleus are not sensitive to these antibiotics, supporting the notion that the mitochondria have their origin in a bacterial ancestor.
Because many of the mitochondrial components are not encoded by the organelle's DNA, but by the nuclear DNA, mitochondria must have mechanisms to take up their components from the surrounding cytoplasm. The mitochondrial proteins encoded by nuclear genes and synthesized on ribosomes in the cytoplasm are transported into the mitochondria by specific machinery found in the mitochondrial membranes. The transport machinery recognizes unique sequences of amino acids found only in mitochondrial proteins.
During cell growth, the contents of the cell approximately double to ensure that both daughter cells receive a full set of organelles and cytoplasm in addition to the correct number of chromosomes at cell division. The growth and division of mitochondria is not linked to the cell cycle ; instead, mitochondria replicate their DNA and divide mainly in response to the energy needs of the cell. When the energy use by a cell is high, the mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive. At cell division, mitochondria are distributed to the daughter cells more or less randomly by partitioning of the cytoplasm when the cell divides. Mitochondria divide by binary fission similar to bacterial cell division. Unlike bacteria, however, mitochondria can also fuse with other mitochondria.
Membranes and Matrix
The mitochondria are unique organelles in that they are surrounded by two membranes, rather than the single membrane that surrounds other non-nuclear organelles in the cell. The outer mitochondrial membrane completely encloses a large internal space called the matrix. The inner mitochondrial membrane is highly folded into structures called cristae, which significantly increase the surface area of the inner membrane. The narrow space between the inner membrane and the outer membranes is known as the intermembrane space. The outer membrane contains a large number of proteins that form pores or channels through the membrane allowing small molecules to pass freely between the intermembrane space and the cytoplasm. These pores make the outer membrane permeable to most ions and small molecules; therefore, the intermembrane space has the same ionic composition as the cytoplasm surrounding the mitochondrion.
The inner membrane, on the other hand, is relatively impermeable and blocks the movement of ions and other small molecules. Both the inner and outer mitochondrial membranes contain specific transport proteins that can move large molecules or ions across each by both passive active transport. Only those large molecules that have specific membrane transporters are able to enter mitochondria. This allows the mitochondria to create a unique biochemical environment within the matrix to carry out the energy production reactions.
ATP is the main source of energy for most processes in the cell, and large quantities must be made for a cell to function. Most cells use simple sugars, such as glucose , as their primary energy source. The metabolism of glucose begins in the cytoplasm with a process called glycolysis . During glycolysis, glucose is processed from a 6-carbon sugar to two molecules of a 3-carbon compound called pyruvate . However, glycolysis is an inefficient process, yielding only two ATP molecules for each molecule of glucose metabolized. The pyruvate formed in glycolysis can be further metabolized in mitochondria to gain another thirty molecules of ATP from a single original glucose molecule. In addition to the metabolism of pyruvate, fatty acids derived from dietary fat can also be used by mitochondria to make ATP.
Krebs Cycle. The metabolic functions of the mitochondrion occur within the matrix and the inner mitochondrial membrane. The matrix contains a highly concentrated mixture of the enzymes of the Krebs, or citric acid, cycle and enzymes for the degradation of fatty acids. Pyruvate and fatty acids from the cytoplasm are actively transported into the mitochondrial matrix by specific membrane transporters that span both the outer and the inner membranes. Inside the matrix, both pyruvate and the fatty acids are first converted to an activated 2-carbon compound called acetyl-Coenzyme A that is the starting point of the Krebs cycle . The enzymes of the Krebs cycle process the acetyl CoA, removing high-energy electrons that will be used as an energy source to produce ATP. The high-energy electrons from the Krebs cycle are stored on the specialized carrier molecules that carry the electrons from the matrix to the inner mitochondrial membrane.
Electron Transport Chain. The high-energy electrons from the Krebs cycle are passed to a series of three large protein complexes located in the inner mitochondrial membrane, known as the electron transport chain. Each complex is made up of several proteins organized to form a pathway that moves electrons through the complex. The electrons from the Krebs cycle enter the chain at a very high energy and gradually give up part of their energy as they move through the electron transport chain. The energy from the electrons is used to pump hydrogen ions across the inner mitochondrial membrane from the matrix to the intermembrane space by active transport. This creates a chemical and electrical gradient across the inner membrane, storing energy in much the same way a battery does.
Defects in mitochondrial genes are responsible for numerous maternally inherited diseases, including a number of muscle diseases (myopathies).
Low-energy electrons that emerge from the end of the electron transport chain are combined with an oxygen atom forming one molecule of water for every two electrons that pass through the chain. Because of the similarity to respiration by the body (inhaling oxygen and exhaling carbon dioxide), the Krebs cycle and the electron transport chain together are sometimes referred to as cellular respiration. The CO2 that humans exhale is a product of the Krebs cycle, and the oxygen humans breathe in is used as the final electron acceptor in the electron transport chain.
ATP Synthesis. The electrochemical gradient is harnessed to produce ATP by an enzyme in the inner membrane called ATP synthase. As the hydrogen ions flow back down their concentration gradient into the matrix through the ATP synthase complex, the energy released is used to add a phosphate group to adenosine diphosphate (ADP ) to make adenosine triphosphate. Because the actions of the electron transport chain and ATPase are tightly linked, the combination of the two is referred to as oxidative phosphorylation. In fact, some types of mitochondria uncouple the movement of electrons through the electron transport chain from ATP syntheses by shuttling electrons back across the inner membrane. Since the energy is not used to make ATP it can be released as heat energy. Such mitochondria are found in "brown fat" in babies and hibernating animals, and are an important source of heat.
The maternal inheritance of mitochondria is being used to trace human evolutionary groups back to their African origins.
Within a cell, mitochondria are typically positioned near areas of high ATP use such as near the contractile apparatus of muscle cells or wrapped around the whiplike tail of the sperm. The positioning of mitochondria within a cell is at least partially due to attachment to the microtubule cytoskeleton . The cytoskeleton is a very dynamic scaffold within the cytoplasm, constantly growing and retracting. As it grows and retracts, it drags along attached organelles such as mitochondria. Destruction of the microtubules with specific drugs leads to a disorganized arrangement of the mitochondria that is restored when the cytoskeleton is allowed to reform.
Mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote nucleus. However, all of the mitochondria, and therefore all the mitochondrial genes, are contributed to the zygote by the egg. At fertilization of an egg, a single sperm enters the egg along with the mitochondria that it uses to provide the energy needed for its swimming behavior. However, the mitochondria provided by the sperm are targeted for destruction very soon after entry into the egg. The egg itself contains relatively few mitochondria, but it is these mitochondria that survive and divide to populate the cells of the adult organism. This type of inheritance is called maternal inheritance and is common to the mitochondria of all animals. Because mitochondria are inherited from the mother only, the sequence of mitochondrial DNA is sometimes used to trace the lineage of families.
see also Cell Evolution; Cytoskeleton; Glycolysis and Fermentation; History of Biology: Biochemistry; Human Evolution; Krebs Cycle; Metabolism, Cellular; Oxidative Phosphorylation; Primate
Stephen A. Adam
Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.
Mitochondria are specialized structures inside a cell that break down food and release energy. If a cell is like a tiny chemical processing plant, then the mitochondria are the power plants of the cell. Without mitochondria, a cell with a nucleus cannot use oxygen and cannot live. Mitochondria are found only in eukaryotic cells (those with a nucleus) and are not found in prokaryotic cells (those without a nucleus).
Mitochondria (singular, mitochondrian) are described as organelles. An organelle is a tiny structure inside a cell that is controlled by the nucleus and has a specific function to play in maintaining the life of the cell. For a cell, mitochondria play a most important role in carrying out aerobic respiration. In other words, mitochondria break down the food a cell takes in and release the energy it contains. Examined under a microscope, mitochondria appear as oval- or sausage-shaped structures that have a double membrane. The outer membrane is smooth and permeable to certain enzyme molecules, meaning that molecules of a certain size
can flow through its walls. The inner membrane consists of many folds that allow it to have an increased surface area and make it able to pack in many more enzymes than would be possible without the folds. A eukaryotic cell typically has anywhere from a dozen to a thousand mitochondria, and animal cells usually have more mitochondria than plant cells. Mitochondria are especially abundant in cells whose functions have high energy demands.
As the "powerhouse" of the cell, mitochondria carry out respiration. Respiration is the chemical process that breaks down food to release energy. Two key ingredients are necessary for respiration—sugar and oxygen. The sugar that cells use is usually in the form of glucose, which contains a great deal of stored energy. Oxygen is used to get the energy out of the glucose. This process takes place in the folds of the mitochondria where a substance known as adenosine triphosphate or ATP is created. ATP is the molecular storage form of energy that mitochondria produce. When a cell needs energy, it draws on the stored ATP, which releases the energy.
Here is a simplified example of how the human body uses mitochondria to release energy from food. For an organism like the human body, most of its energy comes from mitochondria, which are the body's power plant that burns the fuel to produce the energy to run it. The food a person eats is the fuel that is "burned" in the body's furnaces (mitochondria). The ATP produced by this process is the energy or "electricity" produced by the power plant (which powers the cells in the body). When a person breathes, he or she is taking in oxygen for the mitochondria to use to release the energy the body needs. This process is similar in all organisms, since all organisms must break down food to get energy. As a result, all life forms have cells that contain mitochondria.
[See alsoCell; Organelle ]
mi·to·chon·dri·on / ˌmītəˈkändrēən/ • n. (pl. -dri·a / -drēə/ ) Biol. an organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. It has a double membrane, the inner layer being folded inward to form layers (cristae).DERIVATIVES: mi·to·chon·dri·al / -drēəl/ adj.