What is Mitochondria? Definition, Functions, Structure, Autonomy, Evolution, Enzymes

Mitochondria

The mitochondrion (plural mitochondria) derived from Greek words; mitos, thread, and chondrion, "granule" or "grain-like", they are the granular or filamentous cell organelles that is present in the cytoplasm of aerobic cells of higher animals, plants and some microorganisms including protozoa, algae and fungi.

Mitochondria are found in nearly all eukaryotic cells and occupy a substantial portion of the cytoplasm. They are absent in prokaryotic cells and anaerobic eukaryotes.

Almost all the eukaryotic cell has mitochondria, though they are lost in the later stages of development of cell like in the red blood cells or in elements of phloem sieve tube. Like in other eukaryotic cells, the mitochondria in plants play an important role in the production of  ATP via the process of oxidative phosphorylation. Mitochondria also play essential roles in other aspects of plant development and performance. It also has various properties which allow the mitochondria to interact with special features of metabolism in plant cell.

Fig.5.1 Structure of plant cell showing mitochondria

Mitochondria are well-defined cytoplasmic organelles of the cell which take part in a variety of cellular metabolic functions. Survival of the cells requires energy to perform different functions. The mitochondria are important as the fact that these organelles supply all the necessary biological energy of the cell, and they obtain this energy by oxidizing the substrates of the Krebs cycle. Energy of the cell is got from the enzymatic oxidation of chemical compounds in the mitochondria. Hence, the mitochondria are referred to as the 'power house' of the cell. Mitochondria generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. 

They contain a battery‘ of enzymes and coenzymes that interact to catalyze cellular energy transformation. The enzymes also produce specific DNA for the cytoplasmic inheritance and ribosomes for protein synthesis.

The energy is derived by the breakdown of carbohydrates, amino acids and fatty acids and is used in the formation of energy rich molecules the ATP (often referred to as the energy currency of the cell) by the process of oxidative phosphorylation. In addition to supplying cellular energy, mitochondria are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining control of the cell cycle and cell growth. Although most of a cell DNA is contained in the cell nucleus, the mitochondrion has its own independent genome that shows substantial similarity to bacterial genomes.

Mitochondrial proteins (proteins transcribed from mitochondrial DNA) vary depending on the tissue and the species. Mitochondria have their own circular DNA and synthesize some of their proteins. Thus, they are said to be "semi-autonomous" organelles. Most of the proteins required by the mitochondria, however, are encoded by the nuclear genes and are imported from the cytosol. The organelle is believed to have originated by the process of endosymbiosis.

The first observations of intracellular structures that probably represented mitochondria were published in the 1840s. Rudolf Kоlliker (1880) was the first who observed the granules (mitochondria) in striated muscle cell of insects. Richard Altmann, (1890) established them as cell organelles and called them "bio blasts". The term "mitochondria" was coined by Carl Benda (1898). In the 1920s, a biochemist Warburg found that oxidative reactions takes place in most tissues in small parts of the cell. Leonor Michaelis discovered that Janus green can be used as a supravital stain for mitochondria in 1900. In 1904, Friedrich Meves, made the first recorded observation of mitochondria in plants, in cells of the white-water lily, Nymphaea alba and in 1908, along with Claudius Regaud, suggested that they contain proteins and lipids.

What is Mitochondria

Mitochondria are cell organelles of aerobic eukaryotes which take part in oxidative phosphorylation and Krebs cycle of aerobic respiration. They are called power houses of a cell because they are the major centers of release of energy in the aerobic respiration. Cells of dormant seeds have very few mitochondria. But that of germinating seeds have several mitochondria. In general, green plant cells contain less number of mitochondria as compared to non-green plant and animal cells.

Fig.5.2 A detailed structure of Mitochondria (section view)

The position of mitochondria in a cell depends upon the requirement of energy and amino acids. In unspecialized cells they are randomly distributed throughout the cytoplasm. In absorptive and secretory cells, they lie in the peripheral cytoplasm. During nuclear division, more of mitochondria come to lie around the spindle. Mitochondria are more abundant at the bases of cilia or flagella to provide them energy for movements. In muscle fibres they occur in rows in the regions of light bands in between the contractile elements.

Morphology of Mitochondria

Mitochondria are very dynamic organelle and may interact extensively with the other cellular structures. These are bean-shaped organelle that occurs free in the cytoplasm. The shape of mitochondria varies according to the functional stages of the cell. In general, these organelles are filamentous or granular in shape. A long mitochondrion may swell at one end to assume the form of a club or be hollowed out to take the form of a tennis racket. At other times, mitochondria may become vesicular by the appearance of a central clear zone.

The size of mitochondria is also variable; however, in most cells the width is relatively constant (0.5 to 3 µm), but vary considerably in length (reaching a maximum of 10 µm) and structure. They are large enough to be resolved in the light microscope (though ultra structure can be studied only under electron microscope) but unless specifically stained, they are generally not visible due to lack of contrast structure. Special stains are used to make them visible, for example Janus Green B. The mitochondria are highly plastic and constantly change their shape and position. In some cells, however, they remain in a fixed position and provide ATP, e.g., the muscle cells and around the flagellum of a sperm.

They are uniformly distributed throughout the cytoplasm, but there may be exceptions. For example, in certain muscle cells, mitochondria are grouped like rings or braces around the I�band of the myofibril or in sperm they are wrapped tightly around the flagellum. The number of mitochondria in a cell varies from one to several and depends upon the type and functional state of the cell (cellular activities). A normal liver cell contains between 1,000 and 1,600 mitochondria but this number diminishes during regeneration and also in cancerous state. The number may be as high as 3, 00,000 in some oocytes, but some algal cells may contain only one mitochondrion.

Structure of Mitochondria

Mitochondria are double membranous organelle; each mitochondrion is bounded by two highly specialized membranes. An outer limiting membrane of about 6 nm thick is surrounding the organelle. Within this membrane, separated by a space of about 6 to 8 nm, is an inner membrane.

The outer membrane is smooth and continuous but the inner membrane projects into the mitochondrial cavity and forms complex infoldings called mitochondrial crests. It folds into finger-like projections called cristae that project into the matrix. The two chemically and physiologically different membranes in mitochondria create two separate mitochondrial compartments; the matrix and the intermembrane space. The two membranes have different properties. Because of this double-membrane organization, there are five distinct parts of a mitochondrion.

1. Outer mitochondrial membrane,
2. Intermembrane space (the space between the outer and inner membranes),
3. Inner mitochondrial membrane,
4. Cristae space (formed by infoldings of the inner membrane), and
5. Matrix (space within the inner membrane).

1. The Outer Membrane

The outer mitochondrial membrane, which encloses the entire organelle, is 60 to 75 angstroms (Å) thick. It has a protein-to-phospholipid ratio similar to that of the cell membrane (about 1:1 by weight). 

The outer membrane is a simple phospholipid bilayer, containing 50% lipid and 50% proteins by weight. A mixture of enzymes is involved in degradation of tryptophan and the elongation of fatty acids. The outer mitochondrial membrane is homologous to an outer membrane present in as part of the cell wall of certain bacterial cells.

In contrast to the inner membrane, outer mitochondrial membrane is highly permeable as it contains large numbers of integral membrane proteins or special protein called porins. It forms an aqueous channel which allows free diffusion of molecules of 5000 Daltons or less. A major trafficking protein is the pore-forming voltage dependent anion channel (VDAC). The VDAC is the primary transporter of nucleotides, ions and metabolites between the cytosol and the intermembrane space.

The outer membrane also contains enzymes involved in such diverse activities as the elongation of fatty acids, mitochondrial lipid synthesis, oxidation of epinephrine, and the degradation of tryptophan. Outer membrane contains many other proteins such as the receptor proteins that recognize the import signal and also the enzymes that are involved in the division and fusion of the mitochondria. Mitochondria stripped of their outer membrane are called mitoplasts.

2. Intermembrane space

The outer chamber or intermembrane is the space between the outer membrane and the inner membrane, usually 60–75 Å wide and filled with watery fluid having a few enzymes. It is also known as peri-mitochondrial space and can be increased by placing the isolated mitochondria in a sucrose solution. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules, such as ions and sugars in the intermembrane space is the same as in the cytosol. 

However, large proteins must have a specific signaling sequence to be transported across the outer membrane; therefore the protein composition of this space is different from the protein composition of the cytosol. One protein that is localized to the intermembrane space in this way is cytochrome c. It has an important role in the primary function of mitochondria, which is oxidative phosphorylation. The intermembrane space contains the enzymes that use the ATP to phosphorylate other nucleotides.

3. Inner Membrane

As mentioned earlier the inner membrane is impermeable to most ions and small charged molecules and thus forms a functional barrier to the free passage of molecules between the cytosol and the matrix. The inner membrane contains more than 100 different polypeptides; it has high protein-to-lipid ratio (80:20). It is rich in unusual phospholipids named cardiolipins (which have four fatty acids rather than two), which is characteristic of bacterial membrane.

Presence of phospholipid cardiolipin makes the inner membrane impermeable to ions though oxygen, carbon dioxide and water can move freely through this layer. This property enables the membrane to maintain the proton gradient that drives the oxidative phosphorylation.

The inner membrane is thus the principal site of ATP synthesis. Another structural feature that enables ATP synthesis is that the inner membrane is folded into finger-like projections called cristae (singular-crista). The folding of the inner membrane increases the total surface area. The number and shape of the cristae is highly variable. Greater the demand of ATP will leads to more the number of cristae.

Fig. 5.3. Production of ATP in the inner membrane of Mitochondria

The inner membrane is highly complex, containing complexes of the electron transport chain, the ATP synthase and transport proteins (Fig. 5.3). The proteins present in the inner membrane can be divided into three principal types, those that are part of the electron transport chain and carry out oxidation reaction, ATP synthase (the enzyme that is involved in synthesis of ATP) and transport proteins involved in the transport of molecules like fatty acids and pyruvate between the cytosol and mitochondria). The main functions of the inner mitochondrial membrane proteins are:

• Oxidative phosphorylation
• ATP synthesis
• Regulation of protein transport
• Protein import
• Mitochondria fusion and fission

Inner membrane divides the mitochondrion into two chambers; the outer chamber contained between the two membranes and in the core of the crests and the inner chamber filled with a relatively dense proteinaceous material usually called the mitochondrial matrix.

4. Cristae

In general, the mitochondrial crests are incomplete septa or ridges that do not interrupt the continuity of the inner chamber; thus the matrix is continuous within the mitochondrion. The cristae of animal cells are usually lamellar or plate-like, but in many protozoa and in steroid synthesizing cells including the adrenal cortex and corpus luteum, they occur as regularly packed tubules.

The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. For typical liver mitochondria, the area of the inner membrane is about five times as large as the outer membrane. This ratio is variable and mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain even more cristae. Cristae and the inner boundary membranes are separated by junctions called cristae junctions. The ends of cristae are partially closed by transmembrane protein complexes that bind head to head and link opposing crista membranes in a bottleneck-like fashion.

Cristae membranes folds are studded with small round protein complexes bodies known as F1 particles or oxysomes, the site of proton-gradient driven ATP synthesis (Fig.5.3.). These are not simple random folds but rather invaginations of the inner membrane, which can affect overall chemiosmotic function of mitochondria. The presence of F1 particles on the matrix side (M-face) confers to the inner mitochondrial membrane, a characteristic asymmetry that is of fundamental importance to its function i.e. formation of ATP.

F1 particles: If a mitochondrion is allowed to swell and break in a hypotonic solution and is then immersed in phosphotungstate, the inner membrane in the crest appears covered by particles of 8.5 nm. These particles have a stem linking each with the membrane (Fig.5.3.). These particles are called elementary particles (F1 or F0-F1 particles) and are regularly spaced at intervals of 10 nm on the inner surface of these membranes.

There may be 104-105 elementary particles per mitochondrion. Actually these particles correspond to a special  ATP synthase involved in the coupling of oxidation and phosphorylation.

Fig.5.5 Schematic diagram of ATP synthase occurring on the inner mitochondrial membrane

Electron micrographs revealed that the inner membrane bound F1 particle or the enzyme ATP synthase consists of two major portions; the F1 head piece, which consists of five different subunits, alpha (α), beta (β), gamma (γ), delta (δ) and epsilon (ε), with the probable ratio of 3α : 3β : 1δ : 1γ : 1ε; and the F0 base piece. The F0 base piece remains embedded in the membrane and consists of three different subunits in the ratio of 1a:2b:12c. The b subunit of F0 extends into the head piece and forms the stalk (stem).

Matrix 

The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total protein in a mitochondrion. It is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix components can easily diffuse to inner membrane or inner chamber because of the folds of the cristae. Matrix forms the core of the mitochondrion which has a pH of about 7.8. The matrix contains a highly concentrated mixture of hundreds of enzymes required for the expression of mitochondrial genes, insoluble inorganic salts, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. In addition, the matrix also contains the enzymes required for the oxidation of pyruvate and fatty acids and for the citric acid cycle. 

Cytoplasmic matrix of mitochondria contains the DNA molecules responsible for cellular respiration. Mitochondrial DNA was discovered by Margit and Sylvan Nass in 1963. One to several double stranded, mainly circular DNA is present in mitochondrial matrix. Mitochondrial DNA is 1% of total DNA of a cell, rich in guanine and cytosine content. The DNA molecules are packaged into nucleoids by proteins. Mitochondrial ribosomes ranging from 55 S to 70 S in nature, For example mammals have 55 S ribosomes. They thus resemble the ribosomes of prokaryotes. Mitochondrial double standard DNA is commonly circular but can be linear. The size of mitochondrial DNA also varies greatly among different species. Mitochondria have their own genetic material, and the machinery to manufacture their own RNAs and proteins. DNA makes the mitochondrion semi-autonomous.

Autonomy of Mitochondria

Mitochondria show a large degree of autonomy or independence in their functioning. 

Mitochondria are considered as autonomous cell organelle due to the following counts:

Mitochondria have their own DNA which can replicate independently.

• The mitochondrial DNA produces its own mRNA, tRNA and rRNA.
• The organelle posses their own ribosomes called mitoribosomes.
• Mitochondria synthesize some of their own structural proteins. However, most of the mitochondrial proteins are synthesized under instructions from cell nucleus.
• The organelles synthesize some of the enzymes required for their functioning, e.g. succinate dehydrogenase.
• They show hypertrophy, i.e. internal growth.
• New mitochondria develop by division or binary fission of pre-existing mitochondria.

However, mitochondria are not fully autonomous, both their structure and functioning are partially controlled by nucleus of the cell and availability of materials from cytoplasm. Hence, they are termed as the 'semi-autonomous' cell organelles. Mitochondria are believed to be symbionts in the eukaryotic cells which became associated with them quite early in the evolution.

Evolution of Mitochondria

Mitochondria are thought to have evolved from free-living bacteria that developed into a symbiotic relationship with a prokaryotic cell, providing it energy in return for a safe place to live. It eventually became an organelle, a specialized structure within the cell, the presence of which is used to distinguish eukaryotic cells from prokaryotic cells. This occurred over a long process of millions of years, and now the mitochondria inside the cell cannot live separately from it. The idea that mitochondria evolved this way is called endosymbiotic theory.

Chemical Composition 

Composition of mitochondrial membrane is just like the plasma membrane, i.e. phospholipids and proteins. Protein is present on two surfaces and a bimolecular layer of lipid in between the two. Outer membrane contains more cholesterol and is higher in phosphatidyl-inositol. The gross chemical composition of the mitochondria varies in different animal and plant cells. In general, mitochondria are found to contain 65 to 70% proteins, 25 to 30% lipids, 0.5% RNA and small amount of DNA. 

The lipid content of mitochondria is composed of 90% phospholipids, 5% or less cholesterol and 5% free fatty acids and triglycerides. The inner membrane is rich in the phospholipid, called cardiolipin that makes the membrane impermeable to various ions and small molecules. Cardiolipin (tetra-acyl-diphophatidyl-glycerol) is a unique phosphoglyceride, which is important in all systems involving electron transport and essential for membranes involved in coupled (oxidative) phosphorylation.

Mitochondria contain sulphur, iron, copper and some vitamins in traces which are mostly related to enzyme activities. When mitochondria are separated from a cellular environment and ruptured, some of the enzymes associated with matrix are released as soluble proteins while the other enzymes remain firmly bound to the membranes. Mitochondria contain about 70 enzymes and about a dozen coenzymes and numerous cofactors. Soluble enzymes from mitochondria include all enzymes of the tricarboxylic acid cycle (TCA) except some dehydrogenases, those that catalyze β-oxidation of fatty acids and others that catalyze transamination of amino-acids and synthesis of protein. Membrane bound enzymes of mitochondria are the essential for electron transport chain and oxidative phosphorylation. Mitochondria do not contain the enzymes for anaerobic glycolysis which occurs in the groundplasm.

Mitochondrial enzymes

All the three major components of food (carbohydrates, proteins and lipids) degraded in cytoplasm, enter mitochondrial Krebs cycle and undergo oxidation. The electrons emitted during Krebs cycle are transported to electron transport system (ETS). Several enzymes and coenzymes are involved in the oxidative phosphorylation and electron transfer mechanism.

Lehninger (1969) classifies them as follows:

Table-1: Enzyme composition of mitochondrial compartments and membranes are

Part of Mitochondria	Enzymes distribution in mitochondria
Outer membrane	Monoamine oxidase, fatty acid CoA ligase, NADH- cytochromes-C reductase, kynurenine hydroxylase.
Outer compartment	Adenylate kinase and nucleoside diphosphokinase
Inner membrane	Respiratory chain enzymes, ATP synthetase oxidase, succinate dehydrogenase, β-Hydroxybutyrate dehydrogenase, carnitine fatty acid acyl transferase, etc.
Matrix	Malate and Isocitrate dehydrogenase, fumarase and aconitase, Citrate synthetase, α-keto acid dehydrogenases, β-oxidation enzymes, enzymes required for expression of mitochondrial genes.

Electron Transfers in Oxidative Phosphorylation

1- An electron from NADH is first accepted by the protein complex NADH-Q reductase, also known as the NADH dehydrogenase complex. The NADH-Q reductase complex accepts an electron from NADH and passes the electron to the next electron carrier, Ubiquinone, which has a higher reduction potential.

2- Ubiquinone, abbreviated as Q, is an organic molecule (not a protein) dissolved in the hydrophobic region of the inner membrane of the mitochondrion. It can move freely within the hydrophobic region of the membrane, by diffusion. It has a higher reduction potential than the NADH-Q reductase. Hence, when ubiquinone in the oxidized form comes in contact with the NADH-Q reductase complex (by a random collision), this mobile electron carrier accepts an electron from NADH-Q reductase.

3- The reduced form of ubiquinone then continues to move through the hydrophobic region of the membrane by diffusion. When the ubiquinone comes in contact with the next carrier in the electron-transport chain, the electron is transferred to this protein complex, known as cytochrome reductase. 

4-From cytochrome reductase, the electron is picked up by another mobile electron carrier, cytochrome C (not to be confused with the cytochrome c1 subunit of cytochrome reductase). 

Cytochrome C is a small protein containing one heme group. When the oxidized form of cytochrome C contacts the cytochrome reductase complex by a random collision, its heme group can accept an electron from the heme group of the cytochrome c1 subunit (in cytochrome reductase).

Cytochrome C then carries this electron until the carrier collides with the final protein carrier in the electron-transport chain, cytochrome oxidase. Like NADH-Q reductase, cytochrome reductase acts as both an electron carrier and a proton pump. As the electron is spontaneously transferred from one group to another in the protein complex, free energy is released. This free energy is used to pump protons from the matrix, across the inner mitochondrial membrane (through cytochrome reductase), and into the intermembrane space. Hence, the proton gradient is increased further.

5-Cytochrome oxidase is the best understood of all the electron-carrier proteins involved in oxidative phosphorylation. Cytochrome oxidase accepts an electron from cytochrome C, and passes it to O2, the final electron acceptor in this chain. As with the other proteins, the free energy from the spontaneous oxidation-reduction reaction is used to pump more protons into the intermembrane space, increasing the proton gradient even further.

Functions of Mitochondria

Mitochondria supply nearly all the required biological energy. Only mitochondria are fully capable of converting pyruvic acid to carbon dioxide and water. They are the respiratory centers of the cell. The ATP molecules produced as a result of cellular respiration accumulate in the mitochondria. The enzymes for Krebs cycle are found in the matrix of the mitochondrion. The enzymes for electron transport are located in the inner membrane of mitochondrion. A set of enzymes that control synthesis of lecithin and phosphatidyl ethanolamine from fatty acids, glycerol and nitrogenous bases is present in most mitochondria. Mitochondrial genes control some hereditary characters, for example male sterility in maize. An organism generally receives mitochondria from its mother (maternal inheritance).

Table-2: Functions of mitochondrial parts in cellular respiration 

Mitochondrial Part	Functions in Cellular Respiration
Outer Mitochondrial Membrane	Separates the contents of the mitochondrion from the rest of the cell.
Matrix	Internal cytosol-like area that contains the enzymes for the link reaction and Krebs Cycle.
Cristae	Tubular regions surrounded by membranes increasing surface area for oxidative phosphorylation.
Inner mitochondrial membrane	Contains the carriers for the ETC and ATP.
Space between Inner and Outer membrane	Reservoir for hydrogen ions (protons), the high concentration of hydrogen ions is necessary for chemiosmosis.

Mitochondria are miniature biochemical factories where food stuffs or respiratory substrates are completely oxidized to carbon dioxide and water. The energy liberated in the process is initially stored in the form of reduced coenzymes and reduced prosthetic groups. The latter soon undergo oxidation and form energy rich ATP molecule, comes out of mitochondria and helps perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc. Because of the formation of  ATP, the mitochondria are called power houses of the cell. Some of the major functions of mitochondria are

1. Production of  ATP: The most important function of the mitochondria is to produce energy. These charged molecules combine with oxygen and produce ATP molecules. This process is known as oxidative phosphorylation. Mitochondria function as energy-transducing organelle into which the major degradation products of cell metabolism penetrate and are converted into chemical energy (ATP) that is used in various activities of the cell.

The processes of energy transformation that occur in mitochondria are based on three coordinated steps:

• Krebs cycle, carried out by a series of soluble enzymes present in the matrix, which produce CO2 by decarboxylation and removes electrons from the metabolites.
• The respiratory chain or electron transport system, which captures the pairs of electrons and transfers them through a series of electron carriers, which finally leads by combination with activated oxygen to the formation of H2O.
• A phosphorylation system, tightly coupled with the respiratory chain, which at three points gives rise to ATP molecules.

2. Synthesis of fatty acid: The matrix or inner chamber of the mitochondria has enzymes for the synthesis of fatty acids. Enzymes required for elongation of fatty acids have been reported in the outer mitochondrial chamber. In the mitochondria of all cells, the outer membrane enzymes mediate the movement of free fatty acids into the mitochondrial matrix. In the matrix, each fatty acid molecule is broken down completely by a cycle of reactions, called β-oxidation that trims two carbons from its carboxyl end, generating one molecule of acetyl-CoA in each turn of cycle. This acetyl-CoA is fed into Krebs cycle for further oxidation.

3. Synthesis of amino acid: Synthesis of many amino acids occurs in the mitochondria. The first 

formed amino acids are glutamic acid and aspartic acid. They are synthesized from α-ketoglutaric acid and oxaloacetic acid respectively. Other amino acids are produced by transformation and transamination or transfer of amino group (-NH2) from glutamic acid and aspartic acid.

4. Synthesis of biochemicals: Mitochondria provide important intermediates for the synthesis of several biochemicals like chlorophyll, cytochromes, pyrimidines, steroids, alkaloids, etc. Besides the ATP production, mitochondria perform certain biosynthetic or anabolic functions also. 

• It contains DNA and the machinery needed for protein synthesis. Therefore, it can make different proteins of its own. These proteins include subunits of ATP synthase, portions of the reductase and three of the seven subunits in cytochrome oxidase.
• The synthesis of haeme (needed for myoglobin, hemoglobin) begins with a mitochondrial reaction catalyzed by the enzyme delta or d-aminolaevulinic acid synthetase. 
• Likewise, some early steps in the conversion of cholesterol to steroid hormones in the adrenal cortex are also catalyzed by mitochondrial enzymes.

5. Sometimes, the mitochondria assume storage functions, for example the mitochondria of ovum store large amounts of yolk proteins and transform them into yolk platelets. Mitochondria help the cells to maintain proper concentration of calcium ions within the compartments of the cell. Mitochondria may store and release calcium when required.

6. The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen.

7. The liver cells mitochondria have enzymes that detoxify ammonia.

8. The mitochondria also play important role in the process of apoptosis or programmed cell death. Abnormal death of cells due to the dysfunction of mitochondria can affect the function of organ.

Table-3: Localization of Metabolic Functions within the Mitochondrion

Membrane or Compartment	Metabolic Functions
Outer membrane	Phospholipid synthesis Fatty acid desaturation Fatty acid elongation
Inner membrane	Electron transport Oxidative phosphorylation Pyruvate import Fatty acyl CoA import Metabolite transport
Matrix	Pyruvate oxidation TCA cycle β-oxidation of fats DNA replication RNA synthesis (transcription) Protein synthesis (translation)

Summary

• Mitochondria, the granular or filamentous cell organelles present in the cytoplasm of aerobic cells of higher animals, plants and some microorganisms including protozoa, algae and fungi. These are bean-shaped organelles, occur free in the cytoplasm, and can vary greatly in both size and number per cell. However, regardless of their size, number per cell, plant or animal origin, they have very similar structures. Mitochondria are found in the cytoplasm of nearly all eukaryotic cells and occupy a substantial portion of the cytoplasm. They are generally not visible as they lack contrast. 
• Mitochondria are membrane bound cell organelles, associated with cellular respiration, the source of energy, being termed as power houses of cell. Mitochondria generate most of the cell's supply of adenosine triphosphate (ATP) used as a source of chemical energy. Energy is derived by the breakdown of carbohydrates, amino acids and fatty acids and is used in the formation of energy rich molecules the ATP (often referred to as the energy currency of the cell) by the process of oxidative phosphorylation. The central set of reactions involved in ATP production is collectively known as citric acid cycle or the Krebs cycle. However, the mitochondrion has many other functions in addition to the production of ATP. They are involved in other tasks, such as signaling, cellular differentiation, and cell death, as well as maintaining the control of cell cycle and cell growth. 
• Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome that shows substantial similarity to bacterial genomes. Mitochondria have their own circular DNA and synthesize some of their proteins. Mitochondrial proteins (proteins transcribed from mitochondrial DNA) vary depending on the tissue and the species. Most of the proteins required by the mitochondria, however, are encoded by the nuclear genes and are imported from the cytosol. Thus, they are said to be "semi-autonomous" organelles. The organelle is believed to have originated by the process of endosymbiosis.
• Unlike other organelles (miniature organs within the cell), they have two membranes, an outer and an inner one. Each membrane has different functions. Mitochondria split into different compartments or regions, each of which carries out distinct roles. Some of the major regions include outer membrane, through which small molecules can pass freely. This outer portion includes proteins called porins which form channels that allow proteins to cross. The outer membrane also hosts a number of enzymes with a wide variety of functions. 
• Intermembrane space is the area between the inner and outer membranes. Inner membrane holds proteins that have several roles because there are no porins in the inner membrane. It is impermeable to most molecules. Molecules can only cross the inner membrane in special membrane transporters. The inner membrane is where most ATP is created. Cristae, the folds of the inner membrane increase the surface area of the membrane; therefore, increase the space available for chemical reactions. Matrix, the space within the inner membrane containing hundreds of enzymes. It is important in the production of ATP. Mitochondrial DNA is housed here.

Glossary

• Amino acid: An organic molecule that is made up of a basic amino group (−NH2), an acidic carboxyl group (−COOH), and an organic R group (or side chain) that is unique to each amino acid.
• Anabolic: The phase of metabolism in which simple substances are synthesized into the complex materials of living tissue.
• ATP synthase: An enzyme that catalyzes the formation of ATP from the phosphorylation of ADP with inorganic phosphate, using a form of energy, such as the energy from a proton gradient.
• ATP: Adenosine triphosphate, coenzyme used as an energy carrier in the cells of all known 
• organisms: the process in which energy is moved throughout the cell.
• Biosynthesis: The production of a complex chemical compound from simpler precursors in a living organism, usually involving enzymes to catalyze the reaction and energy source (ATP)
• Cardiolipin: An important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. A phospholipid used in combination with phosphatidylcholine and cholesterol as an antigen to diagnose syphilis.
• Cristae junction: A tubular structure of relatively uniform size that connects a mitochondrial crista to the mitochondrial inner boundary membrane.
• Cristae: One of the inward projections or folds of the inner membrane of a mitochondrion.
• Cytochrome c: A type of cytochrome, a protein which carries electrons, that is central to the process of respiration in mitochondria
• Cytosol: The liquid component of the cytoplasm surrounding the organelles and other insoluble cytoplasmic structures in an intact cell where a wide variety of cell processes take place.
• Electron transport chain: A group of compounds that pass electron from one to another via redox reactions coupled with the transfer of proton across a membrane to create a proton gradient that drives ATP synthesis
• Endosymbiosis: A symbiotic association in which one or more organisms live inside another, such as bacteria normally present in human intestines.
• Enzyme: Any of numerous compounds that are produced by living organisms and function as biochemical catalysts.
• Fatty acid: Any of the group of a long chain of hydrocarbon with a carboxylic acid at the beginning and a methyl end and derived from the breakdown of fats.
• Krebs cycle: Also known as citric acid cycle or TCA (tricarboxylic acid) cycle is a series of enzymatic reactions in aerobic organisms involving oxidative metabolism of acetyl units and producing high-energy phosphate compounds such as ATP, which serve as the main source of cellular energy. 
• Matrix: The substance occupying the space enclosed by the inner membrane of a mitochondrion. It contains enzymes, filaments of DNA, granules, and inclusions of protein crystals, glycogen and lipid.
• Metabolite: A substance that is a product of metabolic action or involved in a metabolic process.
• Mitochondrion: A spherical or elongated organelle in the cytoplasm of nearly all eukaryotic cells, containing genetic material and many enzymes important for cell metabolism, including those responsible for the conversion of food to usable energy.
• Mitoplasts: A mitochondrion that has been stripped of its outer membrane leaving the inner membrane intact.
• Oxidative phosphorylation: The process in cell metabolism by which respiratory enzymes in the mitochondria synthesize ATP from ADP and inorganic phosphate during the oxidation of NADH by molecular oxygen.
• Oxysomes: Oxysomes are the structures which are present on the surface of the folded Inner membrane of the mitochondria. They are also called F0-F1 particles or ATP synthase.
• Porins: Proteins found in the outer membrane of a double membrane that allow permeability in most small molecules.
• Pyruvate: A salt, ester or anion of pyruvic acid. Pyruvate is the end product of glycolysis and may be metabolized to lactate or to acetyl CoA.
• Respiration: A process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
• Tryptophan: A naturally occurring, one of the essential amino acids; it is a precursor of serotonin. Adequate levels in the diet may mitigate pellagra by compensating for deficiencies of niacin.
• VDAC: Voltage-dependent anion channel is a pore located at the outer membrane of the mitochondrion. It allows the entry and exit of numerous ions and metabolites between the cytosol and the mitochondrion.

FAQ :

The term "mitochondria" was coined by?

Benda 

Where is the mitochondrial matrix located?

Within the inner membrane

Mitochondria are the store houses or power house of?

ATP

Mitochondria are usually found in?

reproductive and vegetative cells 

Which is a function of mitochondria?

(a) Regulating metabolism (b) Producing ATP (c) Storing calcium

In which part of mitochondria, ATP is generated?

Oxysomes