MITOCHONDRIA

 

Mitochondria

I INTRODUCTION

mitochondria

Mitochondria

Mitochondria, minute sausage-shaped structures found in the hyaloplasm (clear cytoplasm) of the cell, are responsible for energy production. Mitochondria contain enzymes that help convert food material into adenosine triphosphate (ATP), which can be used directly by the cell as an energy source. Mitochondria tend to be concentrated near cellular structures that require large inputs of energy, such as the flagellum, which is responsible for movement in sperm cells and single-celled plants and animals.

Don Fawcett-Keith Porter/Photo Researchers, Inc.

Mitochondria, small cellular structures, or organelles, found in the cytoplasm of eukaryotic cells (cells with a nucleus). Mitochondria are responsible for converting nutrients into the energy-yielding molecule adenosine triphosphate (ATP) to fuel the cell’s activities. This function, known as aerobic respiration, is the reason mitochondria are frequently referred to as the powerhouse of the cell.

Mitochondria are unusual organelles in that they contain deoxyribonucleic acid (DNA), typically found in the cell’s nucleus, and ribosomes, protein-producing organelles abundant in the cytoplasm. Within the mitochondria, the DNA directs the ribosomes to produce proteins, many of which function as enzymes, or biological catalysts, in ATP production. The number of mitochondria in a cell depends on the cell’s function. Cells with particularly heavy energy demands, such as muscle cells, have more mitochondria than other cells.

II MITOCHONDRIAL STRUCTURE

A mitochondrion is typically long and slender, but it can appear bean-shaped or oval-shaped under the electron microscope. Ranging in size from 0.5 micrometer (0.00005 in) to 1 micrometer (0.0001 in) in length, a mitochondrion has a double membrane that forms a sac within a sac. The smooth outer membrane holds numerous transport proteins, which shuttle materials in and out of the mitochondrion. The region between the outer and inner membranes, which is filled with liquid, is known as the outer compartment. The inner membrane has numerous folds called cristae. Cristae are the sites of ATP synthesis, and their folded structure greatly increases the surface area where ATP synthesis occurs. Transport proteins, molecules called electron transport chains, and enzymes that synthesize ATP are among the molecules embedded in the cristae. The cristae enclose a liquid-filled region known as the inner compartment, or matrix, which contains a large number of enzymes that are used in the process of aerobic respiration.

III MITOCHONDRIAL FUNCTION

The chief function of the mitochondria is to create energy for cellular activity by the process of aerobic respiration. In this process, glucose is broken down in the cell’s cytoplasm to form pyruvic acid, which is transported into the mitochondrion. In a series of reactions, part of which is called the citric acid cycle or Krebs cycle, the pyruvic acid reacts with water to produce carbon dioxide and ten hydrogen atoms. These hydrogen atoms are transported on special carrier molecules called coenzymes to the cristae, where they are donated to the electron transport chain.

The electron transport chain separates the electron and proton in each of the ten hydrogen atoms. The ten electrons are sent through the electron transport chain and some eventually combine with oxygen and the protons to form water.

Energy is released as the electrons flow from the coenzymes down the electron transport chain to the oxygen atoms, and this energy is trapped by the components of the electron transport chain. As the electrons flow from one component to another, the components pump random protons from the matrix to the outer compartment. The protons cannot return to the matrix except by one pathway—through the enzyme ATPase, which is embedded in the inner membrane. As the protons flow back into the matrix, ATPase adds a phosphate group to a molecule in the matrix, adenosine diphosphate (ADP). The addition of a phosphate group to ADP forms ATP.
Aerobic respiration is an ongoing process, and mitochondria can produce hundreds of thousands of ATP molecules each minute in a typical cell. The ATP is transported to the cytoplasm of the cell, where it is used for virtually every energy-requiring reaction it performs. As ATP is used, it is converted into ADP, which is returned by the cell to the mitochondrion and is used to build more ATP.

IV ORIGIN OF MITOCHONDRIA

Mitochondria have significant features that resemble those of prokaryotes, primitive cells that lack a nucleus. Mitochondrial DNA is circular, like the DNA of prokaryotes, and mitochondrial ribosomes are similar to prokaryotic ribosomes. Mitochondria divide independently of the cell through binary fission, the method of cell division typical of prokaryotes.

The prokaryote-like features of mitochondria lead many scientists to support the endosymbiosis hypothesis. This hypothesis states that millions of years ago, free-living prokaryotes capable of aerobic respiration were engulfed by other, larger prokaryotes but not digested, possibly because they were able to resist digestive enzymes. The two cells developed a symbiotic, or cooperative, relationship in which the host cell provided nutrients and the engulfed cell used these nutrients to carry out aerobic respiration, which provided the host cell with an abundant supply of ATP. The engulfed cells evolved into mitochondria, which retain the DNA and ribosomes characteristic of their prokaryotic ancestors.

V RECENT MITOCHONDRIAL RESEARCH

The DNA in mitochondria is used to track certain genetic diseases, and to trace the ancestry of organisms that contain eukaryotic cells. In many animal species, mitochondria tend to follow a pattern of maternal inheritance. When a cell divides, the mitochondria replicate independently of the nucleus. The two daughter cells formed after cell division each receive half of the mitochondria as the cytoplasm divides. When an egg is fertilized by a sperm, the sperm’s mitochondria are left outside the egg. The fertilized zygote inherits only the mother’s mitochondria. This maternal inheritance creates a family tree that is not affected by the typical shuffling of genes that occurs between a mother and father.

While the DNA within mitochondria directs the synthesis of enzymes for aerobic respiration, it also codes for proteins important in the nervous system, circulatory system, and other body functions. A number of genetic diseases, including diabetes mellitus, deafness, heart disease, Alzheimer’s disease, Parkinson disease, and Leber’s Hereditary Optic Neuropathy, a condition of complete or partial blindness, are associated with mutations in mitochondrial DNA. A relatively new medical specialty, mitochondrial medicine, seeks to understand the role of mitochondrial DNA mutations in genetic diseases.

A recent comparison of samples of human mitochondrial DNA suggests that humans have descended from a woman who lived in Africa 140,000 to 290,000 years ago. Genetic samples taken from African, Asian, Australian, European, and New Guinean ethnic groups revealed a specific number of mitochondrial DNA types. Comparison of these mitochondrial DNA types enabled scientists to construct a family tree that shows when each group probably began evolving away from one another. On this tree, the African mitochondrial DNA occupies the longest and oldest of the branches, giving rise to the other ethnic groups. There were likely many other women alive at the time of the so-called mitochondrial Eve, but their lines of maternal inheritance have died out. This commonly occurs when one generation in a family fails to have a daughter.

Another use of mitochondrial DNA analysis is in forensic science. The identities of the skeletons alleged to be those of Tsar Nicholas II, the last Russian tsar, and his family were recently established using mitochondrial DNA. The mitochondrial DNA of a living maternal relative of the tsar’s family was found to be an exact match to the suspected remains of the tsar’s wife, Alexandra, and three children. Because mitochondrial DNA is inherited through the mother, the mitochondrial DNA of Tsar Nicholas II’s skeleton did not match that of his wife and children.

In eukaryotic cells most of the generation of metabolic energy occurs in the organelle called mitochondria (singular: mitochondrion) often referred to as the power house of the cell. 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.

Mitochondria are found in the cytoplasm of nearly all eukaryotic cells and occupy a substantial portion of the cytoplasm. They are large enough to be resolved in the light microscope but are generally not visible as they lack contrast. Special stains are used to make them visible, for e.g. Janus Green B.

Each cell contains hundreds to thousands of mitochondria (e.g., the liver cell contains 1000-2000 mitochondria occupying one fifth of the cell volume). Mitochonria are double membrane organelles. Their number and size varies in metabolically different cells. 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.

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.

Historical Background

1894- Richard Altmann, discovered the cell organelle and called it as “bioblasts”.

1898- Carl Benda coined the term “mitochondria”. The word mitochondrion is derived from the Greek word mitos– thread, and chondrion– granule.

1900 – L. Michaelis (of enzyme kinetics) found that mitochondria in living cells could be specifically stained green by the dye ‘Janus Green B’. Because the dye must be oxidized to give the green colour, Michaelis proposed that mitochondria are cellular oxidizing organelles.

1904- Friedrich Meves for the first time observed mitochondria in plants (Nymphaea alba).

1908- Friedrich Meves and Claudius Regaud suggested that proteins and lipids are present in mitochondria.

1912- B. F. Kingsbury related this organelle with cell respiration.

1913- O.H.Warburg extracted particles from guinea-pig liver and conformed that enzymes catalyzing oxidative reactions are present in the particles identified as mitochondria, and thus, linked them to respiration.

1925- David Keilin discovered cytochromes in mitochondria.

1937 – Based on work of many scientists such as Szent-Gyorgyi, Martius, Knoop and others and his own work, Hans Krebs presented the complete tricarboxylic acid cycle and was awarded a Nobel prize in 1953.

1939- It was demonstrated that in minced muscle cells one molecule of oxygen can lead to formation of two ATP.

1941- F.A.Lipmann gave the Unifying concept of ATP as the energy currency or the primary and universal carrier of chemical energy in cells. energy rich phosphate bond in cellular metabolism. He shared the Nobel prize with Krebs in 1953.

1946- A.Claude isolated mitochondria from other cell fraction and did biochemical analysis.

He also isolated cytochrome oxidase and other enzymes responsible for the respiratory chain.

1956 – George Palade and Fritjof Sjostrand published high resolution electron micrographs showing the presence of two mitochondrial membranes and the cristae formed by the folded inner mitochondrial membrane.

1957- Philip Siekevitz named the mitochondria as “the powerhouse of the cell”, because it generates adenosine triphosphate (ATP).

1960- Efraim Racker gave evidence for presence of particles involved in coupling ATP synthesis to electron transport and called them coupling factors or F1 particles.

1961- Peter D. Mitchell proposed the chemi-osmotic mechanism to explain biosynthesis of ATP and was awarded a Nobel prize in 1978.

1964 – H. Fernandez-Moran – presented electron micrographs of negatively stained inner mitochondrial membrane containing sub-mitochondrial particles which he named as “elementary particles”.

1967- Ribosomes were isolated from mitochondria.

1968- The mapping of mitochondrial genes was done.

1976- The complete genetic and physical map of yeast mitochondria was developed.

1978- Peter Mitchell was awarded noble prize for proposing chemiosmotic mechanism of oxidative phosphorylation.

The size of mitochondria varies from 0.5 to 1.0  μm in diameter and 1 to 4 μm in length. As mentioned earlier these organelles tend to change their shapes frequently and are usually seen as elongated cylinders. These also occur as highly branched, interconnected tubular network. The number of mitochondria varies considerably. These are present where ATP is needed the most for example the metabolically active tissues. In many unicellular organisms only single mitochondrion is present. Mitochondria can fuse with one another or divide in two. The balance between fission and fusion determines the number, size and degree of interconnection. The number of mitochondria can be as high as several thousand e.g., in human liver cells 1000-2000 mitochondria/cell are present. The exception is human mature red blood cells, which lacks mitochondria. The RBCs of vertebrates lose their mitochondria during differentiation; it is a rare example of a vertebrate cell generating all of its ATP by glycolysis alone. In sperm cell the mitochondria is located in the central region, just behind the nucleus. The movement of sperm is energized by ATP produced in mitochondria. In plants, they are the primary supplier of ATP in non-photosynthetic tissues as well as source of energy in photosynthetic tissues during dark. Therefore the mitochondria is present in both chemotropic and phototropic tissues in animal as well as plant cells. Mitochondria are double membranous organelle. The outer membrane is smooth and continuous but the inner membrane is folded 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.mitochondria

The outer membrane is a simple phospholipid bilayer, containing 50% lipid and 50% proteins by weight. And mixture of enzymes involved in degradation of tryptophan and the elongation of fatty acids.The outer membrane is a simple phospholipid bilayer, containing 50% lipid and 50% proteins by weight. And mixture of enzymes 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 the outer mitochondrial membrane is highly permeable as it contains special protein called porins that form aqueous channels which allows free diffusion of molecules of 5000 Daltons or less.The proteins of outer membrane also contains proteins include a number of enzymes involved in mitochondrial lipid synthesis, degradation of tryptophan, elongation of fatty acids, in division and fusion of mitochondria etc., and 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.

the 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/lipid ratio. It is rich in unusual phospholipids named cardiolipins (which have four fatty acids rather than two), which is characteristic of bacterial membrane. Presence of the 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. The greater the demand of ATP, more the number of cristae.  The inner membrane is highly complex, containing complexes of the electron transport chain, the ATP synthase and transport proteins. 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
  • Transport proteins that are 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
  • The intermembrane space
  • The outer chamber or intermembrane space is 40-70 Ao in width and filled with watery fluid. This space can be increased by placing the isolated mitochondria in a sucrose solution. The sucrose can penetrate only in this chamber but not in the inner chamber. Since, the outer membrane is permeable and the constitution of the inner chamber resembles that of the cytosol. 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.
  • Matrix
  • The matrix contains insoluble inorganic salts having binding sites for divalent cations (Mg2+ and Ca2+) in the form of dense granules (300-500Ao), ribosomes, tRNAs, several copies of mitochondrial DNA and enzymes required for the expression of mitochondrial genes. In addition, the matrix also contains the enzymes required for the oxidation of pyruvate and fatty acids and for the citric acid cycle.  The matrix components can easily diffuse to inner membrane or inner chamber because of the folds of the cristae.

the mitochondria contain their own genetic material which is distinct from the nuclear genome. The mitochondrial DNA (mtDNA) and protein synthesizing machinery resembles the prokaryotic genome supporting the endosymbiotic theory that postulated that organelles like chloroplast and mitochondria originated from bacteria that eventually established a symbiotic relationship with the host cell. Mitochondrial DNA originated from aerobic bacteria and during evolution most of the genes were either lost or transferred to the nucleus of host cell.

Mitochondria are dynamic organelles frequently dividing, fusing and changing in number and shape. The process of formation of new mitochondria in the cell is known as mitochondrial biogenesis. This process is activated by different signals during cellular stress or in response to environmental stimuli. Mitochondrial biogenesis involves all the processes required in mitochondrial maintenance, growth, division and segregation during cell cycle. Although mitochondrion has its own genome but more than 95% of mitochondrial proteins are synthesized by nucleus genome. The nuclear encoded proteins are transported into the mitochondria by specialized transport processes
Name of the enzyme Location in mitochondria
Rotenone-insensitive NADH cytochrome c reductaseKynurenine hydroxylase 

Monoamine oxidase

Succinate cytochrome c reductase

Rotenone-sensitive NADH cytochrome c reductase

Adenylate kinase

NAD+ malate dehydrogenase

NAD+ glutamate dehydrogenase

Citrate synthase

outer mitochondrial membraneouter mitochondrial membrane

outer mitochondrial membrane

inner mitochondrial membrane

inner mitochondrial membrane

intermembrane space(outer chamber)

mitochondrial matrix(inner chamber)

mitochondrial matrix

mitochondrial matrix

Cellular respiration

The foremost important role of mitochondria is cellular respiration involving the oxidative breakdown of glucose and fatty acids. Mitochondria can use both pyruvate (which comes from the glucose and other sugars by glycolysis) and fatty acids (from fats). These molecules are transported into the mitochondrial matrix and converted to acetyl CoA. The acetyl CoA is then oxidized by the enzymes of the mitochondrial matrix to CO2 (Krebs cycle) along with the generation of high energy electrons (released from the NADH and FADH2). These electrons are transported to the inner mitochondrial membrane and enter into the electron transport chain. The cellular respiration consists of three main steps:

  • Glycolysis
  • Krebs cycle
  • Oxidative phosphorylation

Glycolysis

The first step of oxidation of carbohydrates – glycolysis occurs in the cytosol; it is the process of conversion of glucose into pyurvate. When glucose (6 carbon compound) enters into the cell, it is broken down into two molecules of pyurvic acid (3 carbon compound) generating 2ATP molecule in    the cytoplasm in the absence of oxygen. Only a small amount of energy is released through glycolysis, most of the energy is stored in pyruvate and NADH.

Pyruvate is then transported into the mitochondrial matrix and decarboxylated to form a two carbon acetyl group. The acetyl group combines with coenzyme A to form acetyl CoA. This reaction is catalyzed by a large multienzyme complex known as pyruvate dehydrogenase.

Krebs cycle/Citric acid cycle/TCA cycle

This newly formed acetyl-CoA binds to oxaloacetic acid (4-carbon molecule) to form citric acid. The citric acid undergoes two cycles of decarboxylation, phosphorylation and using one glucose molecule generates 2 ATP, 6 NADH+H+, 2 FADH2 and 4 CO2. This cyclic pathway is called the tricarboxylic acid (TCA) cycle. This is also referred as Kerb’s cycle after the British biochemist Hans Kerbs, who elucidated the pathway in the 1930s.

Oxidative phosphorylation

Oxidative phosphorylation refers to the chemiosmotic process that converts the oxidation energy into ATP. The initial energy released from burning of carbohydrates and fats etc., are stored initially in the form of high energy electrons in NADH and FADH2. The NADH2+ and FADH2 molecules give up their electrons to electron transport complexes located in the inner mitochondrial membrane, which in turn move protons from inner compartment to outer compartment. This proton gradient creates free energy potential which activates turbine like ATPase pump to generate ATP (oxidative phosphorylation). The final acceptor of electrons is oxygen that in turn reacts with hydrogen and forms water

Fig:the diagram showing location and connection of electron transport chain and citric acid cycle in the mitochondria.

The electron transport chain contains five types of carriers; flavoproteins, cytochromes, copper atoms, ubiquinone and iron-sulfur proteins. These electron carriers are present in inner mitochondrial membrane in the form of four complexes, named as complexes I, II, III and IV. Two component of the electron transport chain, Ubiquinone and cytochrome c, are not part of any of these four complexes.

A series of electron moves through electron transport chain. These reactions are coupled with conformational           changes in electron carrier that move proton outwards across the inner mitochondrial membrane. At the end low energy electron are transferred to the terminal electron acceptor, molecular oxygen (O2), which becomes reduced to water.

The coupling of the mechanism of electron transport chain and ATP synthesis is referred to as chemi-osmotic coupling. The outward movement of proton creates a potential difference across the mitochondrial inner membrane. The proton moves down the electrochemical gradient, through ATP-synthesizing complex- ATP synthase. This was first proposed by Peter Mitchell in 1961, and this is known as chemiosmotic mechanism. Mitochondria utilizes gradient across inner membrane to synthesize ATP. This process of ATP formation driven by electron movement generated by substrate oxidation is known as oxidative phosphorylation.

On the matrix side of the inner membrane stalked particles called inner-membrane spheres are present. These spheres are ATP-synthesizing enzyme – ATP synthase. The enzyme ATP synthase is considered to be the nature’s smallest rotary motors that provides the chemical energy. ATP synthase is a multisubunit protein complex that has two parts F0 and F1. The F0 is embedded in the membrane whereas the F1 containing the catalytic site. The F0 and F1 together carry out the ATP synthesis as protons are carried along the electrochemical gradient.

In addition to supplying energy, mitochondria play role in regulating self-destruction of cells known as apoptosis. Mitochondria are also involved in cell signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth.

Mitochondria are considered to have originated from the group of bacteria- the proteobacteria (most likely from Rickettsiales) through endosymbiosis. The primitive heterotrophic eukaryotic cell got symbiotically associated with autotrophic bacterium and gradually became dependent on the host cell.

Most of the mitochondrial proteins are encoded by the nucleus and are imported into the mitochondria. These proteins are recognized by their specific signal sequences are imported into the mitochondria by protein-lined channels.mitochondria

Structure and distribution of mitochondria:

  •  Usually, mitochondrion is like a particle or rod and others
  • composed of proteins (65-70%) and lipids (25-30%) at 0.5~1μm diameter and 1.5~3.0μm length.
  • The mitochondria in the cells of pancreas can be 10~20μm at length called huge mitochondrion.
  • The number of mitochondrion in a cell can be hundreds to thousands, and less in plant cell than in animal cell because of chloroplast.
  • Some unicellular organism contains 500,000 mitochondria inside
  • mammalian erythrocytes contain no any mitochondrion inside.
  • Mitochondrion can migrate in cell along micro tube, and motorprotein supplies energy for that.
  1. Outer membrane
  • Outer membrane contains lipids (40%) and proteins (60%).
  • There are the hydrophilic tunnels composed of porin that allows the molecules lighter than 5KD passed through.

2. Inner membrane

  •    Inner membrane contains more than 100 types of polypeptide with a low permeability.
  •   The electron transmission chain of oxidative phosphorylation is located in inner    membrane.
  •   Cytochrome C reductase is the marker enzyme for inner membrane.
  •    Inner membrane can be pleated into inside to form cristae.
  •    The cristaes enlarge the area of inner membrane to 5 – 10 folds.
  •    Cristae can be two types of shape: lamella or tube.
  •    Elementary particles are located on cristae, and composed of head part (F1 conjugate  factor) and elemantary part (F0 conjugate        factor).
  •   F0 inserts into inner membrane.

3. Intermembrane space

  •    It is between inner membrane and outer membrane  with 6-8nm width.
  •    Adenylate kinase is the marker enzyme for  intermembrane space.

4. Matrix

  • The area surrounded by cristae, intermembrane space and inner membrane.
  • Excepting glycolysis (in plasma), other bio-oxidation reactions are carried out in mitochondrion (in matrix).
  • Malic dehydrogenase (MDH) is the marker enzyme for the matrix of mitochondrion.

The matrix contains a complete system for transcription and translation including mitochondrion DNA (mtDNA), 70s ribosome, tRNA, rRNA, and DNA polymerase.mitochondria

Mitochondria The organelle that releases energy in the cell. (The powerhouse of the cell) Only found in ANIMAL cells.
Mitochondria produce ATP using energy stored in food molecules.

STRUCTURE

  • Mitochondria are the primary energy producers in cells.
  • Mitochondria have a double membrane structure
  • There is a single outer membrane and a folded inner membrane
  • Sac with two inner compartments which are separated by the inner membrane.
  • The first compartment is between the outer and inner membranes.
  • The outer compartment is inside the inner membrane.
  • The outer mitochondrial membrane is composed of about 50% phospholipids by weight and contains a variety of enzymes involved in such diverse activities as the elongation of fatty acids, oxidation of epinephrine (adrenaline), and the degradation of tryptophan.
  • Mitochondria are the site of most of the energy production in eukaryotic cells .
  • Mitochondria are very abundant in cells that require lots of energy.
  • Ex:- Muscle
  • The inner membrane contains proteins with three types of functions [Alberts, 1994]:
  • those that carry out the oxidation reactions of the respiratory chain
  • ATP synthase, which makes ATP in the matrix
  • specific transport proteins that regulate the passage of metabolites into and out of the matrix.
  • They use complex molecules and oxygen to produce a high energy molecule know as ATP  (Adenosine Triphosphate) process called aerobic respiration.

UNIQUE FEATURES

    • Mitochondria are very unique in several regards
    • have their own circular DNA
    • have their own  Ribosomes.

(The DNA in the cell nucleus does not code for the construction of mitochondria. )

    • All the mitochondria in your body came from your mother.
    • Mitochondria are not part of the genetic code in the nucleus of your cells.
    • Fathers only give genes to their children.
  • Mothers give genes and cytoplasm to their children in their egg cells.
  • Since mitochondria are in the cytoplasm and reproduce themselves they only are inherited from mothers.
  • Geneticists have used this curious feature of mitochondria to study maternal family lines and rates of evolution.

Some important points

  • Although the primary function of mitochondria is to convert organic materials into cellular energy in the form of ATP, mitochondria play an important role in many metabolic tasks, such as:
  • Apoptosis-Programmed cell death
  • Glutamate-mediated excitotoxic neuronal injury
  • Cellular proliferation
  • Regulation of the cellular redox state
  • Heme synthesis
  • Steroid synthesis
  1. Heat production (enabling the organism to stay warm).
  2. Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in a variety of mitochondrial diseases.

origin of mitochondria

Mitochondrion was originated from a bacterium that parasited in cell probably because mitochondrion has many features are almost same to bacteria, for examples, morphology, staining, chemical components, genetic system, etc.

The following genetic features are same to a bacterium’s:

① circle DNA without intron.

② 70S ribosome.

③ RNA polymerase can be inhibited by EB, not by actinomycin D.

④ Sensitive to chloromycetin that inhibits bacterial protein synthesis, not to actidione that inhibits the cellular protein synthesis.

  • The genetic codons of mammalian mtDNA are different from universal genetic codons:
  • ① UGA is not a stop codon here, it is a codon for Tryptophan.
  • ②  Methionine is encoded by codon AUG, AUA, AUU and AUC.
  • ③ AGA and AGG are not codons for arginine, they are stop codons here. There are 4 stop codons in mitochondrion: UAA, UAG, AGA, and AGG.
  • mtDNA is transferred to new generation from parental generation with a matrilinear inheritance way, and its mutation rate is higher than nucleus DNA (nDNA) without efficient repairing function.
  • So, mtDNA is easy to be mutated and cause mutation genetic diseases, such as Leber optic nerve disease (optic nerve denaturation and atrophy) and myoclonus epilepsy (convulsive seizure and loss of consciousness).
  • Here’s the simplest way to explain what happens. The food we eat gets broken down and assigned in various fashion.
  • The fats and sugars go through processing, and there’s quite a bit involved in this. If you get into this stuff more, you’ll hear all about the respiratory chain and ATP, which is the end result, or energy.
  • The mitochondria in a cell have to go through five “complexes” to create energy.
  • An error in any of those complexes is bad, but obviously there can be varying degrees of how big the error is, and where it occurs in the energy making process.
  • Mitochondria are responsible for producing 95% of the energy that’s needed for our cells to function.
  • In fact, they provide such an important source of energy that a typical human cell contains hundreds of them.

WHAT ARE THE BIOCHEMICAL REACTIONS THAT OCCUR IN THE MITOCHONDRIA?

  1. The biochemical processes which occur in the mitochondria and produce energy are known as the “mitochondrial respiratory chain”.
  2. This “chain” is made up of five components called Complex I, II, III, IV and V. Each of these complexes are made up of a number of proteins. The instructions for these proteins to be produced by the cells are contained in a number of different genes.
  3. There are over 80 different genes needed to produce the components of the mitochondrial respiratory chain. Some of these genes are found in mitochondria rather than in the nucleus.
  4. Changes (mutations) in any of these mitochondrial genes that make them faulty can result in biochemical problems due to absence or malfunctioning of the enzymes involved in the respiratory chain complexes.
  5. This leads to a reduction in the supply of ATP.
  6. This can have severe consequences, resulting in interference of body functions including any of the following, either in isolation or in various combinations.mitochondria

EXAMPLES OF THE IMPACT OF FAULTY (MUTATED) MITOCHONDRIAL GENES
General: small stature and poor appetite.
Central nervous system: developmental delay / intellectual disability, progressive neurological deterioration (dementia such as the late-onset form of Alzheimer disease), seizures, stroke-like episodes (often reversible), difficulty swallowing, visual difficulties and deafness
Skeletal and muscle: floppiness, weakness and exercise intolerance
Heart: heart failure (cardiomyopathy) and cardiac rhythm conditions
Kidney: problems in kidney function