Mitochondria: Distribution, Morphology, Functions and Origin of Mitochondria

Mitochondria: Distribution, Morphology, Functions and Origin of Mitochondria!

Kоlliker (1880) was the first who observed the granules (mitochondria) in muscle cell of insects. Flemming (1882) named the mitochondria as fila. Altmann in 1894 observed them and they were called Altmann’s granules bioplasts.

The term mitochondria were applied by Benda (1897-98) to these granules which were described as cytomicrosomes by Velette St. George. Benda staind mitochondria with alizarin and crystal violet. Kingsbury (1912) related them to cellular respiration and Warburg (1913) observed the presence of respiratory enzymes. In 1934 Bensley and Horr isolated mitochondria from liver cells and Porter and Palade described their electron microscopic structure.


Ordinarily mitochondria are evenly distributed in the cytoplasm. They may, however, be localized in certain regions. In proximal convoluted tubules of the kidney they are found in the basal region of the cell, opposite the renal capillaries. In skeletal muscles they lie between the myofibrils. In insect flight muscle several large mitochondria are in contact with each fibril.


In cardiac muscle the mitochondria are situated in clefts between the myofibrils, numerous lipid droplets are associated with the mitochondria. In many sperms the mitochondria fuse into one or two structures which lie in the middle piece of the sperm, surrounding the axil filament. In columnar or prismatic cells they are oriented parallel to the long axis of the cells. In leucocytes they are radially arranged.


Mitochondria may have a more or less definite orientation. For example in cylindrical cells they are generally oriented in the basal-apical direction, parallel to the main axis. In leukocytes, mitochondria are arranged radially with respect to the centrioles. It has been suggested that these orientations depend upon the direction of the diffusion currents within cells and are related to the submicroscopic organization of the cytoplasmic matrix and vacuolar system.

Plasticity of Mitochondria in a Cell:

Lewis and Lewis (1914-15) concluded that the mitochondria are extremely variable bodies, which are continually moving and changing shape in cytoplasm. There are no definite types of mitochondria, as any one type may change into another. They appear to arise in cytoplasm and to be used up by cellular activity.


The shape may change fifteen to twenty times in ten minutes; it can by potassium permanganate and osmotic changes. Frederic (1958), Littre (1954), Tobioka and Biesels (1956) studied the effect of large number of chemical and physical agents on mitochondrial behavior. Some material, such as detergents show some effect in vivo as on mitochondria isolated from homogenates.

Rapid changes of shape in living cells



The shape is variable but is characteristic for a cell or tissue type, this too is dependable upon environment or physiological conditions. In general they are filamentous or granular. They may swell out at one end to become club- shaped or hollow out at one end to assume a shape of tennis-racket. They may become vesicular by the appearance of central clear zone. Rod-shaped mitochondria are also observable.


The size of mitochondria also varies. In majority of the cells, width is relatively constant, about 0.5µ, but the length varies and, sometimes, reaches a maximum of 7µ. The size of cell also depends on the functional stage of the cell. Very thin mitochondria, about 0.2µ, or thick rods 2µ are also seen.


The size and shape of the fixed mitochondria are determined by the osmotic pressure and pH of the fixative. In acid, mitochondria are fragmented and become vesicular. Mitochondria, in the rat-liver, are usually 3.3µ in length; in mammalian exocrine pancreas, they are about 10µ in length and in oocytes of Amphibis, they are approximately of the length of 20 to 40µ.

Different shapes and sizes of mitochondria


Mitochondria are found in the cytoplasm of all aerobically respiring cells with the exception of bacteria in which the respiratory enzymes are located in the plasma membrane. The mitochondria content of a cell is difficult to determine, but in general, it varies with the cell type and functional stage.

It is estimated that in liver mitochondria constitute 30 to 3 5 per cent of the total content of the cell and in kidney, 20 per cent. In lymphoid tissue the value is much lower. In mouse liver homogenates there are about 8.7×10’° mitochondria per gram of fresh tissue. A normal liver cell contains about 100 to 1600 mitochondria, but this number diminishes during regeneration and also in cancerous tissue.


This last observation may be related to decreased oxidation that accompanies the increase to anaerobic glycolysis in cancer. Another interesting finding is that there is an increase in the number of mitochondria in the muscle after repeated administration of the thyroid hormone, thyroxin. An increased number of mitochondria have also been found in human hyperthyroidism.

Thus cells with high metabolic activity have a high number of mitochondria, while those with low metabolic activity have a lower number. Large sea urchin eggs have 13,000-14,000, while renal tubules have 300-400. In the sperm there are as few as 20- 24 mitochondria while in some oocytes there are about 300,000. In the protozoon Chaos chaos there are about 500,000 mitochondria. Some algal cells contain only one mitochondrion.

Structure of Mitochondria:

A typical mitochondrion in sausage shaped with an average diameter of about 0.5(i. When it is properly fixed in osmium containing fluid and studied under electron microscope which reveals that there is hardly any difference between plant and animal mitochondria. In both the cases the mitochondrion is bounded by two membranes, the outer membrane and the inner membrane.

The space between the two membranes is called the outer chamber or inters- membranous space. It is filled with a watery fluid, and is 40-70A0 in width. The space bounded by the inner membrane is called the inner chamber or inner membrane space.

The inner membrane space is filled with a matrix which contains dense granules (300-500A0), ribosomes and mitochondrial DNA. The granules consist of insoluble inorganic salts and believed to be the binding sites of divalent ions like Mg ++and Ca++.

Showing the structure of mitochondria In some cases, they apparently contain polymers of sugars. The side of the inner membrane facing the matrix side is called the M-side, while the side facing the outer chamber is called die C-side. Two to six circular DNA molecules have been identified with mitochondrion. These rings may either be in the open or in the twisted configuration. They may be present free in the matrix or may be attached to the membrane. The enzymes of the Krebs cycle are located in the matrix.

The inner membrane is thrown up into a series of folds, called cristae mitochondrial, which project into the inner chamber. The cavity of the cristae is called the inter-cristae space, and in continuous with the inter-membrane space.

The space and arrangement of crests is variable and may be of the following types:

Structural variations in mitochondrial Cristae

(i) Parallel to the long axis of mitochondrion as in the neurons and the striated muscle cells.

(ii) Concentrically arranged as in the matrix of certain spermatids.

(iii) Interlaced to form villi as in Amoeba.

(iv) Cristae in the form of vesicles which form a network of interconnected chambers as in the cells of parathyroid gland and W.В. C. of man.

(v) Arranged in a tubular fashion but perpendicular to mitochondrial axis as in the cells of adrenal gland.

(vi) Haphazardly distributed as in the cells of kidney of insects and hepatic cells.

(vii) Cristae extremely small and irregular as in the interstitial cells of Opossum.

(viii) Rarely the mitochondrial wall is smooth with no cristae. The number and size of cristae in a mitochondria directly affects in efficiency. The greater and larger are the cristae, the faster is the speed of oxidation reaction.

(ix) Perpendicular to the long axis of mitochondrion.

Mitochondrial Particles:

According to the descriptions (Green and Perdue, 1966) the outer surface of the outer membrane and the inner surface of the inner membrane were supposed to be covered with thousands of small particles. Those on the outer membrane were described as being stalk less and were called the subunits of Parson.

There may be as many as 10,000 to 100,000 particles per mitochondrion. However, recent studies have shown that stalk less particles are absent. The stalked inner membrane particles were called the subunits of Femandez-Moran, Elementary particles, FI particles or the oxiosomes or ETP or electron transport particles. These particles are about 84A° diameter and are regularly spaced at intervals of 10 nm on the inner membrane. There may be as many as 104 to 105 elementary particles per mitochondrion.

Structure and measurements of the mitochondrial particles

Isolation of Mitochondria:

Mitochondria can be isolated from the cell in the living form for their physiological studies. The cells first treated with deoxycholate for their break down. Then they are passed in sucrose solution. The homogenate should be centrifuged for 10 minutes at the speed of 6000 X g. From this homogenate upper substance is centrifuged at the speed of 8500 X g for 10 minutes.

After this centrifugation, the upper microsomal fraction is discarded while the lower fraction consists of mitochondria and other particles like lysosomes. This fraction is passed through sucrose gradient. The mitochondrial fraction then centrifuged at the speed of 10,000 X g up to 3 hrs. The upper part of this centrifuged material have mitochondria and lower part of lysosomes.

Inner membrane of mitochondrion showing distribution of respiratory chain

The Respirating Chain Complex’s:

Green et al. have recognized five main complexes which, if mixed in correct ratios, can reconstitute to form ETC.

These complexes are:

1. Complex 1 (NADH-Q-reductase):

This is the largest complex, with a molecular weight about 500,000 and structure consisting of 15 subunits. It contains as prosthetic groups flavin mononucleotide (FMN) and six iron-sulphur centers. The NADH reaction site lies at M side of mitochondrion.

Contact between NADH and CoQ is apparently made in the middle of the membrane. Complex 1 spans the inner mitochondrial membrane and is able to translocate the protons across it, from M side to С side of mitochondrion.

2. Complex II (Succinate-Q-reductase):

This complex is made up to two polypeptides with a molecular weight of 97000. It contains flavin adenine dinucleotide (FAD) and three iron-sulphur centers (Fe-SS1, Fe-SS2 and Fe-SS3). The succinate binding site occurs on the M side.

The three Fe-S centers are on the M side and there is a close interaction between Fe-S centre 3 and CoQ. In contrast to complex I, succnate-Q- reductase apparently is unable to translocate protons across the membrane.

3. Complex III (Q H2-cytochrome-C-reductase):

This complex contains a number submit with a molecular weight 280,000. It contains cytochrome b, cytochrome C, and iron sulphur protein. There are two types of cytochrome b, these are but (transducing cytochrome b) and bk (Keilin-type cytochrome b). The heme containing part of cytochrome C,’ which transfers the electrons to cytochrome – c, is located on the С side of mitochondrion.

4. Complex IV (Cylochrome-C-oxidase):

The arrengement of the respiratory chain in cristae

It has two cytochromes, a and a3, and two copper atoms. The molecular weight is about 200,000. The a and a3, however, never been separated and, therefore, both must be considered to be parts of the same complex. Complex IV is thought to traverse the mitochondrial membrane, protruding on both surfaces.

In experiments with yeast mitochondria it has been shown that cytochrome- c- oxidase is made of seven subunits. The seven subunits are arranged the membrane in a functional sequence, being in contact with cytochrome с on the С-side. The electrons then pass to cytochrome a then to Cu++ and finally to cytochrome a., oxygen at the M side.

5. Complex V (ATPase complex):

Towards the M side, the inner membrane contains rounded stalked particles- known as F, particles or Fernandez-Moran particles. Each F, particle is made up of a head, stalk and base. It has been shown that complex V is identical with F1 particles. These are four compiling factors on the M side designated as F 1 F2 F3 or OSCP (oligomycin-sensitive conferring protein) and F6.

The headpiece, coupling factor F1 is the ATPase proper. The F, contains five types of subunits, a with a molecular weight 53,000 daltons, β with a molecular weight 50,000 daltons, y with a molecular weight 33,000 deltons, β with a molecular weight 17,000 daltons and e with a molecular weight 7,000 deltons. In addition to these subunits there is ATPase inhibitor (I) with a molecular weight 10,000 daltons. This inhibitor can be removed while treating with trypsin.

Current view of the location of the enzymes

The stalk made up of protein connect the head – pieces to the base. This portion corresponds to the OSCP (oilgomycin –sensitive conferring protein) and F6.These is needed to bind F1 to the membrane .Treatment with ammonia releases OSCP. While treatment with silicotungustate removes F6.

The bases-piece lies within the inner mitochondrial membrane. It contains the portion translocating mechanism. This corresponds to FO in the figure.

Biochemistry of Mitochondria:

Lindberg and Ernster (1954) have given the data of chemical composition of mitochondria as follows: proteins 70 to 75%, Lipids 25-30% and RNA 5% of the dry weight.

But the recent biochemical analysis shows the following components:

(i) Proteins

The proteins are the main constituent which is insoluble in water. The outer limiting membrane of mitochondria contains less than 10 per cent of the total protein. There are about 14 different proteins having molecular weight from 12,000 to 22,000.

The inner membrane contains about 60% protein having molecular weight varying from 10,000 to 90,000. The protein composition of mitochondrial membranes is not fully known.

Localization of Enzymes, Obtained in Fractionation Studies


Mitochondrial fraction

Enzymes located


Outer membrane.

Monoamine oxidase,

“Rotenone-insensitive “NADH-cyto- chrome-C-reductase, Kynurenine hydro­xylase, Fatty acid CoA ligase, Glycero­phosphate acyl transferase, Nucleoside diphosphokinase,


Inter-membranes space.

Adenylate kinase, Nucleoside diphos­phokinase, Nucleoside monopho- sphokinase,


Inner membrane.

Respiratory chain enzymes, β-Hydroxybutyrate dehydrogenase, Ferrochelatase, Carnitine palmityl-transferase, Fatty acid oxidation system, Xylitol dehydrogenase,



Malate, isocitrate and Glutamate dehydrogenases, Fumarse, Aconitase, Citrate synthetases. Ornithine-Carbony 1 transferase, Fatty acid oxidation systems, Pyruvate carboxylase,

(ii) Lipid:

The lipid forms about 1/5th of the weight of the membranes. It is present almost entirely in the form of the molecules known as phospholipid. It has been reported by Meluick and Packer in 1971 that the outer membrane fraction has a 40% lipid contents as compared to 20% in the inner membrane.

(iii) Enzymes:

About 70 enzymes and 12 co-enzymes have been recognized in the mitochondria. Enzymes lie in a non-aqueous membranous region as solid arrays, with perhaps as many as 5000 to 20,000 such assemblages in a single liver or heart mitochondrion.

(iv) Mitochondrial DNA:

Recently the DNA is also reported from mitochondria. The mitochondrial DNA is double stranded like the nuclear DNA. Each mitochondrion may contain one or more DNA molecules depending on its size, if, the mitochondrion is larger than that may have more DNA molecules. DNA has a circular shape.

Mitochondrial DNA differs from nuclear DNA in several respects. The guanine and cytocine content is higher in mitochondrial DNA, and consequently the buoyant density is also higher. The amount of genetic information carried by mitochondrial DNA is not sufficient to provide specifications for all the proteins and enzymes present in this organoid. The most likely possibility is that mitochondrial DNA codes for some structural proteins.

Mitochondrial DNA

Yeast mitochondria have been shown to contain DNA polymerase (Winters Berger, 1966) and more recently Kalfl968 has succeeded in isolating the enzymes from rat liver mitochondria; Mitochondrial DNA polymerase appears to be involved in DNA replication rather than repair (Karol and Simpson 1968) and possesses properties which are different from those of the nuclear enzymes.

These include a differing requirement for metal ion (Meyr and Simpson 1968). Yeast mitochondrial DNA polymerase appears to be smaller than its nuclear counterpart, and is active at different stages of the cell cycle (Iwashima and Rabinowitz, 1969). Visual evidence showing what appears to be rat liver mitochondrial DNA in the process of replication has been presented by Kirschner, Wolsten Holme and Gross (1968).

Mitochondrial DNA does not appear to have histones associated with it as has nuclear DNA of higher organisms. In this respect mitochondrial DNA resembles with the bacterial DNA.

(v) Mitochondrial RNA (mt RNA):

South and Mehlear (1968) suggested that the amount of mt-RNA is about 10 to 20 times that of mt-DNA. All sorts of RNA have been identified in mitochondria. The present evidences point out conclusively that mitochondria contain complete set of nRNA (Wintersberger and Tuppy. 1965), aminoacyl RNA synthetases (Barnett, Braun and Epler, 1967), as well as ribosomal RNA (Rogers, Preston, Titchener and Linnane, 1967).

All these components differ from their respective counterparts in the groundplasm. The presence of m- RNA, transcribed from mitochondrial DNA is still uncertain. However, there are authorities who suggest its presence. The ribosomal RNA’s are coded for by mitochondrial DNA and are thus apparently synthesized within the mitochondria by a mitochondrial DNA-dependent RNA polymerase system (Wintersberger, 1964).

(vi) Mitochondrial Ribosomes:

Mitochondria appear to contain ribosomes which are smaller in diameter than cytoplasmic ribosomes {Swift, 1965) and Yeast mitochondria contain RNA species of 23 s and 16 s (Winter Berger 1966) which would correspond to a 70s ribosome of bacterial type rather than the SOS ribosome of the cytoplasm.

Ribosome like particles with sedimentation values of 8IS and 55S have also been reported, and the extent of degradation suffered by the particles during isolating is not yet clear. Polysome like aggregation of ribosomes have been observed in sections of yeast mitochondria by Vignais, Huet and Andre in 1969.

High molecular weight and RNA species associated with mitochondria which differ in sedimentation value from cytoplasmic ribosomal RNA have been reported in Yeast, Neurospora and He-La cells. Mitochondrial ribosomes require a higher concentration of Mg++ ions to maintain their integrity than do cytoplasmic ribosomes.

Protein Synthesis:

In general, mitochondria can code and synthesize protein, but the DNA present therein is insufficient to code for all the proteins. It is suggested that mitochondria can synthesize the proteins of structural nature (cytochrome oxidase), but much of the proteins if not all, of the soluble proteins of the matrix as well as the proteins of the outer membrane and a number of proteins located in the criptae (Borst, 1972) are under the control of nuclear.

DNA of the proteins coded by nuclear DNA, it is generally agreed that the m-RNA derived from nucleus are translated in the cytoplasm, and the resulting proteins are then transported into the mitochondria. How these proteins enter the mitochondria?

Two methods have been proposed:

(1) The precursors enter the mitochondrion and inside are changed into the end products, thereby affecting a unidirectional flow of material into the mitochondrion.

(2) There is the synthesis of lipoprotein vesicles which they merge and combine with the growing mitochondrion.


1. Role of Mitochondria in Yolk Formation:

There have been a good number of investigations, whose account reveals that, mitochondria help in the formation of yolk in a developing ovum. The first study in this field was made by Loyez (1911) and latest probably by M.D.L. Srivastava (1965), with the help of light microscope. The evidence adduced depends upon the topographical, and size relationship, and staining reactions of mitochondria and early protein yolk.

In the modern cytology with the investigation of electron microscope a new era has started and the studies of yolk formation do not remain away from electron microscope. With the help of electron microscope Farvard and Carasso (1958) come to the conclusion that mitochondria transformed into yolk granules in the egg of Planorbis coneus

The main structural changes which they observed in the mitochondria are as follows:

(i) The cristae become disorganised in a few membranes, remaining con­centric to the outer membrane before dropping completely.

(ii) In the matrix appeared a few minute granules which have scattered first, but become aggregated eventually in masses in regular pattern.

2. During Cell division and Spermatogenesis:

Early cytologists, Benda, Dulberg and Meves were of opinion that mitochondria also divide equally during the cytoplasmic division and perhaps playing a part in inheritance. Wilson (1928) commented that not the slightest proof has been produced of a fusion between the paternal and maternal chondriosomes. Frederic (1958) has briefly summarized various changes in mitochondria during cell division.

The first phase shows a decrease in the total volume of mitochondrial material; gradually ceases its movements, pronounced thinning, fragmentation into small spheres, loss of optical density and finally assimilation into the cytoplasm.

In the second phase, when the cell divides into two, the modified mitochondria are separated passively, into the daughter cells: In the third phase, the modified mitochondria are reconstituted by addition of elements assimilated in the cytoplasm.

Wilson found that in Opisthacanthus, during spermatogenesis, the number of mitochondria gradually reduces. Pollister (1930) describes in Gems that mitochondria arranged themselves into a well-defined ring, but without fusion. Modem microscopic studies provide firm conclusion regarding mitochondrial division during mitosis. Pune (1952) noted in low1 adrenal cortex the mitochondria frequently appeared as pairs.

This suggested that division, rather than fusion, was occurring. In the transformation of spermatids into spermatozoa many mitochondrial changes are observed. Franzen (1956) observed in those sperms, which are shed directly in water that mitochondria are present in form generally of four or five spheres below the sperm head, and in the case of sperms discharged in viscous medium these spheres transform into two elongated ribbon like filamentous mitochondria.

Sometimes these develop into ‘nebenkern spheres’ which may elongate and twist around the axial filament to form the mitochondrial sheath. Yasuzumi (1958) found an electron Indistinguishable from a lipid droplet.’

Role of Mitochondria during spermiogensis

3. Role of mitochondria in the production of energy:

Mitochondria play a very important role in cellular respiration or the produ­ction of energy. Energy is produced inside the cell, partly outside the mitochondria and mainly inside the mitochondria. The ATP molecules that are produced in the non-mitochondrial cytoplasm are generated by a process referred to as anaerobic respiration.

4. Role of mitochondria in haem synthesis:

In rat liver cells and avian red blood corpuscles δ-amino levulinate is synthesized from succiny 1 Co-A and glycine by enzymatic action of δ-amino levulinate synthatase. This enzyme is present in mitochondrial fraction. δ-amino levulinate is an important intermediate in porphyry syntheses. Thus mitochondria help in haem synthesis.

Mechanism of Gluconeogenesis

5. Role in gluconeogenesis:

The gluconeogenesis is the conversion of non-carbohydrates to glucose from pyruvic acid. It is well-known that pyruvic acid is converted into oxaloacetic acid of the presence of pyruvic acid carboxylase. This intermediate may escape mitochondria and become converted into phosphoenol pyruvic acid by phosphoenol pyruvate corboxykinase: Phosphoenol pyruvic acid occupies a place in Embden-Mayerhoff pathway or glycolytic pathway from where the pathway is reversible up to glucose.

Glucogenic amino acids, lactic acid, glycerol and in some cases propionate can, after suitable modification, be fed at one or the other point in Krebs’ cycle. Oxaloacetic acid and malic acid can come out of mitochondria to be finally converted into glucose.

6. Role of mitochondria in amino acid metabolism:

The enzymes for oxidative domination of amino acids are present in mitochondria. These are glutamate dehydrogenase, proline dehydrogenase, 8- amino levulinate synthetase etc.

7. Role of mitochondria in lipid metabolism:

They are able to oxidize fatty acids. Oxidation of fatty acid requires complete oxidation of acetyl Co A in the Krebs’ cycle so that free Co A may be generates. Reversal of fatty acid oxidation leads to fatty acid synthesis. During starvation, the mitochondria utilize fat to produce energy.

Origin of Mitochondria:

Our understanding of the process whereby mitochondria are produced, is still very incomplete.

Lehninger (1964) classified the various theories of possible routes of mitochondrial genesis into three main groups:

1. Formation from other membranous structures in the cell.

2. Growth and division of pre-existing mitochondria.

3. De novo synthesis from submicroscopic precursors.

1. Formation from other membranous structures in the cell:

The formation of mitochondria by “pinching off” or budding from pre­existing cell structures has been suggested for a range of cell membranes including those of the plasmalmma (Robertson, 1959), endoplasmic reticulum, nuclear envelope and Golgi complex (Novikoff, 1961). But the support for such evidence, in the absence of supporting biochemical data, cannot be wholly conclusive.

Part of the problem undoubtedly lies in our fragmentary knowledge of the structure and composition of, and differences between cell membranes in general. Indeed the similarities discussed in the literature between the mitochondrial membrane and the endoplasmic reticulum, could lend weight to the idea that probably mitochondria are formed when cytoplasm pushes into a cavity surrounded by an internal membrane, which they, pinches off and separates from the continuous system.

2. Growth and division of pre-existing mitochondria:

Electron microscopic evidence for mitochondrial division by fission, although plentiful, is difficult to assess the danger of producing artifacts is very real because of the harsh chemical and physical agents brought to bear on the test material during processing.

Interpretation is not made easier by the ability of mitochondria to undergo extreme changes in shape in vivo which may or may not be associated with mitochondrial fission. There are numerous reports of mitochondria connected to each other by narrow bridges of membrane especially in rapidly melopolizing tissue, and it is thought that such figures may represent mitochondria in an early stage of fission.

By observing serial sections of rat liver Stempak (1967) was able to show that “dumb bell-shaped” mitochondria can be the sections of cup-shaped bodies. Such bodies have also been observed in rapidly growing tissues of fern and may represent the beginning stages of division.

A possible Mechanism for the formation of Mitochondria from the plasma Membrane

Diagram Showing teh Dividing Mitocondria in devloping callus tissus of fem

An early stage in mitochondrial division may involve the separation of mitochondrial contents into two or more compartments. The presence of mitochondria with internal “partitions” has been formed in several cell types (Tandler et. al., 1969) although the possibility that they are manifestations of mitochondrial fusion cannot easily be ruled out.

Lafontaine and Allard (1964) have presented election micrographs of rat liver mitochondria which exhibit what appear to be partitions dividing the inner membrane complex into two masses, the whole being surrounded by continuous outer membrane Tandler (1969) has demonstrated the partitions of mitochondria in liver which was recovering from riboflavin deficiency.

3. Do novo synthesis:

The possibility of de-novo synthesis of mitochondria arose with experiments in the early part of the century, when mitochondria containing larvae were seen to develop from sea urchin egg cytoplasm which had apparently been freed of mitochondria by centrifugation (Novikoff, 1961).

Using the greater resolving power of the electron microscope, it was later shown, that mitochondria could not be dislodged by centrifugation of the egg (Lansing, Hiller, Rosenthall 1952). In the experiments, mitochondria had probably been present in the “centripetal end” of the egg cell after all, and these mitochondria could have served as precursors in subsequent mitochondrial production.

In the above description, a number of views were described, regarding the generation of mitochondria in the cytoplasm of different types of cells. But it is perhaps unwise to group the evidence to suit one or other of a limited number of clear cut methods by which mitochondria could replicate.

The actual situation is probably complex, and it may well be that different methods of replication take place in different tissues, and at different stages in development. One could imagine the early mitochondria being formed from membrane structures in the developing embryo concentration of mitochondria around the nuclear membrane have been noted in embryonic tissues from several phyla (North and Poliak, 1961) and formation of mitochondria from this membrane could involve the transfer of nuclear genetic information essential for subsequent mitochondrial growth and multiplication by division.

The multiplication of mitochondria could then proceed by the incorporation of large pre-fabricated molecules and association of molecules, with division by fission, when the mitochondria reached a critical stage.

A giant dividing Mitochondrion from the liver of a mouse

4. Prokaryotic Origin of Mitochondria:

This fact that mitochondria can grow, divide and are capable of mutations support a long-held view that mitochondria originated with their host. Bacteria would have originated the mitochondria and blue green algae, the chloroplasts.

There are numerous homologies between mitochondria and bacteria. In bacteria, the electron transport system is localized in the plasma membrane which can be compared with the inner membrane of mitochondria.

Some bacteria even have membranous projections extending from the plasma membrane (Fitz- James, 1960) which are comparable to mitochondrial cristae since both contain the respiratory chain (Salton and Chapman, 1962).

The inner membrane and matrix, it has been postulated may represent the original symbiont which may become enclosed within a membrane of cellular origin (ER). Further mitochondrial DNA is circular, it replicates and divides like that of bacteria.

The ribosomes are also found which are, however, smaller than those of the bacteria. In mitochondria and bacteria, the protein synthesis is inhibited by chloramphenicol. From these similarities, one can easily conceive of mitochondria as being evolved from an ancient prokaryote (Swift, 1965), possessing all the attributes of an independent probably aerobic organism.

However, with the adaptation over a long period it became an essential and dependent symbiont, lost some of its identity to the cell, and conversely, the host cell lost some of its functions, deriving it now from the endosymbiont or mitochondrion. As a results, both became obligatory symbionts to each other.

This symbiont hypothesis for the origin of mitochondria and plastids has achieved wide popularity, but all biologists do not necessarily accept it. Raff and Mahler (1972) concluded that, while the symbiotic theory may be esthetically pleasing, it is not compelling.

They presented a lot of evidences and proposed that mitochondria arose by inward blebbing from plasma membrane, by the acquisition somehow of an outer membrane, and by the additional acquisition of a DNA genophore from the DNA of the proto-eukaryote in which the evolution of mitochondrion occurred. Borst (1972) proposed an episode theory and supposed that the DNA of mitochondrion left the ‘nuclear’ DNA by a sort of amplification to become mapped within a membrane containing the respiratory chain.

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