Mitochondria the Powerhouse of The Cell & It’s Function

Mitochondria also are known as the powerhouse of the cell. It is filamentous or granular cytoplasmic organelles of all aerobic cells of higher animals and plants and also of certain micro-organisms including Algae, Protozoa, and Fungi. These are absent in bacterial cells.

The mitochondria have a lipoprotein framework which contains many enzymes and coenzymes required for energy metabolism. They also contain a specific DNA for the cytoplasmic inheritance and ribosomes for protein synthesis.

History and Discovery of Mitochondria

The mitochondria were first observed by Kolliker in 1850 as granular structures in the striated muscles. In 1888, he isolated them from insect muscles (which contain many slab-like mitochondria; and showed that they swelled in water and contain a membrane around them).

In 1882, Flemming named them as fila. Richard Altmann (1890) developed a specific stain that had useful specificity for the mitochondria. He named this organelle, the bioblast. Altmann correctly speculated that bioblasts were autonomous elementary living particles that made a genetic and metabolic impact on the cells.

The present name mitochondria were assigned by Benda (1897–98) to them. He stained mitochondria with alizarin and crystal violet. Michaelis (1900) used the supravital stain Janus green to demonstrate that mitochondria were oxidation-reduction sites in the cell.

In 1912, Kingsbury suggested that the oxidation reactions mediated by mitochondria were normal cellular processes. Otto Warburg (1883–1970), who is considered as ‘the father of respirometry’, in 1910 isolated mitochondria (“large granules”) by low-speed centrifugation of tissues disrupted by grinding.

He showed that these granules contained enzymes catalyzing oxidative cellular reactions. Various steps of glycolysis were discovered by two German biochemists Embden and Meyerhof.

Meyerhof got Nobel Prize in 1922 along with English biophysicist A.V. Hill, for the discovery of oxygen and metabolism of lactic acid in the muscle (i.e., production of heat in muscle). Lohmann (1931) discovered ATP in muscle.

Lipmann (German biochemist in U.S.) discovered co-enzyme A and showed its significance in intermediary metabolism. In 1941, he introduced the concept of “high energy phosphates” and “high energy phosphate bonds” (i.e., ATP) in bioenergetics.

Warburg linked the phenomenon of ATP formation to the oxidation of glyceraldehyde phosphate. Meyerhof showed the formation of ATP from phosphopyruvate and Kalckar related oxidative phosphorylation to respiration.

Sir Hans Adolph Krebs (German biochemist in England), in 1937, worked out various reactions of the citric acid cycle (or tricarboxylic acid or TCA cycle). His contribution was remarkable, because, up to that time radioactively labeled compounds were not available for biological studies and cellular sites of the reactions were not known with certainty. Krebs received the Nobel Prize in 1953 along with Lipmann for his discovery of the citric acid cycle.

Kennedy and Lehninger (1948–1950) showed that the citric acid cycle (Krebs cycle), oxidative phosphorylation, and fatty acid oxidation took place in the mitochondria. In 1951, Lehninger proved that oxidative phosphorylation requires electron transport.

Among these early investigators of ETS the Nobel Prize recipients were Warburg, Szent-Gyorgyi, and Kuhn. In 1961, Mitchell proposed the highly acclaimed “chemiosmotic-coupling hypothesis” for the ATP-production in mitochondria.

He got the Nobel Prize in 1978 for the development of this model. Palade (1954) described the ultra-structure of cristale. In 1963, Nass and Nass demonstrated the presence of DNA fibers in the matrix of mitochondria of embryonic cells.

Attardi, Attardi, and Aloni (1971) reported the 70S type ribosomes inside the mitochondria. Previously the mitochondria have been known by various names such as fuchsinophilic granules, parabasal bodies, plasmosomes, plastosomes, fila, vermicules, bioblasts, and chondriosomes.

Distribution or Localization

The mitochondria move autonomously in the cytoplasm, so they generally have a uniform distribution in the cytoplasm, but in many cells, their distribution is very restricted. The distribution and number of mitochondria (and also of mitochondrial cristae) are often correlated with the type of function the cell performs.

Typically, mitochondria with many cristae are associated with mechanical and osmotic work situations, where there are sustained demands for ATP and where space is at a premium, e.g., between muscle fibres, in the basal infolding of kidney tubule cells, and in a portion of inner segment of rod and cone cells of retina.

Myocardial muscle cells have numerous large mitochondria called sarcosomes, that reflect the great amount of work done by these cells. Since the work of hepatic cells is mainly biosynthetic and degradative, and work locations are spread throughout the cell, in these cells, it may be more efficient to have a large number of “low key” sources of ATP production distributed throughout the cell.

Often mitochondria occur in greater concentrations at work sites, for example, in the oocyte of Thyone briaeus, rows of mitochondria are closely associated with RER membranes, where ATP is required for protein biosynthesis. Mitochondria are particularly numerous in regions where ATP-driven osmotic work occurs, e.g., brush border of kidney proximal tubules, the infolding of the plasma membrane of dogfish salt glands, and Malpighian tubules of insects, the contractile vacuoles of some protozoans (Paramecium).

Non-myelinated axons contain many mitochondria that are poor ATP factories since each has only a single crista. In this case, there is a great requirement for monoamine oxidase, an enzyme present in the outer mitochondrial membrane that oxidatively deaminates monoamines including neurotransmitters (acetylcholine).

Orientation

The mitochondria have a definite orientation. For example, in cylindrical cells, the mitochondria usually remain orientated in basal apical direction and lie parallel to the main axis. In leucocytes, the mitochondria remain arranged radially with respect to the centrioles.

As they move about in the mitochondria form long moving filaments or chains, while in others they remain fixed in one position where they provide ATP directly to a site of high ATP utilization, e.g., they are packed between adjacent myofibrils in a cardiac muscle cell or wrapped tightly around the flagellum of sperm.

Morphological structure

How many Mitochondria present in the cell?

The number of mitochondria in a cell depends on the type and functional state of the cell. It varies from cell to cell and from species to species. Certain cells contain an exceptionally large number of the mitochondria, e.g., the Amoeba, Chaos chaos contains 50,000; eggs of sea urchin contain 140,000 to 150,000, and oocytes of amphibians contain 300,000 mitochondria.

Certain cells, viz., liver cells of rat contain only 500 to 1600 mitochondria. The cells of green plants contain fewer mitochondria in comparison to animal cells because in plant cells the function of mitochondria is taken over by the chloroplasts. Some algal cells may contain only one mitochondrion.

What is the shape of Mitochondria?

The mitochondria may be filamentous or granular in shape and may change from one form to another depending upon the physiological conditions of the cells. Thus, they may be of club, racket, vesicular, ring, or round-shape. Mitochondria are granular in primary spermatocyte or rat, or club-shaped in liver cells.

The time-lapse micro cinematography of living cells shows that mitochondria are remarkably mobile and plastic organelles, constantly changing their shape. They sometimes fuse with one another and then separate again.

For example, in certain euglenoid cells, the mitochondria fuse into a reticulate structure during the day and dissociate during darkness. Similar changes have been reported in yeast species, apparently in response to culture conditions.

What are the Size of Mitochondria?

Normally mitochondria vary in size from 0.5 μm to 2.0 μm and, therefore, are not distinctly visible under the light microscope. Sometimes their length may reach up to 7 μm.

Structure of Mitochondria

Structure of Mitochondria with labeled diagram.

Each mitochondrion is bound by two highly specialized membranes that play a crucial part in its activities. Each of the mitochondrial membranes is 6 nm in thickness and fluid mosaic in ultrastructure.

The outer membrane is quite smooth and has many copies of a transport protein called porin which forms large aqueous channels through the lipid bilayer. This membrane, thus, resembles a sieve that is permeable to all molecules of 10,000 daltons or less, including small proteins.

Inside and separated from the outer membrane by a 6–8 nm wide space is present the inner membrane. The inner membrane is not smooth but is impermeable and highly convoluted, forming a series of infoldings, known as cristae, in the matrix space.

Thus, mitochondria are double-membrane envelopes in which the inner membrane divides the mitochondrial space into two distinct chambers:

  1. The outer compartment, peri-mitochondrial space, or the inter-membrane space between the outer membrane and inner membrane. This space is continuous into the core of the crests or cristae.
  2. The inner compartment, inner chamber, or matrix space, which is filled with a dense, homogeneous, gel-like proteinaceous material, called the mitochondrial matrix.

The mitochondrial matrix contains lipids, proteins, circular DNA molecules, 55S ribosomes, and certain granules which are related to the ability of mitochondria to accumulate ions. Granules are prominent in the mitochondria of cells concerned with the transport of ions and water, including kidney tubule cells, epithelial cells of the small intestine, and the osteoblasts of bone-forming cells.

Further, the inner membrane has an outer cytosol or C face toward the peri mitochondrial space and an inner matrix of M face toward the matrix. In general, the cristae of plant mitochondria are tubular, while those of animal mitochondria are lamellar or plate-like, but, in many Protozoa and in steroid synthesizing tissues including the adrenal cortex and corpus luteum, they occur as regularly packed tubules.

The cristae greatly increase the area of the inner membrane, so that in liver cell mitochondria, the cristae membrane is 3–4 times greater than the outer membrane area. Some mitochondria, particularly those from heart, kidney and skeletal muscles have more extensive cristae arrangements than liver mitochondria.

In comparison to these, other mitochondria (e.g., from fibroblasts, nerve axons, and most plant tissues) have relatively few cristae. For example, mitochondria in epithelial cells of carotid bodies have only four to five cristae and mitochondria from non-myelinated axons of the rabbit brain have only a single crista.

Attached to M faces of the inner mitochondrial membrane are repeated units of stalked particles, called elementary particles, inner membrane subunits or oxysomes. They are also identified as F1 particles or F0-F1 particles and are meant for ATP synthesis (phosphorylation) and also for ATP oxidation (i.e., acting as ATP synthetase and ATPase).

F0-F1 particles are regularly spaced at intervals of 10 nm on the inner surface of the inner mitochondrial membrane. According to some estimates, there are 104 to 105 elementary particles per mitochondrion.

When the mitochondrial cristae are disrupted by sonic vibrations or by detergent action, they produce submitochondrial vesicles of inverted orientation. In these vesicles, F0-F1 particles are seen attached on their outer surface.

These submitochondrial vesicles are able to perform respiratory chain phosphorylation. However, in the absence of F0-F1 particles, these vesicles lose their capacity of phosphorylation as shown by resolution (i.e., removal by urea or trypsin treatment) and reconstitution of these particles

Chemical Composition of Mitochondria

The gross chemical composition of the mitochondria varies in different animal and plant cells. However, the mitochondria are found to contain 65 to 70% proteins, 25 to 30% lipids, 0.5% RNA, and a small amount of the DNA. The lipid contents of the mitochondria are composed of 90% phospholipids (lecithin and cephalin), 5% or less cholesterol, and 5% free fatty acids and triglycerides.

The inner membrane is rich in one type of phospholipid, called cardiolipin which makes this membrane impermeable to a variety of ions and small molecules (e.g., Na+, K+, Cl, NAD+, AMP, GTP, CoA, and so on).

The outer mitochondrial membrane has a typical ratio of 50% proteins and 50% phospholipids of ‘unit membrane’. However, it contains more unsaturated fatty acids and less cholesterol.

It has been estimated that in the mitochondria of the liver 67% of the total mitochondrial protein is located in the matrix, 21% is located in the inner membrane, 6% is situated in the outer membrane and 6% is found in the outer chamber. Each of these four mitochondrial regions contains a special set of proteins that mediate distinct functions:

1. Enzymes of outer membrane.

Besides porin, other proteins of this membrane include enzymes involved in mitochondrial lipid synthesis and those enzymes that convert lipid substrates into forms that are subsequently metabolized in the matrix.

Certain important enzymes of this membrane are monoamine oxidase, rotenone-insensitive NADH-cytochrome-C-reductase, kynurenine hydroxylase, and fatty acid CoA ligase.

2. Enzymes of intermembrane space.

This space contains several enzymes that use the ATP molecules passing out of the matrix to phosphorylate other nucleotides. The main enzymes of this part are adenylate kinase and nucleoside diphosphokinase.

3. Enzymes of inner membrane.

This membrane contains proteins with three types of functions:

  1. those that carry out the oxidation reactions of the respiratory chain;
  2. an enzyme complex, called ATP synthetase that makes ATP in a matrix;
  3. specific transport proteins that regulate the passage of metabolites into and out of the matrix.

Since an electrochemical gradient, that drives ATP synthetase, is established across this membrane by the respiratory chain, it is important that the membrane be impermeable to small ions.

The significant enzymes of the inner membrane are enzymes of electron transport pathways, viz., nicotinamide adenine dinucleotide (NAD), flavin adenine dinucleotide (FAD), diphosphopyridine nucleotide (DPN) dehydrogenase, four cytochromes (Cyt. b, Cyt. c, Cyt.c1, Cyt. a and Cyt. a3), ubiquinone or coenzyme Q10, non-heme copper and iron, ATP synthetase, succinate dehydrogenase; β-hydroxybutyrate dehydrogenase; carnitive fatty acid acyltransferase.

4. Enzymes of mitochondrial matrix.

The mitochondrial matrix contains a highly concentrated mixture of hundreds of enzymes, including those required for the oxidation of pyruvate and fatty acids and for the citric acid cycle or Krebs cycle.

The matrix also contains several identical copies of the mitochondrial DNA, special 55S mitochondrial ribosomes, tRNAs, and various enzymes required for the expression of mitochondrial genes. Thus, the mitochondrial matrix contains the following enzymes:

  • malate dehydrogenase,
  • isocitrate dehydrogenase,
  • fumarase,
  • aconitase,
  • citrate synthetase,
  • α-keto acid dehydrogenase,
  • β-oxidation enzymes.

Moreover, the mitochondrial matrix contains different nucleotides, nucleotide coenzymes, and inorganic electrolytes K+, HPO4, Mg++, Cl and SO4 .

Mitochondria and Chloroplast as Transducing Systems

In cells, energy transformation takes place through the agency of two main transducing systems (i.e., systems that produce energy transformation) represented by mitochondria and chloroplasts.

These two organelles of eukaryotic cells in some respects operate in opposite directions. For example, chloroplasts are present only in plant cells and specially adapted to capture light energy and to transduce it into chemical energy, which is stored in covalent bonds between atoms in the different nutrients or fuel molecules.

In contrast, the mitochondria are the “power plants” or “powerhouses” that by oxidation, release the energy contained in the fuel molecules and make other forms of chemical energy.

The main function of chloroplasts is photosynthesis, while that of mitochondria is oxidative phosphorylation. Finally, photosynthesis is an endergonic reaction, which means that it captures energy; oxidative phosphorylation is an exergonic reaction, meaning that it releases energy.

Function of Mitochondria

The mitochondria perform the most important functions such as oxidation, dehydrogenation, oxidative phosphorylation, and respiratory chain of the cell. Their structure and enzymatic system are fully adapted for their different functions.

They are the actual respiratory organs of the cells where the foodstuffs, i.e., carbohydrates and fats are completely oxidized into CO2 and H2O.

 Power house of the cell (ATP Synthesis)

During the biological oxidation of the carbohydrates and fats, a large amount of energy is released which is utilized by the mitochondria for the synthesis of the energy-rich compound known as adenosine triphosphate or ATP.

Because mitochondria synthesize energy-rich compound ATP, they are also known as “powerhouses” of the cell.

In animal cells mitochondria produce 95% of ATP molecules, the remaining 5% is being produced during anaerobic respiration outside the mitochondria. In-plant cells, ATP is also produced by the chloroplasts.

Adenosine triphosphate or ATP

The ATP consists of a purine base adenine, a pentose sugar ribose, and three molecules of the phosphoric acids. The adenine and ribose sugar collectively constitute the nucleoside adenosine which by having one, two, or three phosphate groups form the adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP) respectively.

In ATP the last phosphate group is linked with ADP by a special bond known as “energy-rich bond” because when the last phosphate group of the ATP is released a large amount of energy is released as shown by the following reaction:

ATP = ADP+ Pi+ 7300 calories

In the above reaction, we have seen that by the breaking of the energy-rich bond about 7300 calories of energy are released, while the common chemical bond releases only 300 calories of energy. The chemical reactions which synthesize the energy-rich bond or ~P bond require a great amount of energy which is supplied by the oxidation of the foodstuffs in the mitochondria.

The utility of the energy-rich phosphate bond (~P) of the ATP is that a great amount of energy is kept stored in the ready state in a very limited space of the cell. The stored chemical energy is disposed of very quickly at the time of the need in various cellular functions such as respiratory cycle, protein, and nucleic acid synthesis, nervous transmission, cell division, transportation, and bioluminescence, etc.

Because the terminal phosphate linkage in ATP is easily cleaved with the release of free energy, ATP acts as an efficient phosphate donor in a large number of different phosphorylation reactions. In this way, ATP acts as a carrier molecule like the acetyl CoA and as coenzymes like the CoA or NAD.

Recently, besides ATP, certain other energy-rich chemical compounds have been found to be active in cellular metabolism. These are cytosine triphosphate (CTP), uridine triphosphate (UTP), and guanosine triphosphate (GTP).

These compounds, however, derive the energy from the ATP by nucleoside diphosphokinases. The energy for the production of ATP or other energy-rich molecules is produced during the breakdown of food molecules including carbohydrates, fats, and proteins (catabolic and exergonic activities).

Other Function of Mitochondria

Besides the ATP production, mitochondria serve the following important functions in animals:

1. Heat productionor thermogenesis.

As we have already discussed earlier that only 45% of the energy released during the oxidation of glucose is captured in the form of ATP, the rest 55% is either lost as heat or used to regulate body temperature of warm-blooded animals.

In some mammals, especially young animals, and hibernating species, there is a specialized tissue called brown fat. This tissue, typically located between the shoulder blades, is especially important in temperature regulation; it produces large quantities of body heat necessary for arousal from hibernation.

The color of brown fat comes from its high concentration of mitochondria, which are sparse in ordinary fat cells. The mitochondria appear to catalyze electron transport in the usual way but are much less efficient at producing ATP.

Hence, a higher than usual fraction of the oxidatively released energy is converted directly to heat (called non-shivering thermogenesis).

2. Biosynthetic or anabolic activities.

Mitochondria also perform certain biosynthetic or anabolic functions. Mitochondria contain DNA and the machinery needed for protein synthesis. Therefore, they can make less than a dozen different proteins.

The proteins so far identified are subunits of the ATPase, portions of the reductase responsible for the transfer of electrons from CoQ to the iron of Cyt c, and three of the seven subunits in cytochrome oxidase.

Altogether, no more than 5–10% of mitochondrial components can be attributed to mitochondrial genes. Some biosynthetic functions of mitochondria are of primary benefit to the rest of the cell.

For example, the synthesis of heme (needed for cytochromes, myoglobin, and hemoglobin) begins with a mitochondrial reaction catalyzed by the enzyme, delta, or δ-aminolevulinic acid synthetase. Likewise, some of the early steps in the conversion of cholesterol to steroid hormones in the adrenal cortex are also catalyzed by mitochondrial enzymes.

3. Accumulation of Ca2+ and phosphate.

In the mitochondria of osteoblasts present in tissues undergoing calcification large amount of Ca2+ and phosphate (PO4) tend to accumulate. In them, microcrystalline, electron-dense deposits may become visible.

Sometimes, the mitochondria assume storage function, e.g., the mitochondria of the ovum store large amounts of yolk proteins and transform into yolk platelets.

Prokaryotic Origin or Symbiont Hypothesis of Mitochondria

Early cytologists such as Altmann and Schimber (1890) have suggested the possibility of the origin of the mitochondria from the prokaryotic cells. According to their hypothesis, the mitochondria and chloroplasts may be considered as intracellular parasites of the cells which have entered the cytoplasm of eukaryotic cells in early evolutionary days and have maintained the symbiotic relations with the eukaryotic cells.

The mitochondria are supposed to be derived from the bacterial cells (purple bacteria) while chloroplasts are supposed to be originated from the blue-green algae. Due to these reasons, Altmann suggested the name “bioblasts” to the mitochondria and he also hinted about their self-replicating nature.

Recent cytological findings have also suggested many homologies between the mitochondria and the bacterial cells. The similarities between the two can be summarized as follows:

1. Similarity in inner mitochondrial membrane and bacterial plasma membrane.

  • In the mitochondria, the enzymes of the respiratory chain are localized on the inner mitochondrial membrane like the bacteria in which they remain localized in the plasma membrane. The bacterial plasma membrane resembles with the inner mitochondrial membrane in certain respects.
  • The plasma membrane of certain bacterial cells gives out finger-like projections in the cytoplasm known as mesosomes. The mesosomes can be compared with mitochondrial crests. Salton (1962) has reported respiratory chain enzymes in the mesosomes.
  • Because the outer mitochondrial membrane resembles the plasma membrane, therefore, it may be assumed that the mitochondrial matrix and the inner mitochondrial membrane represent the symbiont that might be enclosed by the membrane of the cellular origin (outer mitochondrial membrane).

2. Similarity in DNA molecule.

The DNA molecule of the mitochondria is circular like the DNA molecule of the bacterial cells. Further the replication process of the mitochondrial DNA is also similar to bacterial DNA.

3. Similarity in ribosomes.

The mitochondrial ribosomes are small in size and resemble the ribosomes of the bacteria.

4. Similarity in the process of protein synthesis.

The process of protein synthesis of both mitochondria and bacteria is fundamentally the same because, in both, the process of protein synthesis can be inhibited by the same inhibitor known as chloramphenicol.

Further, the mitochondria for the process of protein synthesis depend partially on the mitochondrial matrix and DNA and partially on the nucleus and cytoplasm of the eukaryotic cells. This shows the symbiotic nature of the mitochondria.

Due to the above-mentioned similarities between the bacteria and mitochondria, the symbiont hypothesis postulated that the host cell (eukaryotic cell) represented an anaerobic organism that derives the required energy from the oxidations of food by the process of glycolysis.

While the mitochondria represent the symbionts, which respire aerobically and contain the enzymes of the Krebs cycle and respiratory chain. The symbionts seem to be capable to get the energy by oxidative phosphorylation from the partially oxidized food (pyruvic acid) of the host cell.

Biogenesis of Mitochondria

Regarding the origin of the mitochondria, several hypotheses have been postulated which are as follows:

1. “de novo” origin.

According to this hypothesis, the mitochondria are originated “de novo” (L. anew) from the simple building blocks such as amino acids and lipids. But there is no direct evidence in support of the “de novo” hypothesis for the origin of the mitochondria therefore, it is discarded now.

2. Origin from the endoplasmic reticulum or plasma membrane.

According to Morrison (1966), the new mitochondria might have been originated from the endoplasmic reticulum or plasma membrane. This hypothesis also could not provide direct evidence; therefore, it is not well accepted at present time.

3. Origin by division of pre-existing mitochondria.

The electron microscopic and radioautographic observations of the culture cells have shown clearly that the new mitochondria are originated by the growth and division of pre-existing mitochondria. On average, each mitochondrion must double in mass and then divide in half once in each cell generation.

Mitochondria are distributed between the daughter cells during mitosis and their number increase during interphase. Electron microscopic studies of Neurospora crass and HeLa cells have suggested that organelle division begins by an inward furrowing of the inner membrane, as occurs in cell division in many bacteria.

After elongating, one or more centrally located cristae from a partition by growing across the matrix and fusing with the opposite inner membrane. This separates the matrix into two compartments. The outer membrane then invaginates at the partition plane, constricting until there is membrane fusion between the two inner membrane walls. Thus, two separable daughter mitochondria are formed.

Mitochondria as semiautonomous organelles.

Recently the study of mitochondrial and chloroplast biogenesis became of great interest because it was demonstrated that these organelles contain DNA as well as ribosomes and are able to synthesize proteins.

The term semiautonomous organelles were applied to the two structures in the recognition of these findings. This term also indicated that the biogenesis was highly dependent on the nuclear genome and the biosynthetic activity of the ground cytoplasm.

It is well established now that the mitochondrial mass grows by the integrated activity of both genetic systems, which cooperate in time and space to synthesize the main components. The mitochondrial DNA codes for the mitochondrial, ribosomal, and transfer RNA and for a few proteins of the inner membrane.

Most of the proteins of the mitochondrion, however, result from the activity of the nuclear genes and are synthesized on ribosomes of the cytosol (cytoplasmic matrix). The cooperation of two genomes has been greatly clarified by studies on the molecular assembly of cytochrome oxidase.

This cytochrome, as studied in Saccharomyces cerevisiae is made up of seven polypeptide subunits for a combined molecular weight of 139,000 daltons. Three of the polypeptides are coded by mt DNA and assembled on mitochondrial ribosomes.

They are very hydrophobic and high in molecular weight (23,000 – 40,000 daltons). The remaining four subunits are coded by nuclear DNA and made on cytoplasmic ribosomes. These are hydrophilic polypeptides of lower molecular weight (4500–14,000 daltons).

Mitochondrial DNA.

Mitochondrial DNA (mt DNA) molecule is relatively small, simple, double-stranded and except for the DNA of some algae and protozoans, it is circular. The size of the mitochondrial genome is very much large in plants than in animals.

Thus, mt DNA varies in length from about 5 μm in most animal species to 30 μm or so in higher plants. The mt DNA is localized in the matrix and is probably attached to the inner membrane at the point where DNA replication starts. This replication is under nuclear control and the enzymes used (i.e., polymerases) are imported from the cytosol.

Mitochondrial ribosomes.

Mitochondria contain ribosomes (called mitoribosomes) and polyribosomes. In yeast and Neurospora, ribosomes have been ascribed to a 70S class similar to that of bacteria; in mammalian cells, however, mitoribosomes are smaller and have a total sedimentation coefficient of 55S, with subunits of 35S and 25S.

In mitochondria, ribosomes appear to be tightly associated with the inner membrane. Mitochondrial protein synthesis. As already described, mitochondria can synthesize about 12 different proteins, which are incorporated into the inner mitochondrial membrane.

These proteins are very hydrophobic (i.e., they are proteolipids). Thus, on the mitoribosomes are made the following proteins: three largest subunits of cytochrome oxidase, one protein subunit of the cytochrome b-c1 complex, four subunits of ATPase, and a few hydrophobic proteins.

One of the best-known differences between the two mechanisms of protein synthesis (i.e., in the cytosol and in the mitochondrial matrix) is in the effect of some inhibitors. The mitochondrial protein synthesis is inhibited by chloramphenicol, while synthesis in the cytosol (cytoplasmic matrix) is not affected by this drug.

In contrast, cycloheximide has the reverse effect. Import mechanism of mitochondrial proteins. Most mitochondrial proteins are coded by nuclear genes and are synthesized on free ribosomes in the cytosol (cytoplasmic matrix).

The import of these polypeptides involves similar mechanisms both in mitochondria, and chloroplasts. The transport processes involved have been most extensively studied in mitochondria, especially in yeasts.

A protein is translocated into the mitochondrial matrix space by passing through sites of adhesion between the outer and inner membrane, called contact sites. Translocation is driven by both ATP hydrolysis and the electrochemical gradient across the inner membrane, and the transported protein is unfolded as it crosses the mitochondrial membranes.

Only proteins that contain a specific signal peptide are translocated into mitochondria and chloroplasts. The signal peptide is usually located at the amino terminus and is cleaved off after import. Transport to the inner mitochondrial membrane can occur as a second step if a hydrophobic signal peptide is also present in the imported protein; this second signal peptide is unmarked when the first signal peptide is cleared.

In the case of chloroplasts, import from the stroma into the thylakoid likewise requires a second signal peptide. Mitochondrial lipid biosynthesis. The biogenesis of new mitochondria and chloroplasts requires lipids in addition to nucleic acids and proteins.

Chloroplasts tend to make the lipids they require. For example, in spinach leaves, all cellular fatty acid synthesis takes place in the chloroplast. The major glycolipids of the chloroplast are also synthesized locally.

Mitochondria, on the other hand, import most of their lipids. In animal cells, the phospholipids — phosphatidyl-choline and phosphatidyl-serine—are synthesized in the ER and then transferred to the outer membrane of mitochondria.

The transfer reactions are believed to be mediated by phospholipid exchange proteins; the imported lipids then move into the inner membrane, presumably at contact sites. Inside mitochondria, some of the imported phospholipids are decarboxylated and converted into cardiolipin (diphosphatidyl glycerol).

Cardiolipin is a “ double” phospholipid that contains four fatty – acid tails; it is found mainly in the inner mitochondrial membrane, where it constitutes about 20% of the total lipids.

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