Origin and Evolution of the Related Organelles:

Mitochondrion, Chloroplast,and Peroxisome

Hakobu Nakamura

Biological Institute, Faculty of Science, Konan University, Kobe 658, JAPAN

(Received February 15, 1996; Accepted April 17, 1996)

1. Introduction

Mitochondrion and chloroplast are eukaryotic organelles that produce ATP and other types of energy to maintain the cellular life and reproduce the descendants. The life activities are the "works" including chemical, physical, and physiological phenomena. On the other hand, although peroxisome differs from the mitochondrion and chloroplast in many ways and contains oxidative enzymes at a higher concentration, it has been assumed to be a figure of the vestigial mitochondrion.

Therefore, these organelles contain specific metabolisms for the production of ATP and other types of energy and they have specific membranous morphologies for these purposes, which can be easily electron-microscopically discriminated.

When the first-living cell was generated in the primitive sea as the result of chemical evolution for billions of years, the atmosphere contained little molecular oxygen and the proto-cells had incorporated ATP as a product during the chemical evolution from the sea. However, they produced ATP themselves through a very primitive metabolism, called fermentation, that did not use oxygen as an oxidant of organic substances for the energy substrates. The metabolism in the present-day cells occurs in the cytosol, which is in a gelled state and the fermentation enzymes are three-dimensionally arranged and self-assembled within it, the particle being called glycosome. It is evolutionarily important to notice that such a primitive metabolism has not been contained in the membranous organelles. Then, the reactions for ATP production are referred to as "substrate-level phosphorylation"

Abundant prokaryotes must have filled the primitive sea by active reproduction under the soup of organic substances, which were produced and accumulated during the chemical evolution. This living world would finally exhaust the nutrients in the sea. At that time, new mutants that could produce organic substances through their own metabolism used sunlight as an energy source, CO2 as carbon source, and H2 or H2S as a hydrogen source, called photosynthesis. In such biological world, the photosynthetic, namely autotrophic organisms must have an absolute, selective advantage. Further, the autotrophic organisms reversely have since been supplying the nutrients for all of the heterotrophic ones. The extant photosynthetic bacteria (purple sulfur bacteria, green sulfur bacteria and purple non-sulfur bacteria) photosynthesize only under anaerobic conditions. This fact has been believed as an evidence to show that the bacterial photosynthesis originated under the anaerobic atmosphere of the primitive earth. However, more evolved photosynthetic prokaryotes, cyanobacteria, can use water ( H2O) as the hydrogen source to reduce C02 in the photosynthesis. As a result, the bacteria produced O2 as by-product of the water-splitting and it gradually changed into aerobic atmosphere, reaching to 21% at present. Such atmospheric change compelled cyanobacterium to adapt to the oxygen because superoxide (O-2) derived from it is very toxic to anaerobic organisms. The adaptation was a mutational acquirement of ability to synthesize an enzyme super-oxide dismutase which transforms O-2 into H2O via H2O2 . All of the aerobic organisms possess the dismutase for O-2 detoxification. On the other hand, they acquired genetic ability to use O2 as the final acceptor of electron-transport system in so-called respiratory metabolism. Therefore, the photo-synthetic and respiratory metabolisms that produce ATP for the cellular life, were generated during the evolution of the prokaryotic world which took 2.5~109 years. Eukaryotic metabolisms all inherited directly from the prokaryotic products. However, in the eukaryotic cell, these metabolisms are more compartmentalized than those in the prokaryotic ones; That is phylogenetic origins of the eukaryotic organelles. Herein, the processes of the generations of the ATP-productive reactions in the prokaryotic world and their compartmentalization during the formation of eukaryotic cell, are discussed.

2. Morphologies characteristic of the mitochondrion, chloroplast, and peroxisome

Both the mitochondrion and chloroplast are double membrane-bounded organelles that specialize in the synthesis of ATP, which use energy derived from the electron transport system and oxidative phosphorylation in the mitochondrion and from photosynthetic phosphorylation in the chloroplast. The organelles contain their own DNAs, ribosomes and machinery for protein synthesis. However, not only most of the proteins but also the membrane components, such as phosphatidyl choline and phosphatidyl serine needed for mitochondrial morphogenesis are synthesized in the endoplasmic reticulum under the genetic control of nucleus and are imported. On the other hand, most of the proteins for morphogenesis of chloroplast are imported from the cytoplasm whereas the chloroplast tends to make the lipids they require. In spinach leaves, for example, all cellular fatty acid synthesis takes place in the chloroplast, although desaturation of the fatty acids occurs elsewhere.

The proteins which are imported into the organelles and reached their particular subcomponents, play the main roles for the specific functions. The mitochondrion contains two subcompartments of the internal matrix space and the intermembrane space. The compartments are in two distinct membranes, namely the inner membrane which encloses the matrix space and the outer membrane which makes an intermembrane space between the inner membrane and it and is in contact with the cytosol. It is morphologically important that the inner membrane invaginates repeatedly into the matrix space without pinching from the membrane. The topology of the inner membrane refers to as cristae (Fig. 1 A). On the other hand, chloroplast has the same two

Fig. 1 Difference in the membranous compartments of the mitochondrion (A) and chloroplast in the higher plant (B). Morphogeneses of these organelles correlate with each other in a way that the invaginated inner membrane of the mitochondrion is pinched off to generate vesicles in the chloroplast. The thylakoid vesicles would evolve in this process.

subcompartments plus an additional subcompartments, called the thylakoid space, which is surrounded by the thylakoid membrane. The thylakoid membrane is originally derived from the invaginations of the inner membrane into the matrix space, stroma, of chloroplast, but the invaginated membranes are pinched off later and make a raft structure, called grana, in higher plant cells. Therefore, the grana is embedded in the stroma and isolated from the cytosol (Fig. 1 B). The subcompartments of mitochondrion and chloroplast contain a respective set of proteins for the production of ATP. The development of mitochondrion and chloroplast by the import of proteins from the cytosol is a major but complex feat, that is, the proteins are translocated across a number of membranes in succession and end up in the specific places. The complexity comes from the fact that the proteins located mostly in the inner membrane of mitochondrion and in the thylakoid membrane of chloroplast are encoded by the nuclear genes. Furthermore, the organelle DNA-encoded polypetides generally form protein complexes with the subunits encoded by nuclear DNA under certain genetic control for balanced composition. Furthermore, mitochondrion and chloroplast are made of membranes which are basically constructed by phospholipid bilayers. The phospholipids are differentially synthesized between the organelle and the cytosol, it depending on biological and cellular species. Peroxisome differs from mitochondrion and chloroplast in many points, morphogenetically. First, it is surrounded by only a single layered membrane and, second, it does not contain DNA and ribosomes. Therefore, it is apparent that peroxisome imports all of its own proteins by a similar and selective process from the cytosol. It has been observed that mitochondrion and chloroplast can self-replicate, although DNA polymerase and RNA polymerase are imported from the cytosol. Interestingly, some membrane-bounded organelles in the eukaryotic cell also can self-replicate without DNA, and peroxisome is a typical one as the endoplasmic reticulum. As stated above, peroxisome has a strong metabolic correlation with the mitochondrion. This problem will be discussed later.

3. Electron transport systems in photosynthetic bacteria, cyanobacteria

and chloroplast in eukaryotic plant.

Fig. 2 shows a comparison of the electron transport systems in photosynthetic bacteria, in cyanobacteria and plant chroloplast, and in the mitochondrion. Evolution is always conservative, taking facts of the old and building upon them to create something of the new. The electron transport system is a typical example. It has come from the oldest photosystem to synthesize ATP and NADPH in the photosynthetic bacteria and evolved to the chloroplast in the higher eukaryotic plants or to the mitochondrion in the higher eukaryotic organisms. The photosynthetic world has been established after the fermentative one, as pointed above. Therefore, the first photosynthetic cell was born by mutation in the fermentative world and predominated during the declining world due to the exhaust of organic nutrients in the sea. The @@@@@@@@@@@@@@@@@@@@@

Fig. 2 The electron transport chains are homologous among (i) cyclic photo-phosphorylation of the purple non-sulfur bacterium (A), (ii) non-cyclic photosystem of the purple non-sulfur bacterium (B), (iii) photosystems (I and II) of the cyanobacterium and eukaryotic plant (C) and respiratory chain of mitochondrion (D).

chl: photosynthetic pigment, chl*: the excited pigment, Q: ubiquinone, b: cytochrome b, c or f: cytochrome c. Cytochromes b and c (or f ) form bc (or f ) complexes in the cells and organelles, and the complexes pump H+ across each of the specific membranes.

Photo-ATP synthetic system, photophosphorylation, is quite different from the fermentative ATP synthetic system, substrate-level phosphorylation, and the former synthesizes ATP as the reaction is coupled with the electron transport-driven proton ( H+ ) pumping whereas the latter's ATP synthesis does not relate with the electron movement but with only a phosphorylation of ADP to ATP, which occurs at two steps of the latter half in the fermentative (Embden-Meyerhof, EM) metabolism, reaction 1,3-bisphosphoglycerate 3-phosphoglycerate and phosphoenolpyruvate pyruvate. The electron transport-derived H+ pump system of the photosynthetic bacteria consists of light-harvesting pigments, several electron carriers which make the H+ gradient across the membrane and ATP + Pi ATP by coupling with the H+ flow. The early fermentation processes must have provided not only ATP but also the reducing power, such as NADH or NADPH, which was required for essential biosynthesis. However, the photo-synthetic processes for the biosynthesis of ATP and NAD(P)H were independent of those of the fermentation ones. Therefore, the photosynthetic process would have not generated by modification of the fermentation processes but seems to have been generated from the beginning.

The most important, new element for the photosynthetic synthesis of ATP and NADPH was the sunlight harvester, bacteriochlorophyll. It has been believed that the pigment molecule was synthesized during chemical evolution processes, though rather complex in structure. The molecule is chemically stable and is contained even in petroleum from the deeply underground sources. Therefore, it had to have been used as the energy source since the proto-cells, being independent of the photosynthetic system stated above. The present-day purple bacteria contain a photochemical reaction center which is a large protein-bacteriochlorophyll complex and is arranged as a transmembrane protein in the plasma membrane and its derivatives. The special pairs of chlorophyll molecules in the reaction center irreversibly trap photons for excitation of electron. The excited electron is immediately passed to a chain of electron acceptors, quinones, that are tightly bound to the protein complex and then the electron is transferred to subsequent photochemical carriers. Such a photochemical center is thought to have evolved in the primitive photosynthetic bacteria much more than 3.5 billion years ago.

Cyanobacteria are thought to have evolved from purple non-sulfur bacteria about 3.5 billion years judging from their oldest fossils. Photosynthesis of cyanobacteria produces both ATP and NADPH by directly so-called non-cyclic photophosphorylation, through using H20, but H2 or H2S as in the purple bacteria, as the electron donor. Because the two processes, called photosystems I and II, are used in series to the electron carriers, the electron can be transferred all the way from H2O to NADPH, some of these energies are siphoned off for ATP synthesis. In the first photosystem, photosystem II, two moles of water are split with an enzyme by spending four photons. As a result, four electrons are removed and one molecule of oxygen is produced as in the following reaction.

2 H2O { 4 photons 4 H+ { 4 e- { O2

Although the electron is easier to be released from H2 or H2S than from H2O, the core of the reaction center of photosystem II has been conserved from cyanobacteria to the chloroplast of the eukaryotic plant. Photosystem II produces strong electron donors in the form of reduced quinone molecules in the cyanobacterial plasma membrane and in the thylakoid membrane of chloroplast. The quinones pass their electrons to a H+ pump, called cytochrome bf complex, which is a type of cytochrome bc indicated in Fig. 2. The bf complex pumps H+ out of the cytosol across the plasma membrane in the cyanobacterial cell or into the thylakoid space across thylakoid membrane of chloroplast. Therefore, the resulting electrochemical gradient drives the ATP synthesis by ATP synthtase, as discussed later. The final electron acceptor in this electron transport chain is the second photosystem, photosystem I.

Electron entered into photosystem I are boosted to very high energy level within the pigment-protein complex activated by the photons and are passed to NADP+ to generate NADPH via iron-sulfur center in ferredoxin. This last step also takes up H+ from the medium. However, the evolved photosystems can make ATP by cyclic photophosphorylation without making NADPH. In this process, the high-energy electrons from photosystem I are transferred back to the b6f complex to make energy for pumping H+ for ATP synthesis.

The photosynthetic metabolism is constructed by the photosystems to synthesize ATP and NADPH, and by the CO2 - fixative metabolism which is reduced pentosephosphate (or Calvin-Benson ) cycle. The former is located in the thylakoid membrane while the latter is done in the stroma.

4. Mitochondrion as electron transporter for ATP synthesis

The O2 | respiration is the third metabolism that was evolutionarily acquired in the prokaryotic world after O2 had been accumulated in the atmosphere as a result of the phylogenesis and the ecological development of cyanobacteria. The mitochondrion was the product of@compartmentalization of the O2 respiratory metabolism when occurred@during generation of eukaryotic cell from the prokaryotic world.@However, the respiration consists of three parts of metabolisms,@fermentative reactions of glucose to pyruvate (EM pathway),@tricarboxylic (citric) acid (TCA) cycle, and respiratory chain from NADH to O2. In the mitochondrion, the respiratory chain is in the cristae (membrane system) and the TCA cycle is divided into the membrane and matrix. The EM pathway is restricted in the@cytosol, and it seems to be the same anaerobic system as that in the@fermentative bacteria. Therefore, it is possible to say that the mitochondrion is an organelle for oxidation by molecular oxygen of@pyruvate, which is the final product of EM pathway, to CO2 + H2O.@The energy released there is harvested so efficiently that about 38 molecules of ATP are produced per molecule of glucose oxidized. @It is a striking contrast to the fact that the fermentation produces 2 molecules of ATP per molecule of glucose.

Mitochondria are usually depicted as bacterial form with a diameter of 0.5 to 1m. However, time-lapse microcinematography of living cells shows that the form always changes with time into very elongated cylinders and they fuse together. The whole figure appears like the roots of a tree. However, in some phases it separates again and the mitochondrial behaviors are dynamic along the microtubule's arrangement in the cytoplasm. The symbiotic theory that the mitochondrion is a transformant of the symbiotic bacterium in the cytoplasm, is incorrect and the mitochondrial morphogenesis is within the cellular life phenomenon. In fact, the mitochondria are provided directly to a site of unusually high consumption of ATP. A typical example is the filamentous mitochondria wraps tightly around the sperm tail.

A single mitochondrion is bounded by two highly differentiated membranes, inner and outer. The inner membrane creates two separate mitochondrial compartments ; the internal matrix space and a much narrower intermembrane space. The outer membrane is not so metabolically active but forms large aqueous channels through the lipid bilayer. Although the outer membrane is permeable to all small molecules including soluble proteins, the inner membrane contains many kinds of proteins for the metabolisms and transports and thus is impermeable to the substances, especially to ions, such as proton (H+). The matrix enzymes include those that metabolize CoA produced from pyruvate and fatty acids through the TCA cycle. The principal end products of the TCA cycle are CO2 and NADH, and the latter flows out to the respiratory chain that is essential to the process of so-called oxidative phosphorylation for generation of ATP. Most of the protein species of the chain are intrinsic components of the inner membrane. The most important one is the ATP synthase, also called Fo F1 ATPase, which constitutes about 15% of the total inner membrane protein. Both the photosynthetic bacterial membrane and the chloroplast have been evidenced also to contain very similar protein complex as the mitochondrial ATP synthase. As shown in Fig.3, the transmembrane portion of the protein complex works as a H+ carrier, and the large head portion, F 1 ATPase, synthesizes ATP when H+ passes through it down the

Fig. 3 F 1 ATP synthase (as lollipop head) and H+ carrier of transmembrane (as F o, H+ pump).

electrochemical gradient. These points have been demonstrated by using the liposome system, which was composed of the purified ATP synthase from mitochondria and the purified bacteriorhodopsin (highly driven H+ pump of an extreme halophilic bacteria which can photophosphorylate ) as sketched in Fig. 4. When the liposome system were exposed to light, the

Fig. 4 ATP synthase is driven by H+ flow. The experimental system is constructed of a purified bacteriorhodopsin (a), an ATP synthase purified from ox heart mitochondria (b), and a liposome of phospholipids. The bacteriorhodopsin is excited by light to pump H+ into the liposome, and the concentrated protons pass through the ATP synthase outside to drive the reaction ADP + Pi ATP.

H+ pumped into the liposomelumen by the bacteriorhodopsin flowed back out through the ATP synthase. Consequently, ATP is made in the medium outside, nevertheless the sources of bacteriorhodopsin and ATP synthase were different one. This means that the H+ pump and the ATP synthase act independently.

5. Proton pumping across the membrane

When isolated mitochondria were suspended with a suitable substrate and oxygen for oxidation, the H+ flow through ATP synthase occurs and results in acidification of the medium. This indicates that free energy released by electron transport in the respiratory chain causes H+ pumping across the inner membrane from the matrix and the H+ flows back through ATP synthase, resulting in reaction ADP + Pi ATP. This mechanism is common in principle to the plasma membrane and its derivatives of the

Fig. 5 Redox potential changes in the respiratory chain. The potentials drop in three large steps that consist of NADH dehydrogenase complex (a), bc1 complex (b), and cytochrome oxidase complex (c). Each the complex works to pump H+ across the inner membrane of the mitochondrion. (After Alberts et al., 1994)

photosynthetic bacteria and cyanobacteria, chloroplast as well as mitochondrion.

For example, the redox potentials in the mitochondrial electron transport system drops in three large steps from NADH to O2, as shown in Fig.5, and the electron passes across each the enzyme complex of NADH degydrogenase complex, cytochromes bc1 complex and cytochrome oxidase complex in series. The change in redox potential between any two electron carriers is directly proportional to the free energy released between them. Each complex is an energy-converter that harnesses the free energy released to make a H+ gradient across the inner membrane, as demenonstrated by an experiment outlined in Fig.4. Fig. 6 explains

Fig. 6 Redox potential changes in the photosystems of cyanobaterium and eukaryotic plant cell. The potentials rise by irradiation of sunlight but drop after the photosynthesis works that include the H+ pump leading to ATP synthesis in photosystem II and the NADPH formation in photosystem I.

a: water splitting enzyme, b: plastoquinone, c: bf complex, d: plastocyanin, e: ferredoxin, f: NADH reductase, chl: chlorophyll, chl *: excited chlorophyll. (After Alberts et al., 1994)

changes in redox potential during photosynthesis, showing a change of the potential along electron flow in photosystemU,in which the H+ pumping occurs in the cytochromes b6f complex to make an electrochemical gradient. In other words, H+ is pumped across the membrane three times in the respiratory flow of electron whereas once in the photosystem flow of it.

6. Evolution of the electron transport system.

The green sulfur bacteria or purple sulfur bacteria may be the most primitive photosynthetic bacteria because the formers are completely anaerobes and the latters microaerobes. The purple non-sulfur bacteria had to evolve beyond the anaerobic bacteria. The purple non-sulfur bacteria can generally respire O2 in the presence of a suitable organic nutrient, but their photosynthesis is performed only anaerobically. However, recently a variant that can aerobically photosynthesize has been found in nature. The purple non-sulfur bacteria have been demonstrated to have the electron transport system, ubiquinone cytochrome b cytochrome c, that is commonly used for both the photosynthesis and the respiration as shown in Fig.7. This is true in the cyanobacterial

Fig. 7 Common usage of the electron transport system between photosynthesis (A) and respiration (B) in the purple non-sulfur bacterium (Rhodopseudomonas). When the respiration is inhibited (for example, anaerobiosis), the cell can live by photosynthesis, and when the photosynthesis is inhibited (for example, darkness), the cell can live by respiration.

photosynthesis and respiration. However, as discussed above, the first producer of oxygen is considered to be cyanobacteria that have evolutionally acquired the water splitting reaction. Therefore, the purple non-sulfur bacteria would have secondarily adapted to oxygen as the photosynthetic product of the cyanobacteria. The cyanobacterial taxonomy is very complex and there seem to be many phylogenic metabolic variants. On the other hand, as the purple bacteria have been of interest also in photosynthetic evolution, the metabolism has been well studied. Therefore, the electron-transport principle in the transitional stage from the photosynthesis to the O2 - respiration would be that as illustrated in Fig.7, and it would be divided into the photosystem and the respiratory chain and compartmentalized to chloroplast and mitochondrion, respectively, by the intracellular membranes of a cyanobacterium during the eukaryotic differentiation about 1.5 billion years ago.

Here, we have to point out the following facts : The present-day chloroplast has a clearly differentiated granum (pl. grana) structure, thylakoid membrane aggregation, in the higher plant cells but not anything in lower plant cells. Therefore, the photosynthetic compartments have evolved even in the plant world. The photosynthetic membranes in the cells also differentiate in the prokaryotic world. In the purple and green sulfur bacteria, there are many types of intracellular membranes involved in the photosynthetic metabolism and derived from their plasma membranes. These intracellular photosynthetic membranes can be seen in the actively photosynthetic cells of the purple non-sulfur bacteria. Two or more names are cytologically given to these prokaryotic intracellular membranes; chlorobium vesicle, mesosome, and chromatophore. However, as their definitions are not clear, we here will use chromatophore, but some investigators think that both the photosynthetic and respiratory metabolisms are contained in the chromatophore. Furthermore, name mesosome is given to an organelle that is electron-microscopically found in the non-photosynthetic bacteria and has been considered to contain metabolisms of O2 - respiration, DNA replication, nucleoid separation in cell division, and others. The photosynthetic intracellular membranes in the cyanobacterial cells are generally referred to as thylakoid, and their non-photosynthetic membranes, which can be discriminated by a histochemical techniques, are separately called mesosome by some investigators. However, it is difficult to differentiate between them. Therefore, we need in maind that the organelles usually called chromatophores may contain not only the electron transport systems of photosynthesis but also that of respiration. This is because both the systems belong to membranous reactions.

Prokaryotic membranous organelles as mesosomes, chromatophores, and thylakoids are derivatives of the plasma membranes, as mentioned above. Then, some ones remain to be continuous with the plasma membranes in all the cell cycle but others are separated to isolate into the cytoplasm. On the other hand, mitochondrial cristae are always continuous with the inner membrane but do not isolate as membrane units. However, when the chloroplast develops from the proplastid that is small double-membraned organelle, the inner membrane occurs invagination in many sites and becomes like cristae finally to cut off from the inner membrane. In the lower plant, the chloroplast maturates by formation of a simple arrangement of flatted thylakoid vesicles from one to several. However, in the chloroplast of higher plants, the thylakoid vesicles increase in number and make so-called grana.

The invaginations of the plasma membrane, as seen in the prokaryotic cells, and of the inner membrane, as seen in the mitochondria, build up a quite different microcompartment for the H+ pumping and ATP synthesis from the thylakoid vesicles isolated from the inner membrane as seen in the chloroplast. That is, the invagination of a membrane generates an intermembrane space between the inner membrane and the outer (or plasma) membrane (Fig. 1A),and the vesicle

formation makes intermembrane space within the closed membrane (Fig. 1B). Such membranous differentiation gives different systems in the flow of H+ and the orientation of ATP synthase, as shown in Fig. 8. Therefore,

Fig. 8. Comparison the H+ flows and ATP syntheses in the membranes of mitochondrion (A) and chloroplast (B). The H+ motive force across the inner membrane of mitochondrion directs from the matrix to intermembrane space (see Fig. 1A), and the H+ motive force across the thylakoid membrane of the chloroplast directs from stroma to thylakoid space (see Fig. 1B). The increased H+ concentration in both spaces decreases the pH and the H+ flow passing through ATP synthase drives ATP synthesis.

a: H+ pump, b: ATP synthase. ATP synthesis occurs within the matrix in the mitochondrion and within the stroma in the chloroplast

we can conclude as follows. (1) A weak H+ gradient across the membrane was naturally formed by the formation of phospholipid bilayer in the proto-cells. (2) A protein evolved to function as transmembrane H+ pumps. Furthermore, when the fermentative metabolism was evolutionally acquired, some of them synthesized organic acid as an end product and excreted H+ outside of the cells. (3) Function of a protein as primitive ATP synthase to catalize a reaction ADP + Pi ATP by passage through H+ across the synthase complex, was developed in the membrane protein. (4) Proteins of the H+ pumping and the ATP synthesis coupled with the electron transfer system driven by the sunlight energy. This was an origin of photosynthetic cell. However, there are two kinds of photophorylative mechanisms in the present-day bacteria; bacteriorhodopsin system (as in Halobacterium halobium) and bacteriochlorophyll system (as in green or purple bacteria). (5)The photophosphorylation mechanism inherited from ancestors of the purple non-sulfur bacteria and cyanobacteria, and differentiated into two kinds of the organelles, mitochondrion and chloroplast, during the process of cellular evolution to the eukaryota, as formerly discussed.

Although the peroxisome differs from the mitochondrion and chloroplast in many ways in the present-day cells, it has been considered to be the devoluted remain of the mitochondrion. However, this idea is a wrong conclusion. Peroxisomes are found in all eukaryotic cells. They possess oxidative enzymes, such as catalase, peroxidase and urate oxidase, at high concentrations. Like the mitochondrion, the peroxisome is a major site of oxygen utilization. One hypothesis explains the presence of peroxisomes in the eukaryotic cells as that they have served to lower the intracellular concentration of oxygen and also to employ its chemical activity for useful oxidative reactions.

However, the most important function of the mitochondrion that produces ATP and reducing power can not be carried out in the peroxisome. Nevertheless many of the same reactions are common in both the organelles. Peroxisomes contain one or more kinds of peroxidases that use molecular oxygen to remove hydrogen atoms from specific organic substrates, producing hydrogen peroxide. Catalase destroys the hydrogen peroxide generated by peroxidases and consequently produces water. These reactions are useful for the detoxication of some organic substances, such as phenols, formic acid, formaldehyde and alcohol.

A major function of the oxidative reactions carried out in the peroxisomes is so-called -oxidation of fatty acids to produce acetyl CoA. Acetyl CoA produced in the peroxisomes is exported to the cytosol for reuse in biosynthetic reactions and, furthermore, to the mitochondria for production of energy through the TCA cycle and respiratory chain. Interestingly, the -oxidation in mammalian cells occurs both in the mitochondria and peroxisomes, whereas in yeast and plant cells, this essential reaction is restricted to the peroxisomes.

Peroxisomes have quite diverse functions even in the different cells of a single organism, that is, they contain very different sets of enzymes. The composition of the reactions is very adaptive to the conditions. For example, yeast cells grown on sugar have small peroxisomes, whereas the peroxisomes of cells grown become large and oxidize methanol. Furthermore, when yeast cells are grown on fatty acids, their large peroxisomes can break down fatty acids through -oxidation. The peroxisomes plays further complex roles in plants. For example, the peroxisomes present in leaves associate with chloroplasts and materials are exchanged between the organelles during photorespiration. It has been reported also that in a fat-storing cotyledon cells of tomato seeds 4 days after germination, the peroxisomes (also called glyoxysomes in plant physiology) are associated with the lipid body and gluconeogenesis, conversion from fatty acids to carbohydrates, occurs during the germination (papers of Gruber, P. J. and Newcomb, E. H., Frederick, S. E. and Newcomb, E.H., cited in Molecular Biology of the Cell, Third Edition, by Alberts, B. et al., 1994). These findings suggest strongly that the morphogenesis and the genetic transformation involved occur in response to species-specific, life-cycle specific, and environment-adaptive differentiation. Such a conclusion will further be strengthened by the dynamic chloroplast behavior.

Chloroplast is the most prominent member of the plastid family. All plastids develop from proplastids, which are small organelles seen in the immature cells of plant meristem. In plants grown in darkness, the proplastids in the cells enlarge and develop into ethioplasts, which have semicrystalline membranes containing a chlorophyll precursor instead of chlorophyll. When exposed to light, the ethioplasts rapidly develop in a matured chloroplasts pigments, new membranes photosynthesizing enzymes, and components for electron-transport, into a matured chloroplast. On the other hand, leucoplasts are plastids that occur in many epidermal and internal tissues that do not become green on exposure to light. A common form of the leucoplast is amyloplast which the accumulates starch in storage tissue. We have to emphasize an important realization that the plastids are not either just sites for photosynthesis or for the deposition of storage materials, or the sites for productions of more than the energy and reducing power (as ATP and NADPH) that is used for the plant-biosynthetic reactions, of purine and pyrimidine, of most amino acids, of all the fatty acids and so on. However, it must be pointed out that the enzymes to produce these compounds are mainly synthesized under nuclear control. On the other hands, in the animal cells, these compounds are produced in the cytosol. Here, we conclude that the arrangements of the genomes in the cells are dynamic according to biological evolutions and environmental conditions.

7. Conclusion

The organelles closely interact to function and to support cell life as a whole. It is natural for the metabolisms compartmentalized into each organelle to be connected. In this sense, the lipid bilayers in the membrane compartments become barriers against communication among the organelles.

The purpose of the present study is to search the differential origin and the evolutional processes of the mitochondrion, chloroplast, and peroxisome through their metabolic facts that many of the same reactions are distributed among these organelles. One example is shown between mitochondrion and chloroplast. There is a transmembrane electrochemical proton gradient that works in the electron transport systems in both mitochondrion and chloroplast; ubiquinone cytochrome b cytochrome c. In the mitochondrion, the cytochromes bc complex, and in the chloroplast cytochromes bf complex contain ATP synthase (Fo F1 ATPase), in which Fo, namely transmembrane H+ pump, makes the proton concentration gradient across the membrane and F1 ATPase synthesizes ATP by reaction ADP + Pi ATP, when the proton passes through it. These phosphorylatory enzymes must have inherited from a prokaryotic ones as common ancestor in the photosynthetic green and purple sulfur bacteria that are evolutionarily generated much more than 3,5 billion years ago. Thereafter, the system developed to that in the photosynthetic purple non-sulfur bacteria and then the cyanobacteria, and was inherited into mitochondrion and chloroplast when the cell morphogenized as eukaryota about 1,5 billion years ago. Fig.9 shows a hypothetical process of the phosphorylation system in the biological evolution.

Fig. 9 A phylogeny of the electron transport system accompanied by the cellular evolution.

The mitochondrion and peroxisome contain many of enzymes in common. One hypothesis says that the peroxisome is a devolutional remain of the mitochondron. However, the synthetic conclusion on the basis of the many lines of evidences demonstrated suggests strongly that the metabolic compartments between the mitochondrion and peroxisome result from genetic arrangement in the genome in the process of the organelle's differentiation during cell division. In conclusion, genetic and then metabolic arrangements are dynamic during the processes of cellular evolution and of cell division in the extant meristematic tissues. Intracellular morphogenesis is not genetically and morphogenically fixed as previously considered by the symbiotic theorists.


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