Endosymbiotic Theory The Modern Or Organelle Biology Essay

Endosymbiosis ; a theory that proposes the development of the eucaryotic cell or yet is it another biological theory that is complete refuse? This is what this essay has been look intoing, whether the grounds gathered over the last 50 old ages is good plenty to back up the endosymbiotic theory and to alter the positions of 1000000s of life scientists who still do non hold with it. The theory was foremost postulated in 1905 by a Russian life scientist, Konstantin Mereschkowsky but was more popularised in the sixtiess by Lynn Margulis. Harmonizing to this hypothesis, approximately two billion old ages ago, a procaryotic being consumed a chemo-organotrophic bacterium giving rise to the eucaryotic cell and its cell organs such as the mitochondrian. Subsequently on this eucaryotic cell so consumed a cyanobacterium, giving rise to the chloroplast ( plastid ) through a secondary endosymbiosis event. Despite it being one of the most controversial and absurd theories when foremost hypothesised, the endosymbiotic theoretical account is now vastly respected by many life scientists and many facets have been confirmed stunningly by ongoing grounds including phyletic analysis, genome analysis and comparings of cell organs to procaryotic beings. With the theory ne’er traveling to be a 100 per centum proven the strong grounds gathered has made it one of the strongest rivals for the development of the eucaryotic cell.


The three major spheres of life including the archaea, bacteriums ( procaryotes ) and eukarya were foremost established by Carl Woese utilizing ribosomal RNA ( rRNA ) for phyletic analysis. The original divergency of line of descent gave rise to bacteriums and archaea about 3.5 billion old ages ago ( bya ) , but non eucaryotes, these diverged about one million millions of old ages subsequently. However, major alterations took topographic point in the Earth ‘s biosphere as effect of the variegation of micro-organisms from bacteriums and archaea, such as the coevals of an oxygenated ambiance. Bacteria took advantage of this O and developed the ability to respire O, with the oxidization of organic compounds they were able to obtain more energy taking to the development of larger cell populations including the eucaryotic cell. Eukaryotic beings contain several interconnected features that distinguish them between procaryotes. Table 1 below, shows the alone differences between procaryotic and eucaryotic beings, despite there being huge differences between the two beings, the endosymbiotic theory still states that there immense significance between the two[ 1 ].




Membrane Enclosed Organelles



Ribosomes mass






Cell Wall constituent

Cellulose ( in workss ) , Chitin ( Fungi )

Peptidoglycan, muramic acid

Histone proteins



Deoxyribonucleic acid Structure

Contained in the karyon and signifiers constructions called chromosomes.

Round and covalently closed. It is bare and lies free in the cytol.

Genes arranged in operons



Nucleus nowadays



Table 1: The differences of certain features of eucaryotic and procaryotic beings

This got scientists examining the beginning of eukarya, inquiring how this 3rd sphere of life which including a membrane enclosed nucleus and cell organs, emerged. Many hypotheses have been proposed on the beginning of the eucaryotic cell, the most supported is the endosymbiotic theory. Symbiosis is defined as an association between two or more different species. This definition clearly excludes associations between persons of the same species. A major event in which mutualism purportedly played a good function was in the development of the eucaryotic cell and its cell organs ( i.e. chloroplast, chondriosome ) through associations affecting different species of procaryotic cells. In this mode, the eucaryotes acquired the metabolic machinery of cellular respiration and photosynthesis from the endosymbionts, an endosymbiont being any being that lives within the organic structure or cell of another being[ 2 ].

Figure 1[ 3 ]– The different symbiotic associations – Organisms that are involved in this procedure can either profit, be harmed or non be affected by this association. In parasitism one of the species benefit, the grade of injury to the host is really terrible and it depends on the host for nutrient e.g. Fungi, viruses and worms. In commensalism, one of the species benefit while the other 1 remains unaffected, e.g. remora suckerfish attach to the shark to derive nutrient. In symbiosis both organisms benefit from the mutualism event.

The endosymbiotic theory claims that the eucaryotic cell is non a individual entity but instead a combination of different procaryote cells that evolved to populate together in a reciprocally good manner. The endosymbiosis theory is best explained in the beginnings of the cell organs of the chondriosome and the chloroplast. It was thought approximately two billion old ages ago, in which merely prokaryotes inhabited the Earth, consecutive mutualism occurred between different procaryotic species as figure 1 on the page below illustrates and explains.

Figure 2[ 4 ]– Endosymbiotic theory – Chloroplasts and chondriosomes may hold descended from a little blue-green algae and aerophilic bacteriums severally that were engulfed by another larger procaryote. A big procaryotic being engulfed a chemo-organotrophic being, but did non digest it and it became the eucaryotic cell with the common cell organ known as the chondriosome ( A ) . This eucaryotic cell, so likewise took up a species of blue-green algae by a similar engulfing event ( B ) . The blue-green algae evolved into a chloroplast, enabling the eucaryotic cell to go photosynthetic ( C ) . This procedure besides involves the extended loss of Deoxyribonucleic acid from the bacteriums symbiont every bit good as transportation of cistrons to the karyon of the eucaryotic host.

As stated above, the cardinal measure that the endosymbiotic theory purportedly played a function in was the development of the two key cell organs, the chloroplast and the chondriosome. Both the cell organs are really similar in footings in both construction and map, but most significantly both of these evolved in a really similar manner. The chondriosome ( figure 2 ) are the cardinal participants in aerophilic respiration, they facilitate the procedure of oxidative phosphorylation in which the flow of negatrons is energetically favorable plenty to bring forth a gradient of protons, which is eventually used to synthesize ATP – arguably one of the most of import molecules of all time to hold existed on Earth.

Figure 3[ 5 ]– Mitochondrion construction – Mitochondrions are surrounded by two membranes. The outer membrane is rather permeable and is composed of proteins and lipoids, the interior membrane is rich in proteins and is less permeable to ions and little organic molecules. The cristae are a series of folded internal membrane that has attached the pedunculate atoms for the inflow of protons and the enzyme ATP synthase that together catalyse the synthesis of ATP via oxidative phosphorylation. The matrix possesses the enzymes needed for the nexus reaction and Krebs rhythm, the reactions that proceed before oxidative phosphorylation.

The chloroplast is the site of photosynthesis in workss, one of the most of import cell organs in the works behind the karyon. In this two measure procedure, visible radiation captured from the Sun is used to excite negatrons in chlorophyll molecules and bring forth a proton gradient merely like of the mitochondrian to bring forth ATP ( Photophosphorylation ) , and so C dioxide is fixed to bring forth glucose and other sugars which are critical for the workss endurance ( Calvin Cycle ) .

Figure 4[ 6 ]– Chloroplast construction – Chloroplast are surrounded by a dual membrane, a permeable outer membrane and a less permeable membrane like that of the chondriosome. The stroma is a fluid filled matrix which houses the Calvin Cycle and besides contains storage constructions such as amylum grains. The grana are tonss of phonograph record like constructions called thylakoids which contain the chloroplast pigments. They are arranged in composites called photosystems which is really of import in capturing visible radiation and bring forthing ATP.

The endosymbiotic theory is strongly supported by findings from structural, molecular and biochemical surveies of the eucaryotic cell. Circumstantial grounds normally used to back up the theory of endosymbiosis is the similarity of the cell organs of the chondriosome and chloroplast compared to the bacterium. However more strict grounds to back up this theory comes from the scrutiny of evolutionary relationships of the chondriosome and chloroplast genomes via phyletic analysis.

Kwang Jeon ‘s surprise find

In 1966, a find was made by microbiologist Kwang Jeon that supported the hypothesis of endosymbiosis ; that bacteriums can germinate into cell organs. Jeon was analyzing a individual cell protest species Amoeba Proteus which is largely aquatic life in fresh H2O pools and moves by widening a finger of living substance called pseudopodium. But accidently one of his civilizations incorporating the being became contaminated and infected by a bacteria. Jeon was intrigued that non all of the septic ameba died directly off as some continued to turn, so he maintained those septic civilizations for farther analysis. About five old ages subsequently, the descendent ameba were host to many bacterial cells, yet they were still healthy and alive. When those amebas received antibiotics that normally do non harm ameba, they died. This unwilled experiment confirmed that those amebas had come to trust on the bacterium that had infected them originally. Through transfering experimentation, Jeon found out that the karyon of the ameba could non populate without the one time infective bacteriums. Surveies had showed the septic ameba had lost the ability to do an indispensable enzyme and depended on the bacterium to do them. The bacterial cells had become critical endosymbionts and Jeon ‘s inadvertent find proved that it was possible for an being to go dependent on and a functional portion of occupying beings[ 7 ]. “ Rather than extinguishing rivals, development eliminated competition itself on the footing of symbiotic relationships[ 8 ].

Symbiosis between present twenty-four hours photosynthetic beings and non-photosynthetic beings besides support the endosymbiotic hypothesis of the formation of the eucaryote. For case, Cyanophora paradoxa, a biflagellate protist, engulfed an endosymbiotic cyanobacterium ( known as a cyanelle ) , which one time taken up maps as a chloroplast and supplied the host cell with photosynthetically reduced C[ 9 ]. These finds fundamentally proved that the theory of endosymbiosis does really be, and it could hold led to the formation of the eucaryotic cells and its cell organs.

Familial Evidence

Supporting the symbiotic beginning of the chondriosome and chloroplast is both the cell organs possess their ain familial stuff, DNA which is clearly different from the atomic Deoxyribonucleic acid of eucaryotic cells. The architecture of most the chondriosome and all of the chloroplast DNA is evidentially procaryotic, being round and holding a individual beginning of reproduction, as figure 4 below clearly indicates. The cistrons of the chondriosomes are besides arranged in operons which are “ one or more cistrons transcribed into a individual messenger RNA and under the control of a individual regulative site[ 10 ]“ which besides reflects procaryotic lineage, with similar agreement of cistrons. Lactose consumption is controlled by an operon in procaryotic beings, with the binding of RNA polymerase blocked by a represser protein, but is non in eucaryotic beings. This organisation of cistrons is with blunt contrast compared to that of the eucaryotic karyon which has multiple, additive chromosomes and single cistrons, each with its ain regulative elements[ 11 ].

Figure 5[ 12 ]– Map of the human mitochondrial genome – The round genome contains over 15,000 base brace. The genome encodes the 16S and 12S rRNA, which correspond to the procaryotic 23S and 16S rRNAs.

Furthermore, grounds suggests over evolutionary clip, these cell organs lost many cistrons that were unneeded for life as an cell organ inside the new host cell, and many cistrons were transferred to the karyon ( Lake and Rivera 1996, Martin 1996 ) . Therefore, non-phototrophic eucaryotic cells are familial contain Deoxyribonucleic acid from two different beginnings, the first from the endosymbiont and the 2nd the host cell nucleus. Photosynthetic eucaryotic contain three beginnings of DNA, the mitochondrial and chloroplast endosymbionts and the karyon. In the mitochondrian of a fresh water protozoon Reclinomona, it bears the largest aggregation of mitochondrial cistrons coding for 97 cistrons and contains 69,034 base brace. However as table 2 ( page 6 ) illustrates, the human chondriosome genome is less than a one-fourth of this size intending the base brace losing were about surely cistrons coding procaryotic features from its bacterial ascendants that were non required. Harmonizing to Anderson and colleagues in 1998 many cistrons staying in Reclinomonas chondriosomes are strikingly similar to that of the obligate intracellular parasite, Rickettsia prowazekii the unicell causation enteric fever disease[ 13 ]. Therefore this provides supportive grounds that some chondriosomes in some present twenty-four hours eucaryotic beings still contain cistrons that caused them to go a procaryote, and therefore shows their bacterial yesteryear merely as the endosymbiotic theory proposes.

Figure 6[ 14 ]– Comparison of the genomes of Reclinomonas Americana and Rickettsia prowazekii – The S10, spc and I±-operons are organized likewise in these two genomes bespeaking that R. prowazekii may hold been one of the endosymbionts of the chondriosome in the development of the eucaryotic cell. The cistrons rplKAJL and rpoBC are besides identically organized in R. prowazekii and the mitochondrial genome of R. Americana.

Name and Symbol

Homosexual sapiens

Reclinomonas Americana

Ribosomal RNA

Small rRNA

Larger rRNA








Transportation RNAs



NADH dehydrogenase ( subunits ND1-ND4L )



ATP Synthase

( fractional monetary units 6, 8, others )

Entire protein coding cistrons



Entire cistrons



Size of DNA ( kbp )



Table 2[ 15 ]: A comparing of the mitochondrial cistrons of the Homo sapiens and Reclinomonas americana. As it clearly indicates above R. Americana contains many more cistrons than Homosexual sapiens in their chondriosome, showing that it did non reassign the cistrons it did non necessitate to the karyon like the human chondriosome, and it is these cistrons that have non been transferred that are really similar to R. prowazekii.

Most Deoxyribonucleic acid is stored in the karyon, protected from the karyon by the dual atomic envelope. The Deoxyribonucleic acid of chloroplasts and chondriosomes are protected from enzymes found in the cytosol by being located within the cell organs themselves. However, one distinct difference from the karyon is that the Deoxyribonucleic acid in chloroplast and chondriosome is histone free. Histone proteins do the additive DNA molecules that make up the eucaryotic chromosomes, to injure around these proteins to organize specialized constructions called nucleosomes. With no histones, the chondriosome is prone to free groups and O reactive species assailing the cell organs and doing harm and mutants, no histone proteins hence history for the high mutant rate in the chondriosome. Histone free DNA is a characteristic of procaryotic beings, bespeaking that there was a high opportunity that mitochondrial and chloroplast endosymbionts are so procaryotic.

Comparisons of both the order and sequences for fractional monetary units of ATP synthase from chloroplasts and Synechococcus ; a unicellular cyanobacterium, are consistent with the hypothesis that the chloroplast genome is portion of the cyanobacterium. ATP synthase is an enzyme that catalyses the production of ATP via ADP and inorganic phosphate in oxidative phosphorylation. The cistrons for the ATP synthase of Synechococcus are arranged in two bunchs, this agreement resembles those found in chloroplasts of eucaryotic cells, differing merely in the absence of the I? and s cistrons.

Binary Fission

Mitochondria and chloroplast reproduce identically to bacterial cells do ; by binary fission, this procedure is explained by figure 6 below. Chloroplasts besides contain hints of ftsZ, a protein which is required in the early phases of cell separation in binary fission. The chloroplast ftsZ is phylogenetically really near to ftsZ of Cyanobacteria, the false bacteriums endosymbiont of chloroplast. This grounds is farther enhanced by the fact that eucaryotic cells divide by mitosis, meaning these cell organs must hold come from a procaryotic beginning since they used the procaryotic manner to reproduce instead than the eucaryotic manner of mitosis.

Figure 7[ 16 ]– The sequence of cell division in Binary Fission – The chondriosome begins by retroflexing a transcript of its DNA to make two duplicate sets, the chondriosome so starts to stretch and increase in size ( 1 ) . The two replicated DNA ‘s migrate to the poles of the cell, cell division begins as the plasma membrane grows inwards ( 2 ) . The plasma membrane grows inwards wholly, and a complete cell wall is produced for both girl cells ( 3 and 4 ) .


Figure 8[ 17 ]– Similarities between ( a ) eucaryote chloroplasts and ( B ) blue-green algae noted by Schimpher. Dividing chloroplasts ( indicated by pointers on a ) are really similar to those of spliting blue-green algae ( indicated by pointers on B ) , both of them carry out the same reproduction procedure to reproduce ; binary fission.

Antibiotic Resistance

It is amazing to happen many singular similarities in the construction and chemical mechanisms both employed by the chondriosome and chloroplasts compared to procaryotes. One peculiar similarity is the response to specific antibiotics, which the chondriosome and chloroplasts portion with procaryotic, but do non portion with the karyon and the balance of the cell organs in the eucaryotic cell. Experimental trials on chondriosomes and chloroplasts reveal that these cell organs are susceptible to antibiotics such as Achromycin similar to that of procaryotes nevertheless eucaryotic cytol remains unaffected by these drugs. Tetracycline is a big household of antibiotics that inhibits about all gram positive and gram negative bacteriums and they work by suppressing protein synthesis by adhering to the little ribosomal fractional monetary units which prevent aminoacyl transportation RNA from come ining the acceptor sites on the ribosome, so the amino acids ca n’t be added to the turning peptide concatenation. The fact that chondriosome and chloroplasts are sensitive to tetracycline, indicates they contain their ain protein synthesizing machinery and it provides major grounds that these cell organs contained bacterial endosymbionts.

Ribosomal Structure

The chondriosome and chloroplast contain their ain 70S type ribosomes which are really similar to the type in procaryotic beings, and are unlike the 80S type ribosomes typical of the cytol of the eucaryotic cell. Ribosomes are organelles that catalyze the interlingual rendition of messenger RNA into an amino acid sequence, it contains two subunits the 30S fractional monetary unit which match the anticodon from the transfer RNA to the codon of the messenger RNA and a larger 50S fractional monetary unit that catalyses the formation of a peptide bond that links the amino acids together.

Another difference between the chondriosome and chloroplast compared to procaryotes is that interlingual rendition is initiated by the same codon. Translation is the ribosome and transfer RNA mediated procedure by which a protein is produced whose aminic acid sequence is derived from the codon sequence of an messenger RNA molecule. The instigator codon in interlingual rendition for both bacteriums and these two cell organs is N-formylmethionine, and non with methionine as in the eucaryotic cell. The fact that chondriosome and chloroplasts start the procedure of interlingual rendition with the same instigator codon as bacteriums, indicate they are likely to be endosymbionts.

Phylogenetic Analysis

Phylogenetic analyses based on ribosomal RNA ( rRNA ) sequences are one of the standard methods of deducing relationships among diverse beings. Although no individual molecule can specify an evolutionary line of descent one time cistron transportation between beings has occurred, but rRNA is the molecule of pick for deducing evolutionary relationships since its nowadays in both eucaryotic and procaryotic cells and is functionally conservative, germinating really easy. In phyletic analysis sequences of comparatively preserved cistrons from chondriosome and chloroplasts genomes of representative beings of the three spheres ( bacteriums, archaea and eukarya ) are aligned, and a phyletic tree is constructed. If the chondriosome and chloroplasts originated endosymbiotically from bacteriums, their cistrons should ramify from the sphere bacterium in the phyletic tree. More significantly, persuasive grounds in support of the endosymbiotic beginning of the plastid and chondriosome has besides been provided from similar analyses based on rRNA sequences.

The survey was based on the little 16S rRNA, which is a little size category of rRNA feature of both procaryotes and chondriosomes and chloroplasts of eucaryotic beings. “ Equally early as the late sixtiess Carl Woese began to assemble catalogs of oligonucleotide sequences released by the in vivo digestion of rRNA isolated from populating procaryotes and eucaryotes ”[ 18 ]. Using the informations derived from the experiment, Woese constructed a phylogenetic hypothesis for the beginnings of the eucaryotes and the phylogenetic relationship among the procaryotes. The rRNA informations received indicated that both the plastid and chondriosomes descended from really different procaryotic beings. More specifically, the 16S rRNA sequences of the plastid closely aligned with those from certain blue-green algaes. Whilst chondriosome portion homologous sequences with the purple bacterium ( proteobacteria ) and their relations, this is a large indicant that these bacteriums were the symbionts of these cardinal cell organs. As figure 3 on the page below clearly indicates blue-green algaes and plastids portion sequence homology in the protein Elongation Factor Tu ( the country in the ruddy box demoing this ) .

Figure 8[ 19 ]– Comparison of the amino acerb sequences of Elongation Factor – Tu in blue-green algae, plastids and proteobacteria – The box in ruddy on figure 8 above shows a protein signature sequence that is similar from both plastids and blue-green algaes bespeaking that blue-green algae could hold been a possible bacterium endosymbiont of the plastid. Many other proteins incorporating similar protein signatures between plastids and blue-green algaes include polymerase 1, sigma factor and ribosome S1 protein.

One significantly fascinating inquiry yet to be answered is whether chondriosome and plastids are monophyletic or polyphyletic? In other words, are the chloroplasts of all life eucaryotic beings descended from a individual engulfed cyanobacterium ( monophyletic ) or are they posterities of several independent engulfed blue-green algae ( polyphyletic ) . Most surveies based on partial 16SrRNA sequences from chloroplasts and blue-green algaes strongly supported a monophyletic beginning of plastids, these include, Giovannoni et Al. 1988 ; Douglas and Turner 1991 ; Maid and Zetsche 1991.

Barnabas and his colleagues performed a phyletic analysis that took both nucleotide sequences from 5S rRNA and amino acid sequences from Ferredoxin and c type cytochromes into history. Since ferredoxin is an Fe S incorporating protein which is used in a figure of oxidative – reductive tracts including oxidative phosphorylation, the duplicating event that occurred early in its development provided a baseline for the production of the phyletic tree. Organisms whose ferredoxins most resemble the sequences from the chloroplast were blue-green algaes, bespeaking the chloroplast had a procaryotic ascendant.

The cytochromes c analysis that Barnabas and his colleagues used showed a evolution in which the eucaryotic cytochromes sequences were similar to that of the cytochromes c2 sequences of violet photosynthetic bacteriums. Since cytochromes maps entirely in the chondriosome, it is really likely that the organelle derives from these bacteriums. The grounds of strong homologies between blue-green algaes and works chloroplasts for their cytochromes and ferredoxin sequences indicate that eucaryotic works cells must hold incorporated a chloroplast organelle that may at one clip have been a procaryotic cyanobacterium[ 20 ].


The karyon besides has grounds proposing it was formed by endosymbiosis, since the karyon was absent in procaryotes, and was foremost seen merely in eucaryotes. In 1994 Lake and Rivera suggested that the atomic membranes were formed from a captured procaryotic being that provided familial stuff. This was backed up consequently by Gupta and his colleagues in 1994, when they found huge similarities in among endoplasmic reticulate cistrons in the eukaryote Giardia and in cistrons of gm negative bacteriums.

Alternate Hypothesis

The endosymbiotic theory has non ever gained credence by some life scientists, and many other hypotheses exist in the biological universe on the development of the eucaryotic cell. The autogenic theory provinces that the cell organs evolved within the cell by progressive compartmentalisation and this led to the formation of other cell constructions such as vacuoles, endoplasmic rectilum and lysosomes. The endosymbiotic theory was criticised in 1998 by Martin and Muller saying that the demand for energy exchange in ATP drove the constitution of mitochondrial endosymbionts, this was known as the Hydrogen Hypothesis.


Many lines of grounds support the endosymbiotic theory in the formation of the eucaryotic cell in which cyanobacterium and chemoorganotrophic bacteria was incorporated into a procaryotic cell. Most of the stating grounds is based on molecular and familial surveies of procaryotic and eucaryotic beings. Many phyletic analyses that were based on 5S rRNA sequences showed aminic acid sequences similar in the chondriosome and plastid to many purple and cyano-bacteria severally. Mitochondria and plastids portion many similar features including antibiotic sensitiveness, cell division by binary fission, round DNA and 70S ribosomes. But despite the theory being absurd when foremost proposed, it has now gained the credence from many top life scientists including Doolittle and Brown in 1994 who stated “ the endosymbiont hypothesis for the beginning of the chondriosome and chloroplasts is as steadfastly established as any fact in biological science ”[ 21 ].