Site author Richard Steane
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Endosymbiont theory

Symbiosis is the interaction between two different organisms in close physical association, from which both organisms gain an advantage.
The endosymbiont theory covers the idea that certain organelles within cells originate from organisms taken in and adapted over a long period of time.
Practically all eukaryotic cells (from animals, higher plants, protoctists and fungi) have mitochondria. These are the site of aerobic respiration, which uses oxygen to release chemical potential energy from organic materials for use in the cell.

Higher plants and algae have chloroplasts in addition to mitochondria in (some of) their cells. These are the site of photosynthesis, which uses external light energy to synthesise organic molecules for use in the plant's cells, including non-photosynthetic cells.

Only prokaryotes (bacteria, cyanobacteria and archaea) do not have mitochondria or chloroplasts.

The endosymbiont theory suggests that these organelles (mitochondria and chloroplasts) represent independent prokaryotic organisms taken into the ancestor of the eukaryotic cell, and then incorporated into it. It is possible that they were taken in during a feeding process by endocytosis/phagocytosis, and incorporated into a membrane-bound vesicle, but not digested. Alternatively, the organisms may have invaded the eukaryotic cell, behaving like intracellular parasites, and were confined within a cellular compartment. And there is the possibility that an invading organism could escape from this into the cytoplasm.

This must have happened (twice - once for mitochondria and once for chloroplasts), a long time ago, before the development of many-celled organisms. It implies that the transformed cells were more efficient in energy terms, and this gave them a definite evolutionary advantage. It is probable that the environment at that time was very different than it is today.

Conditions were originally quite challenging: anoxic (reducing) conditions favoured anaerobic organisms gaining energy from chemical transformations in the environment.

Free living Cyanobacteria were the first organisms to perform (oxygenic) photosynthesis, starting to produce oxygen 2.7 - 2.8 billon years ago. It was originally absorbed by dissolving in the sea and reacting with iron compounds, and then started to accumulate in the atmosphere about 2.45 billion years ago. This would give an advantage to aerobic organisms and selection pressure for more efficient organisms containing mitochondria.

Later on, single-celled animal life would occupy the same aquatic environment as single-celled plants, and the plant cells would be consumed by animals .

It is thought that mitochondria are descended from a bacterial cell, and that chloroplasts are descended from a cyanobacterial cell, which continued to exist inside their host cells, carrying out their own chemical conversions and dividing within the host cell cytoplasm. Over time there has been much change, so there are a number of differences in shape and function between species. Some genetic material has been transferred to the host cell's nucleus, increasing the dependence of these organelles on the host cell's activities.

It is probable that the adoption of external organisms as internal organelles only occurred once (a "primary endosymbiotic event"), as several biochemical features of mitochondria are shared by almost all eukaryotic organisms, and similarly there are features of chloroplasts common to a wide range of plant groups, but they show gradual changes in the course of evolution.

It is difficult to trace the first organisms to arise from the new combination of host plus endosymbionts, which presumably soon evolved into many-celled organisms. And some organisms which seem to be ideal candidates for the first host appear to have lost their endosymbiont partner as a secondary event, as their DNA shows.
Glaucocystis spp Glaucocystis (23K)

Glaucophytes are a group of freshwater single-celled algae with chloroplasts (known as 'cyanelles' or 'cyanoplasts') that have a peptidoglycan layer, like cyanobacteria, but other plant groups appear to have lost this feature. As such, glaucophytes seem to represent an early phase in the colonisation/adaptation process.

Evidence for endosymbiosis

This is expanded further in the other column.


Eukaryotic cells are larger than prokaryotic cells. Mitochondria and chloroplasts are about the same size as prokaryotic cells.


The double membranes on the outside of these organelles have different chemical characteristics. In mitochondria, the outer one resembles the plasma membrane of the host cell, but the inner one is different. In chloroplasts the outer and inner membranes have a similar constitution to the thylakoids, not the cell plasma membrane.


These organelles have their own DNA. This is not arranged into chromosomes, and shows chemical differences from the main nuclear DNA of the host cell.


Eukaryotic cells have larger ribosomes than prokaryotes. Mitochondria and chloroplasts have smaller mitochondria.



A mitochondrion (TEM) mitochondrionem (50K)
Mitochondria range from 0.75 to 3 µm in diameter.
Prokaryotic cells range from 0.5 to 3 µm in size.
Eukaryotic cells range from 10 to 100 µm in size.


The outer mitochondrial membrane has a protein-to-phospholipid ratio similar to that of the eukaryotic plasma membrane (about 1:1 by weight), and proteins called porins give it permeability.

The inner membrane is rich in an unusual phospholipid, cardiolipin, which contains four fatty acids rather than two. To see the 3-D interactive molecular structure on this site, follow the link below. This compound (so called because it was first found in heart muscle, a rich source of mitochondria) is widely found in mitochondria in animal and plant cells. It stabilises the membrane structure and may be responsible for its infolding into cristae, also anchoring cytochromes of the respiratory electron transport chain into a functional unit, and acting as a reservoir for H+ ions which build up in the intermembrane space.
Cardiolipin is also found in plasma membranes of certain bacteria - including Rickettsia prowazekii
See below.


Mitochondria contain (one or possibly several copies of) a single, circular DNA molecule, not associated with protein like DNA in the nucleus.

This contains 37 genes, including genes for redox proteins of the respiratory chain.
It also codes for some RNAs of (mitochondrial) ribosomes, and the transfer RNAs necessary for the translation of messenger RNAs into protein. Mitochondrial transfer RNA genes have different sequences from normal nuclear equivalents and there are minor differences in the translation of codons into amino acids.

Mitochondrial DNA generally lacks introns, although there are exceptions such as yeast.

It is thought that differences in the DNA polymerase systems operating in cells may result in selection pressure for different nucleotide pairings, possibly based on the different numbers of hydrogen bonds (2 for A-T, 3 for G-C). This can be quite marked in non-coding sections of DNA. Studies of mitochondrial DNA show these effects of GC pressure.

The circular DNA structure (not associated with protein) is also found in many prokaryotes. Prokaryotic DNA includes many more than the 37 genes in mitochondria, but it is thought that some have been transferred into the eukaryotic cell's nucleus.


Mitochondrial ribosomes are a distinct size - 70S (20 nm in diameter) - the same as prokaryotic cells.
Ribosomes in other parts of the eukaryotic cell, e.g. rough endoplasmic reticulum, are larger: 80S (25-35 nm in diameter)

Possible candidates

It is thought that this organism - the proto-mitochondrion - was an aerobic alpha-proteobacterium breaking down lipids, glycerol and other compounds provided by the host.
It is likely that the host had a metabolism based on glycolysis. The rise in atmospheric oxygen produced as a result of photosynthesis by Cyanobacteria resulted in environmental pockets in which obligately anaerobic organisms could not live. The proto-mitochondrion functioned as an oxygen scavenger which gave an ecological and energetic advantage to the combination.

Alphaproteobacteria are a diverse class of bacteria in the phylum Proteobacteria, including a number of species of biological significance, often with symbiotic/parasitic lifestyles.
Click to see/ hide some examples:
Rickettsia typhi (TEM) rtyphi (254K)
Photo credit: David H. Walker (UTMB), George Weinstock (BCM-HGSC)

One possible candidate is in the genus Rickettsia - possibly closely related to Rickettsia prowazekii or Rickettsia typhi.
Rickettsia spp are obligate intracellular parasites. They cause a number of diseases affecting humans, animals and plants.
Rickettsia typhi is a small (0.3 by 1 µm), gram-negative, obligately intracellular parasitic aerobic bacterium.
Rickettsia prowazekii is the causative organism of typhus, spread by the human body louse.

Pelagibacter ubique (SEM) Pelagibacter_ubique (13K)
Photo credit: Kehau Manoi; C-MORE, Hawai'i.)
An alternative candidate may be in the order Pelagibacterales - formerly known as the SAR11 clade. These are extremely common in the ocean. The main species Pelagibacter ubique - only isolated and named in 2002 - is a very small organism: a gram negative curved rod, 0.37-0.89 µm in length and only 0.12-0.20 µm in diameter, with the smallest genome of any free living organism. It has a number of biochemical abilities, requiring organic carbon and nitrogen which it scavenges from the environment. It performs the glyoxylate bypass version of the TCA cycle, and possesses a gluconeogenesis pathway but does not do not carry out glycolysis. Its abundance is phenomenal: in summer multiplying to form about half of the cells in temperate ocean surface waters, thus playing a major role in the Earth's carbon cycle.



A chloroplast (TEM) chloroplast-micrograph (6K)
Length of a chloroplast: 5 µm
Length of a cyanobacterial cell: Synechococcus - a unicellular cyanobacterium that is very widespread in the marine environment - size varies from 0.8 to 1.5 µm.
Eukaryotic plant cells: 10 - 100 µm (includes vacuole)


Plant cell membranes are similar to other eukaryotic cell membranes, i.e. mostly phospholipid and protein.

However the chloroplast outer membrane, inner membrane and thylakoid membrane (in spinach chloroplasts) differ from this but have fairly similar lipid constituents to one another:

outer membrane 48% phospholipids, 46% galactolipids and 6% sulfolipids
inner membrane 16% phospholipids, 79% galactolipids and 5% sulfolipids
thylakoid membrane 15.5% phospholipids, 78% galactolipids, and 6.5% sulfolipids.

Galactolipids are not common components of plant cell membranes but they seem to be required for photosynthetic light reactions and act as constituents of the photosystems I and II.

Cyanobacteria also have thylakoid membranes that are rich in galactolipids.

Interestingly, cyanobacteria and plants use different pathways for the synthesis of galactolipids.

Cyanobacteria are classified as gram-negative bacteria, in which the cell envelope is composed of outer and inner (plasma or cytoplasmic) membranes with a peptidoglycan layer in between. This feature is retained in the chloroplasts of glaucophytes.


Chloroplasts contain a single copy of a single, circular DNA molecule, not associated with protein like DNA in the nucleus.

Chloroplast DNA includes genes for thylakoid proteins and the large Rubisco subunit - other genes being transferred to the nucleus. Like mitochondria, chloroplast DNA codes for some (chloroplast) RNAs and proteins of ribosomes, and the transfer RNAs and RNA polymerases necessary for the translation of messenger RNAs into protein. This also contains genes for electron-recycling proteins.

Like mitochondrial DNA, there appears to be a selection pressure in favour of G-C nucleotides, possibly a consequence of the way circular DNA replicates, leaving single-stranded sections liable to amination, causing a shift from A to G.


Chloroplast ribosomes are similar in size to mitochondrial and bacterial ribosomes (70S) but in the single-cell green alga Chlamydomonas reinhardtii there are several distinctive 'chloroplast-unique' ribosomal proteins which contribute to the smaller subunit, making it slightly larger.

Possible candidates

This is likely to be a free-living cyanobacterium with the capacity to store starch through oxygenic CO2 fixation, i.e. normal photosynthesis, and to fix atmospheric N2 : Order Chroococcales.

Microcystis aeruginosa (light mic) microcystis_aeruginosa (30K)

The principal organism in this order today is Microcystis aeruginosa.

This organism is well known for forming harmful algal blooms in eutrophic fresh water.

Synechococcus aeruginosus and Prochlorococcus spp, major components of marine phytoplankton, have also been compared with chloroplasts, in relation to their galactolipid metabolism.
Prochlorococcus spp Prochlorococcus (274K)
Synechococcus aeruginosus Synechococcus (27K)
[syn Cyanothece aeruginosus]
The main photosynthetic pigment in Synechococcus is chlorophyll a. Freshwater species have also been described.
Prochlorococcus spp is probably the most abundant photosynthetic organism on Earth, and its cells display a wide range of genomic diversity. It has evolved a unique light-harvesting complex, consisting of divinyl derivatives of both chlorophyll a and b. There are low light and high light groups, differing in the ratio of these pigments.

Other related topics on this site

(also accessible from the drop-down menu above)

Eukaryotic cells
Prokaryotic cells.html
Virus particles.html

Interactive 3-D molecular graphic models on this site

(also accessible from the drop-down menu above)
Phospholipid molecule

Web references

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