Mitochondria are energy-producing organelles that are thought to have
once been a type of free-living alpha-proteobacterium.
LEARNING OBJECTIVE [ edit ]
Explain the relationship between endosymbiosis and mitochondria to the evolution of eukaryotes
KEY POINTS [ edit ]
Eukaryotic cells contain varying amounts of mitochondria, depending on the cells' energy needs.
Mitochondria have many features that suggest they were formerly independent organisms,
including their own DNA, cell-independent division, and physical characteristics similar toalphaproteobacteria.
Some mitochondrial genes transferred to the nuclear genomeover time, yet mitochondria
retained some genetic material for reasons not completely understood.
The hypothesized transfer of genes from mitochondria to the host cell's nucleus likely explains
why mitochondria are not able to survive outside the host cell.
TERMS [ edit ]
endosymbiosis
when one symbiotic species is taken inside the cytoplasm of another symbiotic species and both
become endosymbiotic
vacuole
a large, membrane-bound, fluid-filled compartment in a cell's cytoplasm
crista
cristae (singular crista) are the internal compartments formed by the inner membrane of a
mitochondrion
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Relationship between Endosymbiosis and Mitochondria
One of the major features
distinguishing prokaryotes from
eukaryotes is the presence of
mitochondria. Eukaryotic cells contain
anywhere from one to several thousand
mitochondria, depending on the cell's level
of energy consumption. Each
mitochondrion measures between 1 to 10
μm in length and exists in the cell as
an organelle that can be ovoid to worm-
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shaped to intricately branched. Mitochondria arise from the division of existing
mitochondria. They may fuse together. They move around inside the cell by interactions with
thecytoskeleton. However, mitochondria cannot survive outside the cell. As the amount of
oxygen increased in the atmosphere billions of years ago and as
successful aerobic prokaryotes evolved, evidence suggests that an ancestral cell with some
membrane compartmentalization engulfed a free-living aerobic prokaryote, specifically an
alpha-proteobacterium, thereby giving the host cell the ability to use oxygen to release energy
stored in nutrients. Alpha-proteobacteria are a large group of bacteria that includes species
symbiotic with plants, disease organisms that can infect humans via ticks, and many freeliving species that use light for energy. Several lines of evidence support the derivation of
mitochondria from this endosymbiotic event. Most mitochondria are shaped like alphaproteobacteria and are surrounded by two membranes, which would result when one
membrane-bound organism engulfs another into avacuole. The mitochondrial inner
membrane involves substantial infoldings called cristae that resemble the textured, outer
surface of alpha-proteobacteria . The matrix and inner membrane are rich
with enzymes necessary for aerobic respiration.
Micrograph of mammaliam mitochondria
In this transmission electron micrograph of mitochondria in a mammalian lung cell, the cristae,
infoldings of the mitochondrial inner membrane, can be seen in cross­section.
Mitochondria divide independently by a process that resemblesbinary fission in prokaryotes.
Specifically, mitochondria are not formed de novo by the eukaryotic cell; they reproduce
within the cell and are distributed between two cells when cells divide. Therefore, although
these organelles are highly integrated into the eukaryotic cell, they still reproduce as if they
are independent organisms within the cell. However, theirreproduction is synchronized with
the activity and division of the cell. Mitochondria have their own circular
DNAchromosome that is stabilized by attachments to the inner membrane and carries genes
similar to genes expressed by alpha-proteobacteria. Mitochondria also have
special ribosomesand transfer RNAs that resemble these components in prokaryotes. These
features all support that mitochondria were once free-living prokaryotes.
Mitochondrial Genes
Mitochondria that carry out aerobic respiration have their own genomes, with genes similar
to those in alpha-proteobacteria. However, many of the genes for respiratory proteins are
located in the nucleus. When these genes are compared to those of other organisms, they
appear to be of alpha-proteobacterial origin. Additionally, in some eukaryotic groups, such
genes are found in the mitochondria, whereas in other groups, they are found in the nucleus.
This has been interpreted as evidence that genes have been transferred from
the endosymbiont chromosome to the host genome. This loss of genes by the endosymbiont
is probably one explanation why mitochondria cannot live without a host.
Despite the transfer of genes between mitochondria and the nucleus, mitochondria retain
much of their own independent genetic material. One possible explanation for mitochondria
retaining control over some genes is that it may be difficult to transport hydrophobic proteins
across the mitochondrial membrane as well as ensure that they are shipped to the correct
location, which suggests that these proteins must be produced within the mitochondria.
Another possible explanation is that there are differences in codon usage between the nucleus
and mitochondria, making it difficult to be able to fully transfer the genes. A third possible
explanation is that mitochondria need to produce their own genetic material so as to ensure
metabolic control in eukaryotic cells, which indicates that mtDNA directly influences the
respiratory chain and the reduction/oxidation processes of the mitochondria.