21.3: Mitochondria

Figure \(\PageIndex{1}\): Structure of the mitochondrion. (Illustration by Mariana Ruiz Villarreal, via Wikimedia Commons.)

Mitochondria are organelles found in many eukaryotes, including animals, plants, fungi, and many varieties of protist. They are bounded by two membranes, the outer one smooth and pill-shaped, the inner one intensely folded into fin-like creases. This gives the inner membrane a very large surface area, crumpled into a small volume of space. On average, they are about 1 \(\mu\)m long. The number of mitochondria per cell can vary between zero (in some anaerobic protists) and upward of 2000. Functionally, mitochondria are critical because they produce the molecule adenosine triphosphate (ATP), a vital molecule used as a conveyance of chemical energy in the cell. To do this, they catalyze the reaction of free oxygen (\(\ce{O2}\)) with pyruvate (a product of the digestion of glucose), and generate carbon dioxide (\(\ce{CO2}\)) and water (\(\ce{H2O}\)) as waste products in addition to the ATP. In most mitochondria in most eukaryotes, about 36 units of ATP are produced as outputs per unit of glucose ingested. In other words, mitochondria are where respiration happens: where energy is derived from the food we eat, “burning” it chemically by oxidizing it, and “charging up the batteries” of our bodies.

Mitochondria are probably descended from a common ancestor shared with modern free-living bacteria, specifically the group called alphaproteobacteria (sometimes written as α-proteobacteria). We think this for several reasons. First, mitochondria only come from other mitochondria. Like bacteria, they reproduce through binary fission. If they are removed from a cell, the cell cannot replace them on its own. Within the inner membrane, the mitochondrion contains DNA, RNA, and ribosomes: all the apparatus for storing genetic information and translating it into proteins. Second, comparison of the ribosomal RNA in mitochondria and other extant groups shows the mitochondria to be most similar to the alphaproteobacteria. The extant alphaproteobacterial genus Rickettsia, a tickborne pathogen, comes closest in relatedness to the proto-mitochondrion. Interestingly, the tetracycline antibiotic medications used to treat tickborne illnesses such as Rocky Mountain spotted fever (such as Doxycycline) can also negatively impact the functioning of mitochondria in animal cell test cultures.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Mitochondria are valuable organelles in eukaryotes because _____________.

a. they help fight off infections during times of cellular stress

b. they are the site of photosynthesis, the chemical process that builds glucose from smaller molecules using the Sun's energy

c. they make the energy-rich molecule ATP and share it with the rest of the surrounding cell

Answer

c. they make the energy-rich molecule ATP and share it with the rest of the surrounding cell

Exercise \(\PageIndex{2}\)

Which group of modern bacteria are most closely related to the proto-mitochondrion?

a. Alphaproteobacteria like Rickettsia.

b. Cyanobacteria like Cylindrospermum.

c. Betaproteobacteria like Neisseria.

Answer

a. Alphaproteobacteria like Rickettsia.

Recent analogues

The modern amoeba species Pelomyxa hosts endosymbiotic bacteria that serve the same function as mitochondria do in other eukaryotes. Interestingly, ribosomal RNA analysis of its endosymbionts reveals not one species but three! Moreover, two of them are bacterial, while one is a methanogenic archeon (Gutiérrez et al., 2017). This set of relationships is probably relatively recent, for it is not sufficiently entrenched as to be obligatory. The microbial symbionts can live freely on their own, and Pelomyxa has not yet evolved a system of passing them to its descendants. So the amoeba must capture fresh bacterial endosymbionts anew with each generation.

Strigomonas is another example. This trypanosomatid is related to the parasite that causes sleeping sickness in humans. It relies on an endosymbiotic betaproteobacterium, and cannot live without it, though the bacteria is capable of life outside Strigomonas. This is an interesting case where one relies on the other, but the feeling is not mutual!

Mitochondrial DNA

Figure \(\PageIndex{2}\): The context of the hoop-shaped DNA hosted by mitochondria. (Public domain; Illustration by National Human Genome Research Institute – National Institutes of Health.)

The DNA of mitochondria is not stored in chromosomes, the way that nuclear DNA is. Instead, it is found in circular “hoops” of DNA within the mitochondrion, similar to the shape in which DNA is organized in bacteria. In animal mitochondria, the length of a one of these hoops ranges from 11–28 thousand base pairs. (For comparison, the number of base pairs in the human genome is ~3.2 billion.)

In animals, mitochondrial DNA is passed from mother to offspring. The father contributes half of the nuclear genetic material to his children, but the mother supplies the other half of the nuclear DNA plus all of the mitochondrial DNA. You got your mitochondria from your mom, who got them from her mom (your maternal grandmother), who got them from her mom (your mom’s maternal great-grandmother). Mitochondrial DNA is thus useful for tracing a line of matrilineal descent.

Studies of human genetics has shown a mitochondrial last common ancestor to all mitochondria in living humans was in a woman who lived in Africa between 120,000 and 156,000 years ago. This individual has been dubbed “mitochondrial Eve.” [LINK TO HUMAN EVOLUTION CASE STUDY?]