Yes, Mitochondria are the powerhouse of the cell. Its function is well known and its existence is crucial to us and anything else that has mitochondria. Mitochondria provide energy to cells via ATP (adenosine triphosphate), which is used to perform a variety of functions. Mitochondria also have its own set of DNA that is inherited from the maternal mitochondria. The size of the DNA is considerably smaller than your genomic DNA, which we will explore the reason of later. Whatever your sex or gender, you inherited your mitochondrial DNA from your biological mom. Our understanding of mitochondria started in 1856 when Albert von Kölliker, a Swiss anatomist and physiologist, discovered the mitochondria while studying muscle cells and called them sacrosomes . Then in 1898, Carl Benda, a microbiologist studying spermatogenesis, saw the sacrosomes and began calling them mitochondria .
From then on, many functions and interactions of the mitochondria were explored and understood. For a full list of developments from the 1890s to 1970s, please take a look at table 1 in the article by Lars Ernster and Gottfried Schatz. Some notable developments included the finding of enzyme interaction between the citric acid cycle and mitochondria by Alfred Kerbs in 1953, the purification of a functional and intact mitochondria by George Palade in 1974 (which really spurred the research of mitochondria), and identifying the role of ATP in mitochondria by Peter Mitchell (in 1978) and then Paul Boyer and John Walker in 1997 . Further developments continued as our ability to look into the mitochondria grew, but questions about its evolutionary origin still persist today. We understood its role in our cells, how it worked, and how it contributed to human diseases, which I will cover in the next article, but its origin remains in question. Here, we will take a look at the current understanding of mitochondria’s origin.
A few notes before we address the origins. There are three domains of life: bacteria, archaea, and eukarya (which includes humans). Archaea were once part of bacteria but enough difference was shown to have them be their own domain. Bacteria and archaea are referred together as prokaryotes and eukarya are eukaryotes.
There are currently two competing theories on the origin of the mitochondria. Both of those theories agree that the mitochondria started as a separate organism that formed a symbiotic relationship with a host and that these two species eventually merged into a single species. The first theory suggests that a prokaryote organism, which was the original mitochondria, formed a symbiotic relationship with an anaerobic (doesn’t use oxygen) and fully fledged eukaryotic organism when the eukaryotic organism ate the mitochondria organism . The theory surmises that they formed a symbiotic relationship because the mitochondria (using oxygen) would remove oxygen from the host (which does not like oxygen). This theory makes two strong assumptions. The first is that eukaryotic organisms were already existing without mitochondria . The issue with this is there are currently no known eukaryotic organisms existing without a mitochondria or a highly modified one (hydrogenosomes) . The other assumption is that mitochondria was aerobic (uses oxygen), which ignores the existence of anaerobic mitochondria . Without explaining the issues of its assumptions, this theory becomes more a guess rather than a theory. Remember, theories in science are established after extensive research.
The second theory suggests that the mitochondria prokaryote formed a symbiotic relationship with another prokaryote, which would be the ancestor of eukaryotes . This relationship would have formed because mitochondria are versatile organisms and were capable of producing hydrogen (H2) to supply energy and electrons to the host prokaryote . To bolster this theory, we see symbiotic relationships between prokaryotes today still and we know that the use of hydrogen as energy is common in microbial communities . Since this theory accounts for the versatility of mitochondria, the eukaryotes that evolved from this symbiotic relationship would also have the same versatility, which is what we see today.
Knowing that there is a consensus that the mitochondria started out as a separate species which formed a symbiotic relationship with a host, it explains why they became a single species (the ancestor of eukaryotes). And also why eukaryotes have chimaera DNA (DNA from multiple sources). As the two species became more symbiotic and dependent on each other, DNA exchange started to occur to the point where enough DNA was moved from the mitochondria to the host so that the host could create its own mitochondria and the mitochondria no longer needed to create itself. It also explains why the mitochondria still have some DNA and most of it is in the genomic DNA. This also contributes to the chimaera trait of our DNA because we have DNA from two different species: the host and the mitochondria . These theories are still being refined and the mitochondria is still being studied as we continue to develop more knowledge about our evolutionary past and better technologies to do so.
Image acquired from Michael W. Davidson and The Florida State University