A Stranger In Your Home

Mitochondria are the powerhouse of the cell. Due to the prominence of the mitochondria in modern high school science education, if there’s one thing that you learned in high school, it’s probably that phrase right there. As you may already know, a mitochondrion is a type of specialized structure or an organelle that resides within a living eukaryotic cell. But the secrets to the origin of the mitochondrion might come as a shock to you.

The structure and functions of mitochondria

The size of a mitochondrion extends within the range of 0.75 to 3 uM in diameter, roughly 50 times smaller than the size of a typical animal cell. If you could recall back to high school biology, you would’ve drawn the mitochondria as a bean-like shape with several invaginations towards the inner portion of the structure. While widely accepted as a basic drawing of a mitochondrion, this is not entirely true to the fact. In reality, the mitochondria’s shape varies and is ever malleable and mostly do not retain a singular fixed structure i.e. not all bean-shaped. With that said, the invaginations are in fact true as this would aid in increasing the total surface area within the mitochondria for maximum efficiency when carrying out their function.

The main function of mitochondria is to supply the cell in which it resides in most of the cellular energy. In the context of biology, energy takes the form of adenosine triphosphate or ATP for short. However, this is not the only function carried out by the mitochondria. In fact, a single mitochondrion is involved in a whole plethora of functions which include the regulation of the cell cycle, cell growth and even cell death! But mainly, the mitochondria convert the oxygen that we breathe in into energy in a process known as oxidative phosphorylation and is therefore indeed, the powerhouse of the cell.

The origins of the mitochondrion

First identified all the way back in the 1840s and first described half a century later, the mitochondrion has always been a fascinating piece of structure in biology. With no less than 3 Nobel prizes shared among 5 well-deserved Nobel laureates, the study of the mitochondria is every-growing with new discoveries constantly being made. Nevertheless, the origins of the mitochondrion have always been shrouded in mystery and controversies.

Back in 1918, a French scientist claimed that the mitochondria came from an extracellular origin i.e. from outside the cell. This claim was further substantiated in the 1920 by Ivan Wallin who declared that the mitochondria was of a bacterial origin. A number of similar theories followed suit, with all of them being dismissed, ignored and criticized for the lack of scientific evidence. It wasn’t until a landmark paper appeared in 1967 by the young evolutionary biologist, Lynn Margulis. Titled “On the origin of mitosing cells”, Lynn gave the first account of microbiological evidence substantiating the claim of a bacterial origin of the mitochondria. The paper was initially rejected by fifteen journals and her concepts were largely ignored for another decade. Only through revolutionary advances in DNA sequencing towards the late 1970s and early 1980s that Lynn’s theory was widely accepted by the scientific community. Through genetic analysis, scientists had found that the mitochondria have their own genome distinct from our own cellular genome and that the mitochondrial genome is mostly related to bacterial origins.

The theory of endosymbiosis

Endosymbiosis or symbiogenesis is a theory that more complex cells such as that of a eukaryote evolved from a symbiotic relationship between less complex cells such as prokaryotes. This theory was first proposed by the Russian biologist, Konstantin Mereschkowski, in 1910 and was further corroborated by Lynn Margulis. It was proposed that the mitochondria represent an ancient prokaryote i.e. bacteria, which around 1.5 billion years ago was taken up by an ancient eukaryotic cell and has since then, formed a symbiotic relationship between the two organisms. In fact, the mitochondria had been found to most closely resemble the proteobacteria, Rickettsiaceae.

The theory of endosymbiosis works perfectly for the origins of mitochondria because as mentioned before, the mitochondrial genome is vastly different than that of the cellular genome. Furthermore, this theory would only hold true if a mutualistic relationship i.e. I help you while you help me, between the two organisms can be established. In this case, the bacteria which would become the mitochondria provides the host cell with cellular energy to allow the host cell to perform innumerable cellular functions that require energy while the host cell provides the bacterial cell shelter and a new safe home.

Source: https://socratic.org/questions/why-did-endosymbiosis-occur

Closing remarks

The mitochondrion is indeed a stranger in your cellular home. It is a stranger that has lived with you for more than a billion years and frankly, you’re okay with that. It is thanks to the undying efforts of biologists like Lynn Margulis that we’re able to understand more about this little stranger. “I greatly admire Lynn Margulis’s sheer courage and stamina in sticking by the endosymbiosis theory…. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it.” – Richard Dawkins, 1995.