Suppose you stop someone on the street to ask them a question. You offer them three words: what are mitochondria? Chances are, they will reply “the powerhouse of the cell” as a near-Pavlovian reflex. It’s amazing to think that almost everyone who went through high school-level biology or life sciences can remember this factoid, regardless of whether or not they remember anything else about the course. Now, suppose you ask this stranger a follow-up question: what else do you know about mitochondria? More than likely, they will simply blink back at you and shrug. I’m here to tell you, if you are one of those people who shrugged, you’re missing out! Sit back and let me tell you the story of my favorite cellular stowaway.
To understand why mitochondria are so important, you first need to know that all cells are not the same. Cells can be classified into two categories: prokaryotes and eukaryotes. Prokaryotes are single-celled organisms that do not have a centralized nucleus, a hub where they keep their genetic information. They also lack specialized regions where cellular reactions take place. Eukaryotes, on the other hand, do have a centralized nucleus, and each of their cellular components, organelles, are individually wrapped by a membrane made up of fats and proteins. However, these two categories have not always existed.
Once upon a time, about 1.45 billion years ago, the ancestors of the modern eukaryotic cell were just getting the hang of being alive. The atmosphere was actively changing. Although oxygen was becoming more abundant, these cells were likely anaerobes, meaning they weren’t very good at surviving where oxygen was present. As the world around them changed, so did their needs for creating energy using the available resources. There were, also, other types of cells, known as facultative aerobes, who could live in both oxygen-rich and oxygen-poor environments. It is thought that these organisms were the first photosynthetic cells, who used the power of the sun to carry out energy-creating processes.
Scientists believe that these two types of cells decided to help each other out. Out in nature, microbial communities often interact with each other to promote their own survival. In some cases, two different types of bacteria may help each other grow by utilizing each other’s waste. For example, one bacteria may produce a certain amino acid, the building block of proteins, as a byproduct. A second bacteria may be in great need of that same amino acid, but does not produce it themselves, so they pick up whatever of the amino acid they can find. So, one bacteria’s trash becomes another’s treasure! This win-win scenario where two organisms cohabitate for their mutual benefit is a type of symbiosis. The theory of how this relationship arose, however, is still up for debate.
Currently, two theories seek to explain the start of this relationship and boil down to a “chicken or egg” dilemma. In this case, the question is: what came first–the eukaryote or the mitochondria? The first theory, the Archaezoan scenario, suggests that there were early eukaryotes, known as archaezoans, that lacked mitochondria. These archaezoans “swallowed”, or engulfed, a type of aerobic α-Proteobacterium, allowing the host to tolerate oxygen. The second theory is known as the symbiogenesis scenario (or, the hydrogen hypothesis). In this case, it is said that both cells in the symbiotic relationship were prokaryotes: one could survive strictly on hydrogen gas, while the other could use oxygen and produced hydrogen gas as a byproduct. The prokaryote that survived on hydrogen gas engulfed the oxygen-tolerant cell. The host cell would provide protection from other bacteria, and in return, the ingested cell would produce hydrogen. This scenario would mean that the existence of eukaryotes arose only as a direct result of this co-evolved relationship between prokaryotes. Most scientists concur that the symbiogenesis scenario is the more likely theory for eukaryotic evolution, and thus, the transformation of the engulfed cell into the modern day mitochondria.
Regardless of how it happened, these two free-living organisms co-evolved until they became one. This included reducing genetic material and incorporating mitochondria-specific genes into the host nuclear DNA, integrating their biochemical functions, and anchoring the mitochondria in place. Nowadays, mitochondria cannot live on their own. However, they retain their own genetic material, passed down from mother to child, and protein-building machinery. Most importantly, they play a role in all major functions of the cell. They regulate intracellular ion concentrations, dictate cell differentiation, and even trigger death protocols of injured and abnormal cells. Dysfunctional mitochondria can also cause serious, even fatal, diseases! So, yeah, mitochondria are “the powerhouse of the cell”, but they are also an ancestral friend that hitched a permanent ride, and we would be doomed without them.