Most people who have learned about biology at school can tell you that the mitochondria is the powerhouse of the cell. But how did mitochondria come to hold this important title in almost every eukaryotic (plant, animal, fungi, and protist) cell?
The answer to this question lies in the Endosymbiotic Theory, which explains how eukaryotic cells are made up of parts of other cells. It all began 4 billion years ago, when prokaryotic cells, some of the earliest forms of life, first appear in the fossil record. Scientists believe that all eukaryotic cells evolved from prokaryotic cells. How did this cellular evolution occur? Mitochondria and chloroplasts were originally types of prokaryotes, as they once lived as self-functioning, single-celled organisms. Endosymbiosis is the process of how these prokaryotes were ingested or engulfed by other cells (host cells), either by phagocytosis or as parasites of the host cells. Eventually, mitochondria and chloroplasts lost the ability to live without the safe habitat of the host cells, and at the same time, the host cells became unable to live without the energy that the organelles provide them with. This loss of independence is called obligate mutualism. The organelles and host cells evolved together to become a single organism, otherwise known as a eukaryotic cell.
What evidence is there to support endosymbiosis? During the 1950s to 1960s, DNA was discovered in mitochondria and plastids. The DNA in the plastids was found to be different than the DNA in the rest of the plant cell. In fact, these genes were similar to prokaryotic genes, supporting the claim that the powerhouses of cells were once prokaryotes that could live on their own. In 1983, Andreas Schimper, a German botanist, observed that new chloroplasts formed through binary fission, which is how prokaryotes reproduce. As well, mitochondria and chloroplasts both have double-membranes, which are made of molecules very similar to those that make up prokaryotic membranes. All these separate discoveries were combined to form the important theory of how eukaryotic cells evolved to have these fundamental organelles. This shows how science is a collaborative process, as every discovery is connected to many more.
The Endosymbiotic Theory explains how mitochondria and chloroplasts were essentially the stepping stones of the evolution of prokaryotic cells to eukaryotic cells. Today, the eukaryotic cells that joined with only mitochondria are animal cells, and those that joined with mitochondria, chloroplasts, and other plastids are plant cells. These organelles are essential to the processes that produce the fuels of life: Mitochondria power cells and all biological processes as the sites of cellular respiration, which converts sugars and oxygen to carbon dioxide, water, and adenosine triphosphate (ATP) (energy-carrying molecules). Chloroplasts are where photosynthesis occurs, converting carbon dioxide, water, and light energy into oxygen and sugar.
Eukaryotic cells have many other organelles that are not found in prokaryotes, including the nucleus, Golgi apparatus, endoplasmic reticulum, and ribosomes. Scientists do not yet know how these organelles evolved. They could have resulted from endosymbiosis as well, or they could have formed within the cell itself. Although learning the definitions and functions of organelles is important, I believe it is just as important to know how these organelles came to be. With an understanding of historical views, an openness to new ideas and explorations, and the ability to connect these different discoveries, perhaps another theory will arise to explain the evolution of different organelles in the eukaryotic cell. But for now, the next time you are asked the definition of a mitochondrion, you can confidently say that the mitochondria is both a past prokaryote and a powerhouse.