by James Street

All living animals, humans included, are inhabited consistently by other organisms. This is a fact that, although not many may want to linger on, is somewhat understandable and unsurprising. What is lesser known, perhaps, is the classification system of the never-ending associations between the inhabitant and its host - known as symbiosis. As shown in figure 1.0, there are three main sections: commensalism (an association where one organism benefits, and the other is not affected), mutualism (where both organisms benefit) and parasitism (where one organism benefits and the other is potentially harmed). As one moves toward mutualism (to the right of the diagram), the dependence of the species on its host increases, and as one moves toward parasitism (down the diagram) the degree of harm to the host increases. Although the general classification of symbiosis is a fascinating subject, it is not one which this article covers; rather what it probably the most extreme example of mutualistic relationships - a mitochondrion in your average eukaryotic cell. However, to understand this, we must first define what a eukaryotic cell is, and what the mitochondria was believed to have evolved from - the prokaryote. Eukaryotes and prokaryotes are the two main types of cell, and in simple terms, bacteria are prokaryotes and other organisms are eukaryotes. There are a number of distinct differences between these types of cell, many of which help to back up the theory that mitochondria used to be prokaryotes. One major difference is the DNA: whereas in eukaryotic cells it is in a nucleus, in prokaryotic cells the DNA is contained in a circular chromosome with no nucleus enclosing it. Obviously, their size is another major factor, prokaryotes being typically much smaller than eukaryotic cells. Figure 1.1 sums up the differences between prokaryotic cells and eukaryotic cells, and also shows the similarity between mitochondria (and chloroplasts) and prokaryotes.

As is shown by the table, the mitochondria have a number of very similar characteristics, most greatly suggested by the similarity of mitochondrial DNA to prokaryotic DNA. Furthermore, the ribosomes in the mitochondria are much more similar to those in prokaryotic cells, being of a smaller size than the eukaryotic equivalents, and certain antibiotics which are designed to kill bacteria can also damage mitochondria (for example, chloramphenicol was designed to stop the action of ribosomes in prokaryotes, but also affected those in the mitochondria and caused cell death). It is also generally accepted how the prokaryotes evolved into mitochondria, through a process called endocytosis. On a basic level, the bacteria was engulfed by an evolutionarily old eukaryotic cell, and developed a mutualistic relationship with it, receiving nutrients and energy substrates, and in return providing the cell with energy in the form of ATP (adenosine tri-phosphate). It lived in the cell as an endosymbiont, or simply an organism in symbiosis with its host, living inside its host cells. Through evolution, the bacteria lose much of their DNA, and the control is largely taken over by the host cell. Although now we may take this for granted, such an accidental contact millions (if not billions) of years ago almost certainly allowed life to evolve in the way it did, and allowed for us to exist today.