Life Together: The Origin of Mitochondria and Chloroplasts

One of the interesting questions about the history of life is the question of where eukaryotic cells came from. Our fossil record shows us pretty clearly that for the first couple of billion years of the existence of life, all of Earth's life forms were prokaryotic. So where did eukaryotic cells come from?

An initial question might be why eukaryotic cells arose. Problems like this rarely have real answers – just good ideas. One important thing that changed in the world about the time that the first eukaryotic fossils appear is that the atmosphere of the planet was beginning to fill up with free oxygen, almost certainly due to the increasing numbers of photosynthesizing organisms in the world. (Photosynthesis produces oxygen as a waste product). The introduction of significant oxygen into the environment would have initially been a disaster for most life forms of the times, since organisms that live without oxygen (anaerobic organisms) are poisoned by oxygen, which is frankly a very destructive substance. However, those organisms that survived the introduction of oxygen (because they were lucky enough to be able to perform some kind of chemical process that would use up oxygen, thus preventing it from messing around with the vital chemistry of the cell) got a bonus. Oxydation chemistry tends to release energy. So we're pretty sure that it was about this time than aerobic cellular respiration arose. The overwhelming difference between anaerobic cellular respiration and aerobic cellular respiration is the amount of energy produced by the two processes. Aerobic respiration produces many times more energy than anaerobic respiration does. Hypothetically, this increase in available energy was at least part of what promoted the development of the much larger and more complex eukaryotic cell.

So where did all of the new structures in eukaryotic cells come from? The probable answer is that there were at least two different kinds of events that added to the complexity of cell structure. Many of the structures in eukaryotic cells probably developed from the elaboration of the membranes of the cell. This is the likely explanation for the origin of the endoplasmic reticulum, golgi apparatus, and nuclear envelope. But the evidence strongly suggests a much more interesting origin for the two great energy processing organelles, the mitochondrion and the chloroplast. These two structures probably arose through a process known as endosymbiosis. [endo=inside; sym=together; bio=life]

Symbiosis is a dependent relationship between two organisms. There are three basic kinds: parasitism, in which one partner (the parasite) benefits and the other (the host) is harmed; commensalism, in which one partner (the commensal) benefits and the other partner (the host) is indifferent, and mutualism, in which both partners benefit. These three states are evolutionarily related to each other: parasitic relationships tend to evolve into commensalistic relationships, and commensalistic relationships tend to evolve into mutualistic relationships. This makes perfect sense. Any accidental genetic change in the host which reduces the harm (or causes benefit) from the parasite would certainly be favored by selection; any accidental genetic change in the parasite which keeps its host, upon which it depends, healthy and alive longer would also be favored. So the selective pressure on both sides is toward less and less damage to the host.

Endosymbiosis is a symbiotic relationship between two organisms in which one of the organisms lives inside the other. The relationship can be any of the three types of symbiosis. A frequent trend in endosymbiotic relationships is for the endosymbiont – the one inside – to become more and more specialized (and thus dependent upon the host). Endosymbiotic relationships are extremely common; there are endosymbionts living inside your body at this very moment.

Almost all biologists believe that this phenomenon explains where mitochondria and chloroplasts came from. Not all organisms developed the ability to perform photosynthesis, or to convert to aerobic cellular respiration. Many of those that didn't make these alterations themselves went into partnership with other organisms that did. If an anaerobic cell could engulf an aerobic one (without digesting it), it could get the benefit of the ATP overflow from its captive partner. Given a couple of billion years to get used to each other, the inside, aerobic partner became so specialized for aerobic cellular respiration that it lost almost all of the basic life skills, depending upon the external host cell to support it. Voila` – mitochondrion. If you tell the same story, but substitute "photosynthesis" for "aerobic cellular respiration," you have a recipe for the invention of chloroplasts.

This is an interesting story, but pretty outrageous unless there's some evidence that indicates that it might be true. Glad that you asked ;^)

There are some very interesting things about mitochondria and chloroplasts that have had biologists scratching their heads for quite a while. For one thing, both of these organelles have their own DNA molecules. Their DNAs are not duplicates of nuclear DNA – they are exclusive to the mitochondrion or the chloroplast. Mitochondrial DNA carries genes necessary to produce some of the molecules vital for the aerobic respiration process, and chloroplast DNA carries the genes for substances necessary for photosynthesis. The nuclear genes can't duplicate these. Unlike the DNA in the nucleus, mitochondrial and chloroplast DNAs are naked and circular – just like a prokaryotic cell's DNA. These two organelles also have their own ribosomes – and they are different from the ribosomes out in the cytoplasm. The proteins coded for by the mitochondrial genes are produced by mitochondrial ribosomes, and those coded for by the chloroplast genes are produced by chloroplast ribosomes.

Another interesting aspect of mitochondrial function is that not all parts of cellular respiration occur in the mitochondria. Aerobic cellular respiration has three parts, but only the second and the third require the presence of oxygen. The first part of cellular respiration (called glycolysis) requires no oxygen, and takes place not in the mitochondria, but out in the cytoplasm of the cell. It is identical to the anaerobic cellular respiration which occurs in cells which cannot use oxygen.

There are two very interesting conclusions to draw from these pieces of information. First, these observations strongly suggest that a mitochondrion or chloroplast is very much like a highly specialized and simplified cell living inside a larger cell. Second, it looks very much like the process of aerobic cellular respiration arose as an "add on" process. The aerobic process performed by the mitochondrion were tacked onto the end of the older, anaerobic respiration process.

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Updated 25 September 2004