In the video "What is Life? Is Death Real?", the subject of mitochondria is raised at 2:58. At 3:12, the narrator says "[mitochondria] are not alive any more: they are dead."
What currents of thought lead to this affirmation?
When I search for mitochondria are dead on Google, I get many links about the role of mitochondria in cell death, but I don't see anywhere that the assertion in this video is discussed.
SolarLunix posted an excellent answer detailing the criteria for being classified as "alive", and showed that by those criteria, mitochondria could be considered as "dead".
However, I would argue that the narrator's statement in your video does not make any sense. The currently-accepted theory of the evolution of mitochondria (and possibly other organelles) from free-living prokaryotes is called symbiogenesis and postulates that the mitochondrial progenitors began living symbiotically with the precursors to eukaryotes some 1.5 billion years ago. Life originated on Earth about 3.5 billion years ago, so eukaryotic cells (carrying mitochondria) have been around for a little less than half of that time.
In that incredibly large span of time (if you counted 1 number per second, you'd reach 1.5 billion in about 47.5 years), what used to be an independent organism (we assume) has become an organelle - an integral part of the eukaryotic cell. It absolutely could not survive outside the cell on its own, and relies on signals from the cell (mainly, the cell's energy needs as detected by "sensor" proteins to key molecules like glucose and ATP) to reproduce.
Therefore, it is most appropriate to think of mitochondria simply as another organelle, just like the endoplasmic reticulum, lysosomes, or the nucleus. It doesn't make sense to think of those other structures as "alive" or "dead", just as it doesn't make sense to think of mitochondria in the same way. Yes, they used to be independent, but are no longer; instead they are a well-integrated part of the whole cell.
Short answer: According to the definition of life, yes, Mitochondria are "dead".
To be considered alive an organism must meet the following criteria:
- organized structure performing a specific function
- an ability to sustain existence, e.g. by nourishment
- an ability to respond to stimuli or to its environment
- capable of adapting
- an ability to germinate or reproduce
Are Mitochondria organised?
Yes, they have a structure that allows them to metabolise energy brought to it by the cell.
Can Mitochondria sustain their own existence?
No, many of the genes that mitochondria need to function are no longer in the mitochondria. They need the host cell to provide much of what they need for them.
Note that I am pointing to the Mitochondria's genetics. They used to be included in the Mitochondria itself and have been moved into the cell host DNA. This is why I consider them to be "dead" because they are no longer their own organism, they are an organelle that helps the cell stay alive.
We can stop here since this disqualifies Mitochondria from being considered alive. However, for completion, I will continue.
Can Mitochondria respond to stimuli?
Yes, in order for it to divide it receives signalling from the cell. I would consider this a response to its environment.
Can Mitochondria reproduce?
Yes, their reproduction is much like a bacteria reproduces - through a process called fission.
Conclusion: Since Mitochondria cannot sustain existence, alone (thanks to no longer holding their full genome) they are dead.
I would like to expand a bit on SolarLunix's post, because the logic used in the conclusion would also mean that endosymbionts, such as https://en.wikipedia.org/wiki/Buchnera_(bacterium), who cannot survive outside their host are also "dead".
I think many of us disagree with that notion, so instead I would say that it's the fact that so many of their genes have been relocated to the nucleus that makes them organelles and thus substructures of the eukaryotic cell and thus "dead".
Ultimately though, I think it's more a matter of semantics, since you could argue that they represent an advanced stage of endosymbiosis.
What a brilliant question for promoting a discussion. Mitochondria definitely reproduce, they maintain a better energy flow than any other part of the cell, and they maintain a very low entropy state by exploiting that energy flow. It may not be a textbook answer but being alive really is about decreasing your local entropy by exploiting a flow of energy. I suspect that mitochondria have ceased to change under the pressures of natural selection, but so what - they have been practically perfect for a billion years.
