I'm working on an assignment for my IB biology class and some assistance would be highly appreciated. I've read several articles and I still haven't quite gotten the answer I'm looking for. I have to write a one page summary on whether or not Mitochondrion are alive or not.

  • $\begingroup$ Think about the definition of a living organism... Does it reproduce, or some kind of growth? Can it live by itself? $\endgroup$
    – TanMath
    Commented Oct 11, 2015 at 20:01
  • $\begingroup$ Mitochondria are symbionts that are no longer free living. Some of there essential genes have been incorporated into the hosts genome and some remain within the mitochondria themselves. After billions of years of coevolution, they have, for the most part become organelles, and are considered to be organelles. You could not lyse a Eukaryotic cell and grow mitochondria. But Eukaryotic cells need mitochondria to function efficiently. $\endgroup$
    – AMR
    Commented Oct 11, 2015 at 20:03
  • 1
    $\begingroup$ The definition is a matter of philosophy and not everybody agree. And typically "can it live by itself?" would be a pretty misleading question given that it would exclude all parasite from the realm of living things. If you push the concept of ecological interdependence a little further it would even yield any living individual to be excluded from the realm of living things. A wolf for example is not a living thing just because it depends on its preys and on the plant that is producing its oxygen (and other species with which it has a relationship). $\endgroup$
    – Remi.b
    Commented Oct 11, 2015 at 20:05
  • $\begingroup$ @TanMath, you have just given a 50-50 scenario. They can reproduce, as that is how the cell gets new mitochondria, but they cannot live by themselves as some of their key genes have been transferred into the host genome, so vital proteins can no longer be synthesized but the mitochondria. $\endgroup$
    – AMR
    Commented Oct 11, 2015 at 20:07
  • $\begingroup$ @Remi.b. Free living in this case implies having a complete genome. Mycoplasmas have extremely small genomes, but they have all of the genes that they need to function. Humans cannot synthesize all 20 amino acids and need oxygen and other things from our environment, but our genome allows us to interact and survive within that environment. Mitochondria have transferred many of their genes to the host genome. They do not have the genes they would need to produce the essential gene products that they would need to survive in say an enucleated cell, such as an RBC. $\endgroup$
    – AMR
    Commented Oct 11, 2015 at 20:13

1 Answer 1


Welcome to Biology.SE

Fields of interest

The assignment you have to write has a little bit to do with Biology and a little bit to do with Philosophy! The question of what is alive and what is not depends on the very definition of what is a living thing and this question is not of interest to biologists but only to philosopher.

How to nail this assignment

Therefore, for your assignment you will need to understand what a mitochondrion is (the Biology part) and to understand how do the different definitions of life apply to the mitochondrion (Philosophy part although you will need some knowledge in Biology to understand the relevance of these definitions).

What is a mitochondrion?

This question is too broad and cannot be answered here. Make sure to have a look at the wikipedia page. In short:

  • mitochondrion contain DNA
  • mitochondrion is the result of a bacteria that has somehow been internalized into a bigger cell (it is a symbiont). This is why today mitochondrion contain DNA. It is therefore not exactly an organelle like others. Note that plants, in addition to having mitochondrion, they have plasmids that are also consequences of a similar process of internalizing other form of life.
  • mitochondrion DNA evolve (of course)
  • mitochondrion are basically the energy factory of the cell.
  • mitochondrion are not able to live by themselves outside a living cell (to my knowledge).

Definitions of life

It is important to understand that the definition of life has absolutely no impact on biology and is nothing but a question of nomenclature.

There are a lot of definitions and I can't cover all of them here. You might want to have a look at this wiki page. The previous link lists the most common properties we think about when thinking about the definition of life.

  • Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature. Organization: Being structurally composed of one or more cells — the basic units of life.
  • Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.[49] Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
  • Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
  • Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms,"with an error rate below the sustainability threshold."

Related questions

The question of whether viruses are alive is a very common one. Here are two Biology.SE posts on the subject:


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