Most eukaryotic organisms have mitochondria in most of their cells

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Most eukaryotic organisms have mitochondria in (most of) their cells.

In particular, most plant cells and most animal cells contain mitochondria.

Justification for presence in most cells of most eukaryotes

Empirical data

Mitochondria have been found in cells of a wide range of living organisms, including plants, animals, protists, and fungi.

Importance of mechanism

Mitochondria are responsible for energy generation in eukaryotic organisms (through cellular respiration, which may be aerobic or anaerobic depending on the organism and its environment). All living organisms need a supply of energy to power their metabolism, and the only known mechanisms for energy generation in eukaryotes are the ones powered by mitochondria and their variants, hydrogenosomes. Therefore, all living organisms are expected to have mitochondria, unless:

  • they are parasitic and use the energy of the host fully to power their own metabolism; or
  • they use a variant of mitochondria, such as the hydrogenosome.

Importance at the cellular level

For the most part, cells take care of their own energy needs, so the same argument as the preceding one should apply at the level of the individual cell.

The exceptions would be cells that have highly specialized roles where the presence of mitochondria interferes with their role, and where they don't have enough energy needs to require production within the cell.

Evolutionary history

Mitochondria are found in several organisms early in the evolutionary lineage. We also expect the behavior of mitochondria to be highly conserved through evolution, therefore we expect to see mitochondria in all living organisms. A few reason to expect evolutionary conservation:

  • Empirical observation: We've found the mitochondrial genome to be pretty similar across species.
  • (?) Maternal inheritance: Mitochondria are (in most organisms) inherited maternally, so there's less mixing up of genes.
  • (?) Less selection pressure: Mitochondria are already pretty good at what they do, and their function is essential to living organisms, so there is less selection pressure for them to "improve".

What's different about prokaryotes?

Prokaryotes also need to generate energy for themselves (except those that are completely parasitic). However, prokaryotes in general don't have membrane-bound organelles, and in particular don't have mitochondria. They do have a similar mechanism as the mitochondrion's for energy production, directly in the plasma membrane.

Exceptions at the organism level

Some parasitic unicellular organisms (that cannot produce energy)

Some parasitic unicellular organisms do not contain mitochondria, instead relying on host cells for their energy needs. An example is the protozoan parasite Entamoeba histolytica.

Many of these organisms contain mitosomes. The mitosome is an organelle structurally similar, and likely evolutionarily descended, from, the mitochondrion, but without the ability to carry out the key metabolic pathways of the mitochondrion, specifically, cellular respiration.

Organisms containing hydrogenosomes (variants of mitochondria that can produce energy)

Some organisms (such as metamonads) have a variant of mitochondria called hydrogenosomes. The hydrogenosome have some functionality like mitochondria; they can produce ATP and hydrogen to create energy for the cell.

More complex counterexamples: parasitic animals

The myxozoa called henneguya lacks mitochondria. This is a parasite of salmon.[1]

Exceptions at the cell level (cells that don't contain mitochondria in organisms whose other cells contain mitochondria)

Red blood cells in mammals

Red blood cells in mammals lack both a mitochondrion and a nucleus.

  • This is helpful because their function is to transport oxygen, and mitochondria would interfere with that function by consuming oxygen.
  • Since RBCs are just transported around by blood, and have a limited lifetime, they don't have a need for within-cell energy production.