Which of the following best supports the endosymbiotic theory of the evolutionary origin of mitochondria?

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Learning Objectives

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• What supports the theory of endosymbiosis for mitochondria?
• What is the endosymbiosis theory for the evolution of mitochondria?
• Which of the following best supports the endosymbiotic theory?
• What supports the theory of endosymbiosis for mitochondria quizlet?

• Briefly describe what is meant by the endosymbiotic theory.
• Give some evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.

It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

Evidence for this is based on the following:

1. Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by binary fission, and the mitochondria of some protists have Fts homologs at their division plane.
2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.
3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.

Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.

Figure \(\PageIndex{1}\): One model for the origin of mitochondria and plastids. This model has an amitochondriate eukaryote engulfing an aerobe and then a cyanobacterium. from Kelvinsong

Example \(\PageIndex{1}\)

1. Briefly describe what is meant by the endosymbiotic theory.
2. Give three points of evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.

Solutions

1. The endosymbiotic theory states that some of the organelles in eukaryotic cells were once prokaryotic microbes.
2. • Mitochondria and chloroplasts are the same size as prokaryotic cells and divide by binary fission.
   • Mitochondria and chloroplasts have their own DNA which is circular, not linear.
   • Mitochondria and chloroplasts have their own ribosomes which have 30S and 50S subunits, not 40S and 60S.

Summary

The endosymbiotic theory states that mitochondria and chlopoplasts in today's eukaryotic cells were once separate prokaryotic microbes.

Biologist Lynn Margulis first made the case for endosymbiosis in the 1960s, but for many years other biologists were skeptical. Although Jeon watched his amoebae become infected with the x-bacteria and then evolve to depend upon them, no one was around over a billion years ago to observe the events of endosymbiosis. Why should we think that a mitochondrion used to be a free-living organism in its own right? It turns out that many lines of evidence support this idea. Most important are the many striking similarities between prokaryotes (like bacteria) and mitochondria:

When you look at it this way, mitochondria really resemble tiny bacteria making their livings inside eukaryotic cells! Based on decades of accumulated evidence, the scientific community supports Margulis’s ideas: endosymbiosis is the best explanation for the evolution of the eukaryotic cell.

What’s more, the evidence for endosymbiosis applies not only to mitochondria, but to other cellular organelles as well. Chloroplasts are like tiny green factories within plant cells that help convert energy from sunlight into sugars, and they have many similarities to mitochondria. The evidence suggests that these chloroplast organelles were also once free-living bacteria.

The endosymbiotic event that generated mitochondria must have happened early in the history of eukaryotes, because all eukaryotes have them. Then, later, a similar event brought chloroplasts into some eukaryotic cells, creating the lineage that led to plants.

Despite their many similarities, mitochondria (and chloroplasts) aren’t free-living bacteria anymore. The first eukaryotic cell evolved more than a billion years ago. Since then, these organelles have become completely dependent on their host cells. For example, many of the key proteins needed by the mitochondrion are imported from the rest of the cell. Sometime during their long-standing relationship, the genes that code for these proteins were transferred from the mitochondrion to its host’s genome. Scientists consider this mixing of genomes to be the irreversible step at which the two independent organisms become a single individual.

Grabbing take-out: Paramecium bursaria packs a lunch

Paramecium bursaria, a single-celled eukaryote that swims around in pond water, may not have its own chloroplasts, but it does manage to “borrow” them in a rather unusual way. P. bursaria swallows photosynthetic green algae, but it stores them instead of digesting them. In fact, the normally clear paramecium can pack so many algae into its body that it even looks green! When P. bursaria swims into the light, the algae photosynthesize sugar, and both cells share lunch on the go. But P. bursaria doesn’t exploit its algae. Not only does the agile paramecium chauffeur its algae into well-lit areas, it also shares the food it finds with its algae if they are forced to live in the dark.