Learning Objectives
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:
- 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.
- Mitochondria and chloroplasts have their own DNA that is circular, not linear.
- Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
- 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.
Example \(\PageIndex{1}\)
- Briefly describe what is meant by the endosymbiotic theory.
- Give three points of evidence supporting the theory that mitochondria and chloroplasts may have arisen from prokaryotic organisms.
Solutions
- The endosymbiotic theory states that some of the organelles in eukaryotic cells were once prokaryotic microbes.
- 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.
All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources
Which of the following is not true of the endosymbiotic theory?
Possible Answers:
The inner membrane of mitochondria displays similarities to bacterial membranes
The mitochondria shows similarities to fossilized microorganisms
Mitochondria have their own genomes that contain similarities to bacterial genomes
None of the answers
Correct answer:
None of the answers
Explanation:
All three of the given answers are true and supportive of the endosymbiotic theory. Because mitochondria have membranes and genomes similar to those contained by bacteria, it is likely that they were once free-living organisms. The mitochondria have also been known to display remarkable similarities to fossilized microorganisms.
Endosymbiotic theory suggests that mitochondria were once free-living prokaryotes that were engulfed by a larger prokaryote. The engulfed cell still generated a proton gradient between its cell membrane and cell wall for energy synthesis, which the larger surrounding cell was able to use. The larger cell provided protection for the engulfed cell. Over time, the engulfed cell lost some of its distinct features, but continued to produce energy, evolving into the modern mitochondrial organelle.
The __________ theory hypothesizes that __________ were originally free-living prokaryotes.
Possible Answers:
ectosymbiotic . . . lysosomes
ectosymbiotic . . . mitochondria
endosymbiotic . . . lysosomes
endosymbiotic . . . mitochondria
Correct answer:
endosymbiotic . . . mitochondria
Explanation:
Ectosymbiosis is a term that refers to an organism that lives on the outside of the host. Both lysosomes and mitochondria are inside of the cell, so these choices cannot be correct.
The endosymbiotic theory postulates that mitochondria were once free-living prokaryotes that have evolved to form a symbiotic relationship with eukaryotic cells. The original prokaryotes were engulfed by larger prokaryotic cells, but continued to generate energy via the membrane gradient. This energy was used by both the smaller engulfed cell and the larger cell, and the smaller cell gained protection and nutrients. Eventually, this relationship evolved into modern eukaryotic cells and mitochondria. There is a plethora of evidence to support this theory. A few examples are that the mitochondrial membranes are more similar in structure to those of bacteria than of eukaryotes, and mitochondria contain their own genomes.
Which if the following organelles is not theorized to have arisen in eukaryotes via an endosymbiotic relationship?
Possible Answers:
Mitochondria
Ribosomes
Chloroplasts
Flagella
Explanation:
The endosymbiotic theory states that ancient prokaryotes may have had a symbiotic relationship with early eukaryotes, leading them to become permanent organelles in the eukaryote. Chloroplasts, mitochondria, and flagella have all been tied to this theory. Ribosomes, however, are organelles found in both prokaryotes and eukaryotes, so they are not a part of the theory.
Which of the following statements is false concerning mitochondria?
Possible Answers:
The outer membrane of mitochondria contain porins
Mitochondria divided separately from the rest of the cell by binary fission
Mitochondria code for their own ribosomes that resemble bacterial ones
Mitochondria contain a small circular genome that is separate from the cellular genome
Mitochondria are surrounded with a thin peptidoglycan layer
Correct answer:
Mitochondria are surrounded with a thin peptidoglycan layer
Explanation:
The endosymbotic theory suggests that mitochondria originated from a bacteria that was engulfed by a proto-eukaryotic cell. Much evidence for this theory is based on the fact that mitochondria resemble bacteria in many ways. They do not however contain a peptidoglycan cell wall like almost all bacterial cells do.