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:
Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.  Example \(\PageIndex{1}\)
Solutions
SummaryThe 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 ResourcesWhich 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. All GRE Subject Test: Biochemistry, Cell, and Molecular Biology ResourcesWhich of the following facts does not provide evidence for the Endosymbiotic theory?Prokaryotes contain peptidoglycan in their cell walls. -This is not evidence supporting the endosymbiotic theory; rather, it highlights a difference between bacteria and mitochondria and chloroplasts.
What evidence exists that supports the Endosymbiotic theory quizlet?Evidence for this theory includes the fact that mitochondria and chloroplasts have two membranes, one being their original membrane and the other being created by the endosome during endocytosis. Further evidence includes the fact that mitochondria and chloroplasts are about the same size as the average prokaryote.
Which of the following statements regarding endocytosis and exocytosis is correct?The correct answer is that a- Endocytosis and exocytosis involve active transport. Both endocytosis and exocytosis require the use of ATP for the cell to enclose the particles being brought in, or to move the vesicle out of the cell, this makes both of these processes active transport.
Which of the following statements is false regarding prokaryotic cells?Which statement is false regarding prokaryotic cells? They contain cytoplasm but never a plasma membrane. Eukaryotic cells may contain all of the following except: a peptidoglycan cell wall.
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Which of the following statements concerning the endosymbiotic theory is false?
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