Question
Researchers have proposed a model of chloroplast evolution. According to the model, chloroplasts evolved from a small prokaryotic organism that was engulfed by an ancestral eukaryote. The engulfed prokaryote then formed an endosymbiotic relationship with the eukaryotic host.
Which of the following observations best supports the model?
A. Chloroplasts are separated from other subcellular compartments by semipermeable membranes.
B. Prokaryotic and eukaryotic organisms both acquire nutrients from the surrounding environment.
C. Eukaryotes evolved after prokaryotes and have more complex structures.
D. Chloroplasts and some prokaryotes share similar photosynthetic reactions.
▶️Answer/Explanation
Ans: D
Photosynthesis is a feature of only a limited number of organisms, including cyanobacteria, algae, and plants. The fact that this specialized process is similar in prokaryotes, such as cyanobacteria, and in eukaryotes, such as algae and plants, provides evidence in support of the model.
Question
Which of the following observations best supports the claim that mitochondria evolved from once-free-living prokaryotic cells by the process of endocytosis?
A. Mitochondria produce ATP.
B. Mitochondria contain proteins.
C. Mitochondria exchange substances with the cytosol.
D. Mitochondria are surrounded by a double membrane.
▶️Answer/Explanation
Ans: D
The double membranes of mitochondria provide evidence that an ancestor of mitochondria, which was most likely a type of free-living aerobic bacterium, was ingested via endocystosis by a primitive eukaryotic cell.
Question
Euglenids are single-cell eukaryotes that live in aquatic environments. The chloroplasts found inside euglenids are enveloped by three membranes, as represented in Figure 1. The inner membrane of euglenid chloroplasts resembles the thylakoid membrane.
Figure 1. Simplified diagram of a euglenid, showing the structure of the chloroplast envelope
Which of the following claims about the origin of the euglenid chloroplast is best supported by the three-membrane structure of the envelope?
A. It originated from the spontaneous assembly of organic molecules into a lipid bilayer inside a free-living prokaryote.
B. It originated from the fusion of the plasma membranes of two different free-living photosynthetic prokaryotes.
C. It originated from the incorporation of a photosynthetic prokaryote into a eukaryotic cell by a single endosymbiotic event.
D. It originated from the incorporation of a photosynthetic prokaryote into a eukaryotic cell by two endosymbiotic events.
▶️Answer/Explanation
Ans: D
The three-membrane structure of the chloroplast envelope best supports the claim presented in this answer choice. The three-membrane structure most likely resulted from two endosymbiotic events, with the middle membrane originating from the first event and the outer membrane originating from the second event.
Question
Which of the following is NOT evidence that supports the
endosymbiosis hypothesis?
(A) Mitochondria and chloroplasts have their own circular DNA.
(B) Mitochondria and chloroplasts have their own ribosomes.
(C) Mitochondria and chloroplasts reproduce by binary fission.
(D) Mitochondria and chloroplasts are found in all eukaryotic cells.
▶️Answer/Explanation
Ans:
(D) Mitochondria and chloroplasts are not found in all eukaryotic cells;
chloroplasts are only found in eukaryotic cells that perform
photosynthesis. Mitochondria and chloroplasts do have their own
circular DNA as well as their own ribosomes, and they both reproduce
by binary fission, which are all characteristics they share with
prokaryotes. Thus, choices (A), (B), and (C) are all evidence of the
endosymbiosis hypothesis.
Question
Mitochondria are found in most eukaryotic cells and contain their own DNA and ribosomes that are similar to those typical of many prokaryotic cells. Which of the following statements is justified by these observations?
(A) The mitochondrion is the only location in which eukaryotic cells can synthesize ATP.
(B) An ancestral cell most likely engulfed an aerobic prokaryote in a relationship that proved beneficial for both cells.
(C) Mitochondrial membranes provide abundant surface area for reactions because of the infoldings called cristae.
(D) The mitochondrion plays a role in respiration, but it also stores the cell’s extra mRNA.
▶️Answer/Explanation
Ans:B