Question

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.

Explanation:

The three - membrane structure of the euglenid chloroplast envelope is best explained by two endosymbiotic events. In the first endosymbiotic event, a eukaryotic cell engulfed a prokaryotic cell. Then, in a second event, the engulfed prokaryote (which was likely photosynthetic) was incorporated, resulting in an extra membrane layer. Options A and B do not account for the three - membrane structure. Option C with a single endosymbiotic event would result in a double - membrane structure like in plant chloroplasts from a single endosymbiotic origin.

Answer:

D. It originated from the incorporation of a photosynthetic prokaryote into a eukaryotic cell by two endosymbiotic events.