[Campbell Biology P.645] According to the text and Figure 28.3, which statement accurately d... | Practice Question

According to the text and Figure 28.3, which statement accurately describes the evolution of plastids through primary endosymbiosis?

  • A: Plastids originated from an alpha proteobacterium and retained four membranes.
  • B: The ancestral heterotrophic eukaryote engulfed a photosynthetic cyanobacterium, and the resulting plastids in red and green algae typically have two membranes, one of which was lost from the original three.
  • C: Plastids evolved from a gram-positive cyanobacterium, resulting in a single membrane.
  • D: The primary endosymbiosis event involved an archaean engulfing a cyanobacterium, leading to three membranes in all descendant plastids.

Explanation

Primary endosymbiosis occurred when an ancestral heterotrophic eukaryote engulfed a photosynthetic cyanobacterium. Cyanobacteria are gram-negative, possessing two membranes. Upon engulfment, a third membrane from the host's food vacuole would have surrounded the endosymbiont. The text explicitly states that "one of these membranes was lost in red and green algal descendants," leading to plastids in red and green algae typically having two membranes.