According to the endosymbiotic theory, how did chloroplasts become part of producers' cells?

• A. Chloroplasts were inherited directly from the nuclear DNA of producers' cells.
• B. Producers' ancestral cells engulfed photosynthetic bacteria, which evolved into chloroplasts.
• C. Chloroplasts formed spontaneously from the cell membrane of producers' cells.
• D. Producers' cells fused with animal cells, resulting in the formation of chloroplasts.

Answer: (B)

Verified step by step guidance

1. Understand the endosymbiotic theory: This theory explains how certain organelles, such as chloroplasts and mitochondria, originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells. These prokaryotes established a symbiotic relationship with the host cell.
2. Focus on chloroplasts: Chloroplasts are organelles responsible for photosynthesis in producers (plants and algae). According to the endosymbiotic theory, chloroplasts evolved from photosynthetic bacteria, specifically cyanobacteria, that were engulfed by ancestral eukaryotic cells.
3. Eliminate incorrect options: Review the provided options and eliminate those that contradict the endosymbiotic theory. For example, chloroplasts were not inherited directly from nuclear DNA, nor did they form spontaneously from the cell membrane. Additionally, producers' cells did not fuse with animal cells to form chloroplasts.
4. Identify the correct explanation: The correct answer aligns with the endosymbiotic theory, which states that producers' ancestral cells engulfed photosynthetic bacteria. Over time, these bacteria evolved into chloroplasts, becoming integral parts of the eukaryotic cells.
5. Summarize the process: The engulfed photosynthetic bacteria provided the host cell with the ability to perform photosynthesis, offering a survival advantage. This mutualistic relationship led to the bacteria evolving into chloroplasts, which are now essential organelles in producers' cells.