Which theory proposes that mitochondria and chloroplasts originated from endosymbiotic relationships with prokaryotes?

• A. Photosynthesis
• B. Endosymbiosis
• C. Phagocytosis
• D. Autophagy

Answer: (B)

The idea being tested is that mitochondria and chloroplasts originated through endosymbiotic relationships with prokaryotes. The endosymbiotic theory explains that an ancestral eukaryotic cell captured free-living bacteria, with the aerobic bacteria eventually becoming mitochondria and the cyanobacteria becoming chloroplasts, establishing a long-term mutualistic partnership. This is supported by strong evidence: mitochondria and chloroplasts contain their own circular DNA and bacterial-like ribosomes, they replicate by binary fission, and they are enclosed by double membranes—patterns expected if they were once independent organisms taken inside a host cell. Other options describe processes rather than origins of these organelles, such as photosynthesis (the energy-harvesting process), phagocytosis (cell eating), and autophagy (self-digestion). Because only endosymbiosis accounts for the origin and characteristics of these organelles, it is the best explanation.