Which concept explains the origin of eukaryotic cells via symbiotic relationships?

• A. Endosymbiotic theory
• B. Law
• C. Metabolism
• D. Cohesion

Answer: (A)

Endosymbiotic theory explains how eukaryotic cells originated through long-term relationships where a larger cell engulfed smaller bacteria that then lived inside and became essential organelles. This idea fits because mitochondria and, in plants, chloroplasts have traits typical of bacteria: circular DNA, ribosomes similar to bacterial ribosomes, and they can replicate by binary fission. Their double membranes reflect the engulfment process. This partnership gave the host cell new capabilities like aerobic respiration and photosynthesis, explaining the origin of complex cells. Other ideas like a law, general metabolism, or cohesion don’t describe how cells arose from such symbiotic relationships.