Which theory proposes that eukaryotic cells formed from a symbiosis among several different prokaryotic organisms?

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

Answer: (B)

The idea being tested is how complex cells with internal compartments could originate from simpler cells through a long-term partnership between prokaryotes. The best answer is the endosymbiotic theory, which says that eukaryotic cells formed when one cell engulfed another and they began living together, with the engulfed cells eventually becoming organelles like mitochondria and chloroplasts. Evidence supporting this includes the fact that mitochondria and chloroplasts have their own circular DNA and ribosomes that resemble bacterial ones, and they replicate by a process similar to binary fission, plus they’re surrounded by double membranes. This explains why eukaryotic cells have energy-producing mitochondria and, in plants and algae, chloroplasts for photosynthesis.

Other terms don’t address how complex cells arose. A law is a rule describing how something works in nature, not a theory about cell origin. Isotonic relates to having equal solute concentrations on both sides of a membrane, which is about cell stability rather than origins. Metabolism refers to the chemical reactions inside living things, not the evolutionary origins of cell structure.