The endosymbiotic theory explains the origin of which cellular organelles?

• A. Nucleus and endoplasmic reticulum
• B. Golgi apparatus and lysosomes
• C. Chloroplasts and mitochondria
• D. Ribosomes and vacuoles

Answer: (C)

The key idea is that some cellular components originated when a primitive cell engulfed free-living bacteria, leading to a long-term partnership. Chloroplasts and mitochondria fit this idea best because they carry their own circular DNA, have ribosomes similar to those of bacteria, and are enclosed by double membranes. They also replicate by splitting on their own, not just by the division of the whole cell, and their genetic material aligns with bacterial lineages (chloroplasts with cyanobacteria, mitochondria with proteobacteria). This combination of features strongly supports their origin through endosymbiosis.

Other organelles don’t share these bacterial traits. They’re explained more by membrane infolding or vesicle trafficking within the cell, rather than a former free-living organism becoming an integrated part of the cell. So, chloroplasts and mitochondria are the organelles whose origin is best explained by the endosymbiotic theory.