The document discusses the endosymbiotic theory of the origin of chloroplasts. It proposes that chloroplasts evolved from cyanobacteria that were engulfed by but not digested by larger eukaryotic cells. Over time, the cyanobacteria established a symbiotic relationship with the host cell, providing it nutrients through photosynthesis. Evidence that supports this includes chloroplast DNA resembling prokaryotic DNA and chloroplasts dividing in a prokaryotic manner. The theory explains the endosymbiotic origins of other plant cell components like mitochondria.