The hypothesis stating that chloroplasts were derived from a photosynthetic procaryote is explored at a genetic and biochemical level. A transfer of genetic material from the endosymbiont to the nucleus of the host cell is proposed along with a corollary argument that the protein products of such transferred genes have remained specific to the chloroplast. This model provides an explanation for the presence of plastid-specific isozymes which are coded by nuclear DNA. It also suggests that the genome of the endosymbiont contributed the information necessary for the biosynthesis of carotenoids and the "essential" amino acids and the assimilation of nitrate-nitrogen and sulfate-sulfur. Animal cells lack these capabilities not because such were lost subsequent to the divergence of the plant and animal lines, but because animal cells did not become host to the appropriate symbionts. Additional implications of this thesis are discussed.