Figure 1.

This figure illustrates the endosymbiotic origin of mitochondria and chloroplasts. α-Proteobacteria, originally phagocytized for their nutrient value by early eukaryotes eventually evolved into mitochondria. Photosynthetic cyanobacteria were likewise phagocytized by eukaryotes and eventually formed chloroplasts. Since plants have both mitochondria and chloroplasts, plant cells generally have higher concentrations of melatonin than do animal cells. Adapted from Reiter et al. (10).