FIG. 27.4

What is the evolutionary origin of chloroplasts?

HYPOTHESIS Chloroplasts evolved from cyanobacteria living as endosymbionts within a eukaryotic cell.

OBSERVATION Chloroplasts and cyanobacteria have closely similar internal membranes that organize the light reactions of photosynthesis. Transmission electron microscopy shows that cyanobacteria (top), red algal chloroplasts (middle), and plant chloroplasts (bottom) have very similar internal membranes that organize the light reactions of photosynthesis.

EXPERIMENT Like cyanobacteria, chloroplasts have DNA organized in a single circular chromosome. Phylogenies based on molecular sequence comparisons place chloroplasts among the cyanobacteria.

CONCLUSION Molecular and electron microscope data support the hypothesis that chloroplasts originated as endosymbiotic cyanobacteria.

SOURCE Giovannoni, S. J., et al. 1988. “Evolutionary Relationships Among Cyanobacteria and Green Chloroplasts.” Journal of Bacteriology 170:3584–3592. Photos (top to bottom): Dr. Kari Lounatmaa/Science Source; Biology Pics/Getty Images; Dr. Jeremy Burgess/Science Source.