Primary endosymbiosis, illustration. Primary endosymbiosis occurs when a eukaryotic cell (purple), which already has membrane-bound organelles, engulfs a prokaryotic cell (blue). During the engulfing process the prokaryote becomes surrounded by the eukaryote's membrane, forming a double membrane. It is thought that chloroplasts, the site of photosynthesis, in plants and algae were formed from photosynthetic bacteria that were engulfed in this way. For this image with text see F034/7128.