The endosymbiont theory suggests that mitochondria and chloroplasts evolved from ancient bacteria that were ingested by primitive nucleated cells. Biologists have proposed that endosymbiosis led to the evolution of other cellular structures, such as flagella and peroxisomes, as well. Over hundreds of millions of years, the ingested bacteria lost features not essential for survival inside the host cell. Part A Based on what you have learned about the membrane systems of organelles, for each of the membrane systems found in mitochondria (two systems) and chloroplasts (three systems), indicate whether it most likely arose from an ingested bacterium or from the nucleated host.