The endomembrane system is composed of the various membranes inside the cytoplasm within cells that have nuclei (which are called "eukaryotic" cells). The membranes divide the cell into functional and structural compartments, the latter being called organelles. The endomembrane system does not include the membranes of mitochondria or chloroplasts because these objects have separate DNA and at one time were actually parasitic bacteria that evolved into symbionts and are not integral to the cell. This amazing realization came to Lynn Margolis, a scientist who was married at one time to Carl Sagan! The evolution of the cell is a fascinating tale!