The endosymbiotic theory states that plastids of green plants are derived from a single primary endosymbiosis involving a eukaryote and a cyanobacterium.Peptidoglycans(PGs)are continuous covalent macromolecular structures found on the outside of the cytoplasmic membrane of almost all eubacteria,which protect bacterial cells from osmotic pressure,maintain the unique shapes of bacterial cells,and are involved in cell division.