The thylakoid is a membranous cellular structure containing the photosynthetic pigments, reaction centers and electron-transport chain. In chloroplast, thylakoids stack up to form the grana or stay as single cisternae and interconnect the grana. Thylakoid, where photosynthesis occurs, are found in chloroplasts, cyanelles and in photosynthetic bacteria where they are the extensive invaginations of the plasma membrane.
