Internal structures of the chloroplast. The interior contains flattened sacs of photosynthetic membranes (thylakoids) formed by the invagination and fusion of the inner membrane. Thylakoids are usually arranged in stacks (grana) and contain the photosynthetic pigment (chlorophyll). The grana are connected to other stacks by simple membranes (lamellae) within the stroma, the fluid proteinaceous portion containing the enzymes essential for the photosynthetic dark reaction, or Calvin cycle.
chloroplast, Microscopic, ellipsoidal organelle in a green plant cell. It is the site of photosynthesis. It is distinguished by its green colour, caused by the presence of chlorophyll. It contains disk-shaped structures called thylakoids that make possible the formation of ATP, an energy-rich storage compound.