Internal structures of the chloroplastThe 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 <strong>stroma</strong>, the fluid proteinaceous portion containing the enzymes essential for the photosynthetic dark reaction, or Calvin cycle.