Stroma is the fluid-filled internal space of chloroplasts that surrounds thylakoids and grana. It is a colorless matrix that contains dissolved enzymes, starch granules, and copies of the chloroplast genome. The stroma is made of an outer membrane and a complex network of inner membranes that go on to form the grana, which are disc-like structures arranged in a stack. Membranous extensions connect various grana together. The stroma contains chloroplast DNA, starch, and ribosomes, along with enzymes needed for the Calvin cycle, which is a light-independent reaction of photosynthesis. The primary role of the stroma in chloroplasts is to provide volume around the thylakoids and grana. It also contains many metabolic enzymes, and like the mitochondrion, the chloroplast has its own genome and genetic system. The stroma, therefore, also contains a special set of ribosomes, RNAs, and the chloroplast DNA.