The chloroplast like the mitochondrion is a double membrane structure with a looped strand of DNA and ribosomes in the gel like solution within the inner membrane. Unlike the mitochondrion there are extensive membrane structures inside the inner membrane, these are the thylakoid membranes. At intervals along their length the thylakoid membranes are stacked in tight layers with chlorophyll molecules trapped between them. The chlorophyll molecules are arranged in groups of about 200 and there are other associated molecules around them. The groups of chlorophyll molecules are referred to as quantasomes or photosystems. The stroma contains all of the enzymes and substances required to run the light independent stage of photosynthesis, while the light dependent stage occurs in the grana. Also present in the stroma (but not shown on the diagram) one may find storage substances such as starch grains or lipid droplets. The stroma has ribosomes and can manufacture many, but not all of the proteins needed for its photosynthetic role within the cell.