Chloroplasts have outer and inner membranes, which are in close association with each other. They also have a closed compartment of stacked membranes—called grana—that lie inside the inner membrane. A chloroplast may contain one hundred or more grana, and each granum (plural for grana) may contain a few to several dozen disk-shaped structures called thylakoids; they, in turn, contain chlorophyll on their surface. The fluid that surrounds the thylakoid is called the stroma.