Talk about the chloroplast structure.

A chloroplast is wrapped by a double membrane. Inside, its cavity is filled with a dense liquid (stroma) and it is crossed by membranous sacs called thylakoids. Stacks of thylakoids form grana (singular granum), their membrane define a compartment, the thylakoid space. The thylakoids are interconnected by stromal thylakoid (or intergranal thylakoids)