A thylakoid's a membrane-bound compartment inside chloroplasts and cyanobacteria. They're the site of the light-dependent reactions of photosynthesis. The word "thylakoid"'s derived from the Greek thylakos, meaning "sac". Thylakoids consists of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as "grana" (singular: granum). "Grana"'s Latin for "stacks of coins". Grana are connected by intergrana or stroma thylakoids, which joi… (More on Thylakoid)