The stacks of membranous sacs found in some chloroplasts are called grana. These stacks are made up of thylakoids, which are tiny hollow discs where photosynthesis actually takes place. Grana are joined together by bridging thylakoids, which are called lamellae.
Grana, together with the bridging lamellae, form structural units within the chloroplast. They are able to change and respond in response to light conditions, optimizing energy creation. Grana only appear in land plants; algae do not possess them. In algae, the thylakoids are arranged laterally, or one next to the other on a single plane, rather than in stacks. Grana appear to be one solution to the problem of separating thylakoids performing different photosynthetic chemical reactions.