Granum

Granum (plural grana) is a structural unit within chloroplasts, the organelles responsible for photosynthesis in plants and algae. It is a stack of flattened, membrane-bound sacs called thylakoids. The thylakoid membranes host chlorophyll and the protein complexes of the light-dependent reactions. Grana are interconnected by stroma lamellae, which are thinner thylakoid membranes that run through the chloroplast stroma. The arrangement of grana and stroma lamellae forms an internal membrane system that increases the surface area available for light capture and electron transport.

During photosynthesis, light energy drives electron transport across the thylakoid membranes, creating a proton gradient across the membrane. The gradient powers ATP synthase to produce ATP, and electrons are transferred to NADP+, forming NADPH. In many plants, photosystem II and the associated electron transport complexes are concentrated in the grana membranes, while photosystem I and ATP synthase are more abundant in the stroma lamellae. Water splitting occurs at the oxygen-evolving complex of PSII, releasing oxygen.

Granum size and the number of thylakoids per stack vary among species and developmental stages. Etymology: granum derives from the Latin granum, meaning grain.