grana

Grana, singular granum, are stacks of thylakoid membranes located inside chloroplasts of plants, algae, and some photosynthetic organisms. Each granum consists of several flattened thylakoid discs arranged in a stack, and a chloroplast may contain many grana. The grana are interconnected by stroma lamellae, thinner thylakoid membranes that bridge adjacent stacks. Together with these lamellae, the grana form the chloroplast’s thylakoid network, the site of the light-dependent reactions of photosynthesis.

Functionally, grana increase the surface area of the thylakoid membranes, providing a concentrated environment for pigment–protein complexes involved in photosynthesis. Within the grana, most of the photosystem II (PSII) complexes, along with the cytochrome b6f complex and associated light-harvesting antennae, reside to capture light energy efficiently. Photosystem I and ATP synthase are more abundant in the stromal lamellae and grana margins, helping facilitate electron transport and ATP production. The proton gradient generated across the thylakoid membrane drives ATP synthesis in the lumen.

The organization of grana supports efficient photosynthetic electron transport by segregating components and enabling rapid transfer of electrons between PSII and the downstream carriers via the connected thylakoid system. Grana size and number can vary with developmental stage, light exposure, and species, reflecting adaptations to different photosynthetic demands.

Etymology and context: The term grana comes from Latin granum, meaning grain. In higher plants and many algae, grana are a defining feature of chloroplast ultrastructure and photosynthetic function.