Tilacoids

Tilacoids, often spelled thylakoids in English, are membrane-bound sacs inside chloroplasts and cyanobacteria. They house the photosynthetic light reactions and enclose the thylakoid lumen. In photosynthetic plant cells, tilacoids are typically organized into grana stacks interconnected by stroma thylakoids.

Each tilacoid membrane contains pigment–protein complexes, including photosystems II and I, the cytochrome b6f complex, ATP synthase, and light-harvesting antennae. The membrane forms a proton gradient when illuminated, pumping protons into the thylakoid lumen.

Water-splitting at photosystem II releases electrons and oxygen; electrons move through plastoquinone and the cytochrome b6f complex to plastocyanin and then to photosystem I, where they reduce ferredoxin and NADP+ to NADPH. The proton motive force across the membrane drives ATP synthesis.

The internal aqueous compartment, the thylakoid lumen, and the surrounding stroma provide distinct environments that regulate photosynthetic chemistry. The thylakoid system is arranged to maximize light capture in plants, with grana and stroma thylakoids supporting efficient energy conversion. Some algae and cyanobacteria exhibit variations in thylakoid organization.

Tilacoids are derived from the chloroplast envelope during evolution and are homologous to cyanobacterial internal membranes. Their composition includes pigments such as chlorophyll a and b, and accessory carotenoids, which participate in light absorption and protection against photodamage.