thylakoide

Thylakoide, also called thylakoid, is a membrane-bound compartment within chloroplasts of plants and algae, and within cyanobacteria. They form the internal photosynthetic membrane system responsible for the light-dependent reactions of photosynthesis.

In higher plants, thylakoids are flattened sacs arranged into stacks called grana, which are connected by stroma lamellae. The grana increase membrane surface area, improving light capture. The thylakoid membrane encloses a continuous lumen, the internal space between the two membranes, while the surrounding chloroplast stroma constitutes the rest of the organelle.

Thylakoid membranes house the photosystems, II and I, together with an electron transport chain composed of plastoquinone, the cytochrome b6f complex, and plastocyanin, as well as ATP synthase and various light-harvesting antennae. They contain pigments such as chlorophyll a and b and carotenoids, which absorb light and funnel energy to reaction centers.

Light-driven reactions take place in the thylakoid membranes. In PSII, water is split, releasing oxygen into the lumen and protons into the lumen, contributing to the proton motive force. Electrons travel through the chain to PSI, transfer energy to ferredoxin, and finally reduce NADP+ to NADPH. The proton gradient across the membrane drives ATP synthase, producing ATP that is used in the Calvin cycle in the stroma.

Thylakoid membranes are thought to have originated from the photosynthetic membranes of ancient cyanobacteria via endosymbiosis and are present in contemporary cyanobacteria as well as in the chloroplasts of algae and plants. The term derives from Greek thylakos (sac) and eidos (form).