thylakoidit

Thylakoids, called thylakoidit in Finnish, are membrane-bound sacs inside chloroplasts and certain cyanobacteria that carry out photosynthesis. They enclose a continuous interior space known as the thylakoid lumen and are the site of the light-dependent reactions of photosynthesis.

In higher plants and green algae, thylakoids stack into grana, connected by stroma lamellae. The thylakoid membrane hosts the photosynthetic apparatus, including photosystem II, the cytochrome b6f complex, photosystem I, ATP synthase, and various electron carriers. The grana and surrounding stroma regions create distinct environments that facilitate different steps of the electron transport chain and ATP synthesis.

Functionally, light energy excites chlorophyll in antenna complexes, transferring energy to reaction centers and driving an electron transport chain. This process pumps protons into the lumen, establishing a proton gradient that powers ATP synthase to produce ATP, while electrons reduce NADP+ to NADPH in the stroma. These energy-rich compounds fuel the Calvin cycle in the chloroplast stroma.

The thylakoid membrane has a specific lipid and protein composition, rich in galactolipids such as monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). It contains pigments, primarily chlorophylls and carotenoids, and a variety of protein complexes that form photosystems, light-harvesting complexes, and the electron transport chain.

Thylakoids occur in chloroplasts of plants and algae and in the photosynthetic membranes of cyanobacteria. Their organization varies among groups, but their proper function is essential for oxygenic photosynthesis and the global carbon cycle, reflecting an ancient endosymbiotic origin.