Tilakoids

Tilakoids are membrane-bound compartments found inside chloroplasts and cyanobacteria. They are the sites of the light-dependent reactions of photosynthesis. In chloroplasts, tilakoids are often arranged in stacks called grana, though they can also exist as single, unstacked lamellae. The thylakoid membrane itself contains chlorophyll and other pigments, as well as electron transport chains and ATP synthase, all crucial for converting light energy into chemical energy in the form of ATP and NADPH.

The internal space enclosed by the thylakoid membrane is called the lumen. Protons accumulate in the lumen during the light-dependent reactions, creating a proton gradient that drives ATP synthesis. The structure of tilakoids is highly organized to maximize the efficiency of light capture and energy conversion. The pigments are arranged in photosystems, which absorb light energy and transfer it to reaction centers. This energy is then used to split water molecules, releasing oxygen, and to energize electrons that move through the electron transport chain.