Thylakoideja

Thylakoideja are membrane-bound compartments inside the chloroplasts of plants and algae and in cyanobacteria, where the light-dependent reactions of photosynthesis take place. They are typically arranged as flattened sacs that form stacks called grana, which are connected by unstacked membranes known as stroma thylakoids.

The thylakoid membranes house the photosynthetic pigment–protein complexes, including photosystem II, the cytochrome b6f complex, plastoquinone and plastocyanin, photosystem I, and ATP synthase. The internal space of a thylakoid, the lumen, is separated from the surrounding stroma by the membrane. The membranes are rich in specific lipids such as monogalactosyldiacylglycerol that support membrane curvature and protein function.

The primary function of thylakoideja is to carry out the light reactions of photosynthesis. Light energy absorbed by chlorophyll drives the oxidation of water at photosystem II, releasing oxygen and electrons. Electrons travel through the electron transport chain, pumping protons into the thylakoid lumen and creating a proton motive force that drives ATP synthesis via ATP synthase. In the stroma, electrons are transferred to NADP+, forming NADPH, which is used in the later stages of carbon fixation.

Organization and variation: In many plants, thylakoideja are organized into grana with stroma thylakoids bridging them; in many algae and cyanobacteria, thylakoids may be more loosely arranged or form multiple membrane systems. Their arrangement can respond to light conditions and developmental cues.

Importance: Thylakoideja are essential for the light reactions of photosynthesis, linking photon capture to the production of ATP and NADPH, which are used in the Calvin cycle to fix carbon.