thylakoid

Thylakoids are membrane-bound, flattened sacs inside the chloroplasts of plants and algae, and in cyanobacteria. They enclose the thylakoid lumen and are the site of the light-dependent reactions of photosynthesis. The thylakoid membrane separates the lumen from the stroma, the fluid surrounding the thylakoids.

In chloroplasts, thylakoids are often arranged into stacks called grana, connected by unstacked regions known as stroma lamellae. The thylakoid membrane contains the photosystems II and I, the cytochrome b6f complex, plastoquinone, plastocyanin, ferredoxin, and ATP synthase, as well as pigment-protein complexes with chlorophyll and carotenoids.

During light reactions, light energy excites chlorophyll in PSII and PSI. Water is oxidized at PSII, releasing electrons, protons, and oxygen. The electron transport chain moves electrons from PSII to PSI, pumping protons into the lumen and creating a proton motive force. ATP synthase uses this gradient to synthesize ATP, while ferredoxin-NADP+ reductase transfers electrons to NADP+, producing NADPH for the Calvin cycle in the stroma.

Thylakoids are derived from ancestral cyanobacteria through endosymbiosis and are present in cyanobacteria in a similar arrangement. Their composition and organization influence photosynthetic efficiency and pigment distribution, and their integrity is essential for proper energy capture and carbon fixation in photosynthetic organisms.