Thylakoidmembraner

Thylakoid membranes are internal membranes of chloroplasts in plants, algae, and cyanobacteria. They form a network of flattened sacs called thylakoids, which are often stacked into grana and linked by stroma lamellae. The membranes enclose the thylakoid lumen.

The thylakoid membrane is a lipid bilayer rich in galactolipids such as monogalactosyldiacylglycerol and digalactosyldiacylglycerol, with sulfolipids and minor phospholipids. It hosts large protein complexes of the photosynthetic electron transport chain, including photosystem II, photosystem I, cytochrome b6f, and ATP synthase, as well as small mobile carriers like plastoquinone and plastocyanin. The lumen and the stroma sides have differing protein compositions.

Functionally, the thylakoid membranes carry out the light-dependent reactions of photosynthesis. Light energy drives water splitting at photosystem II, releasing oxygen and electrons that pass through the electron transport chain. Protons are pumped into the lumen, creating a proton motive force that drives ATP synthesis by ATP synthase; electrons eventually reduce NADP+ to NADPH in the stroma via ferredoxin and ferredoxin-NADP+ reductase.

Organization and dynamics characterize thylakoid membranes. In many plants, thylakoids form grana, stacks that maximize light-harvesting area, connected by stroma lamellae. The architecture is dynamic and can rearrange in response to light conditions, affecting energy balance and photoprotection. The thylakoid lumen participates in pH-dependent regulation and processes like the xanthophyll cycle.

Significance and study: Thylakoid membranes are central to photosynthetic bioenergetics. They are isolated for biochemical and structural analyses using techniques such as electron microscopy, spectroscopy, and cryo-EM to illuminate the workings of photosystems and related complexes.