thylakoider

Thylakoiders, also known as thylakoids, are membrane-bound compartments inside chloroplasts of plants and algae and within cyanobacteria. They enclose the thylakoid lumen and host the light-dependent reactions of photosynthesis. The internal membrane system forms flattened sacs that are often arranged in stacked grana linked by stroma lamellae. The thylakoid membrane contains the major protein complexes of the photosynthetic electron transport chain, including photosystem II (PSII), the cytochrome b6f complex, photosystem I (PSI), and the ATP synthase complex. The lumen is the aqueous space inside the thylakoids; protons pumped across the membrane during electron transport create a proton motive gradient that drives ATP synthesis.

In plants and algae, the thylakoid membranes also hold light-harvesting antenna pigments such as chlorophyll a, chlorophyll b, and carotenoids, which enable capture of light energy. The thylakoid membrane system is organized into two main domains: the grana, which are stacks of thylakoid discs, and the stroma lamellae, which connect grana. This organization optimizes the efficiency and distribution of excitation energy between PSII and PSI and allows regulation of electron flow.

Functionally, thylakoiders are the primary site of the light-dependent reactions of photosynthesis. Water splitting at PSII releases oxygen, electrons are transported through the chain, a proton gradient is generated across the membrane, ATP is synthesized by ATP synthase, and electrons are ultimately transferred to ferredoxin and NADP+, forming NADPH in the stroma.

In cyanobacteria, thylakoids form internal membrane networks that serve a similar role, although their organization is not within chloroplasts. The thylakoid system is evolutionarily linked to cyanobacterial membranes and is essential for energy capture and carbon fixation in photosynthesis.