thylakoidal

Thylakoidal is an adjective relating to thylakoids, the membrane-bound compartments inside chloroplasts of plants and algae, and in some bacteria such as cyanobacteria. Thylakoids are internal membranes that form flattened sacs and often organize into stacked structures called grana, connected by stroma lamellae. The thylakoid membrane encloses the thylakoid lumen and houses most of the pigment–protein complexes essential for photosynthesis, including photosystems I and II, the cytochrome b6f complex, electron carriers, and ATP synthase.

Thylakoidal membranes are the site of the light-dependent reactions of photosynthesis. Light energy absorbed by chlorophyll and other pigments drives electron transfer through the photosystems and associated carriers, creating a proton gradient across the thylakoid membrane. This proton motive force powers ATP synthesis, while electrons ultimately reduce NADP+ to NADPH. Water splitting at Photosystem II releases protons, electrons, and molecular oxygen, contributing to the proton gradient and the chemical energy captured in ATP and NADPH.

The term thylakoidal is commonly used to describe features, processes, or structures related to thylakoids, such as thylakoidal membranes, the thylakoid lumen, or thylakoid-associated proteins. In cyanobacteria, thylakoid membranes serve a similar photosynthetic role, though their organization differs from that found in plant chloroplasts. Overall, thylakoidal structures are central to the light reactions that convert light energy into chemical energy in photosynthetic organisms.