tilacoidi

Tilacoidi, or thylakoids, are flattened membrane-bound sacs inside the chloroplasts of plants and algae, and within the thylakoid membranes of cyanobacteria. They enclose the thylakoid lumen and host the light-dependent reactions of photosynthesis, forming the functional surface where light energy is converted into chemical energy.

In higher plants and algae, thylakoids are organized into stacks called grana, which are connected by stroma thylakoids (stroma lamellae). This arrangement increases the membrane area available for photosynthetic protein complexes. The thylakoid membrane contains pigments and proteins that make up photosystems I and II, the cytochrome b6f complex, and ATP synthase, along with electron carriers such as plastoquinone and plastocyanin.

During photosynthesis, light reactions take place across the thylakoid membrane. The electron transport chain pumps protons into the thylakoid lumen, creating a proton motive force. ATP synthase uses this gradient to produce ATP, while electrons are transferred to ferredoxin and ultimately to NADP+, forming NADPH.

The internal thylakoid lumen is separate from the chloroplast stroma; its transient acidification is essential for electron transport and energy storage. In cyanobacteria, thylakoid membranes form an extensive internal network within the cytoplasm and may not organize into grana as in plant chloroplasts.

Compositionally, thylakoids are rich in chlorophyll, carotenoids, and galactolipids; their proteins and pigments are organized into light-harvesting complexes that optimize photon capture.