Thylakoid

Inside plant cells and other eukaryotic cells that perform photosynthesis, tiny, bacteria-sized organelles, called chloroplasts, contain, within the inner membrane of their dual membrane structure, an extensive system of single-membrane-bound disk-shaped sacs called thylakoids, their interior spaces (lumens) interconnected, their membranes housing the pigment molecules that absorb the energy of photons of particular frequencies emitted by the sun, an event that initiates the physico-chemical sequence of steps culminating in the products of photosynthesis.

In prokaryotic cells that carry out photosynthesis (e.g., cyanobacteria), the thylakoid membranes are not contained within chloroplasts, but are layered in the cytoplasm.

In general terms, the photosynthetic process enables the energy of photons radiated from the sun to energize electrons in special photon-absorbing pigment molecules embedded in the thylakoid membranes, electrons ultimately supplied by the splitting of water molecules in a reaction that also converts water's oxygen atoms to molecular oxygen for release into the atmosphere and for use by the plant.

The thylakoid membranes also contain molecular clusters that transfer the energy of the energized electrons to energy-carrier molecules, the energy therein used in metabolic reactions in the stroma to synthesize organic compounds using the inorganic carbon compound, carbon dioxide, as the carbon source starting material.

(CC) Diagram: Emmanuel Boutet
Schematic of a chloroplast: 1. outer membrane; 2. intermembrane space; 3. inner membrane (1+2+3: envelope); 4. [stroma (aqueous fluid); 5. thylakoid lumen (inside of thylakoid); 6. thylakoid membrane; 7. granum (stack of thylakoids); 8. thylakoid (lamella); 9. Starch; 10. Ribosome; 11. plastidial DNA; 12. plastoglobule (drop of lipids).

The Internet has numerous images of thylakoids of particular instructive value. You will find links to those images on the Gallery subpage.