Thylakoid

Interior to plant cells and other Eukaryotic cells that carry out photosynthesis, tiny, bacteria-sized organelles, called chloroplasts, contain, within the inner membrane of their dual membrane structure, the early photosynthesizing apparatus, an extensive system of membrane-bound disk-shaped sacs called thylakoids, the interior spaces (lumens) of the sacs possibly all interconnected, the membranes of the sacs housing the pigment molecules that absorb energy of photons of particular energy frequencies emitted by the sun, thereby initiating the physico-chemical sequence of steps in the photosynthesizing process. A semi-fluid matrix called the stroma surrounds the thylakoids.

In general terms, in the familiar example of green plants, 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: click Gallery tab in the banner at the top of this page.