Lumen
The lumen refers to the inner fluid-filled space of a Thylakoid in the Chloroplast. It plays a key role in the Light-Dependent Reactions of Photosynthesis, particularly in the generation of a Proton Gradient that drives the synthesis of ATP.
-
Role in Photophosphorylation: During the Light-Dependent Reactions, as electrons travel through the Electron Transport Chain (ETC), protons (H+) are pumped from the Stroma into the lumen. This builds a high concentration of protons in the lumen compared to the stroma, creating an electrochemical gradient. The potential energy in this gradient is used by ATP Synthase to produce ATP as protons flow back into the stroma.
-
Photolysis: In Photosystem II, the splitting of water molecules (Photolysis) occurs in the lumen. Water is broken down into oxygen, protons, and electrons. The electrons replenish those lost by PSII, and the protons contribute to the proton gradient across the Thylakoid membrane. Oxygen is released as a byproduct.
The lumen’s role in establishing proton gradients is essential for the efficient production of ATP and NADPH, which are used in the Calvin Cycle to synthesize sugars.