Final answer:
The main reservoir for protons that are pumped across the membrane in the electron transport chain is the intermembrane space in eukaryotic cells. The accumulation of protons in this space creates an electrochemical gradient that is used to produce ATP through ATP synthase.
Step-by-step explanation:
In the electron transport chain, the main reservoir for protons that are pumped across the membrane is the space between the inner and outer mitochondrial membranes or the intermembrane space in eukaryotic cells. It is primarily created by the accumulation of protons due to the movement of electrons down the electron transport chain. This uneven distribution of protons establishes an electrochemical gradient, which is utilized by ATP synthase to produce ATP.