What is the role of ADP in the electron transport chain?

ADP, or adenosine diphosphate, plays a crucial role in the electron transport chain by serving as a substrate that gets converted into ATP, or adenosine triphosphate. In the process of oxidative phosphorylation, which occurs in the mitochondria, the energy released from the transfer of electrons through protein complexes in the electron transport chain is used to generate ATP from ADP and inorganic phosphate (Pi).

As electrons move through the chain, energy is released, which pumps protons (H+) across the inner mitochondrial membrane, creating a proton gradient. This gradient generates a form of potential energy. ATP synthase, an enzyme located in the membrane, then harnesses this proton motive force to synthesize ATP from ADP and Pi. This process is essential for cellular energy supply, as ATP serves as the primary energy currency of the cell, powering various biological processes.

The other options do not accurately describe the role of ADP in the electron transport chain, as it does not transport electrons, carry carbon dioxide, or act as a catalyst for reactions in this context. The conversion of ADP into ATP is fundamental for cellular respiration and energy production.