Most multi-cellular organisms obtain energy for the synthesis of ATP during oxidative phosphorylation from

ANS:B - a proton gradient across the inner mitochondrial membrane

During TCA, electron carriers are generated in the cell, examples of this are the conversion of NAD+ to NADH and a proton & FAD to FADH2. These electron carriers can drive the movement of protons from the mitochondrial matrix to the space between the inner and outer membranes of the mitochondria via a chain of associated complexes in the inner mitochondrial membrane, we know this as the Electron transport chain.

Because the inner membrane is not permeable to protons, a electrochemical gradient is generated. The protons may pass back into the mitochondrial matrix via embedded ATPases in the inner mitochondrial matrix. The movement of protons will cause rotation in different subunits, resulting in the conversion of ADP and an inorganic phosphate into ATP.