The electron transport chain is used to pump protons into the intermembrane space. This establishes a proton gradient, allowing protons to be pumped through ATP synthase in order to create ATP. This method of ATP production is called oxidative phosphorylation.

The other two metabolic processes, glycolysis and the citric acid cycle, use substrate-level phosphorylation to generate ATP. Substrate-level phosphorylation uses enzymes to create ATP from ADP, while oxidative phosphorylation uses the proton gradient to drive ATP synthase.

NADH and FADH₂ are produced during glycolysis and the citric acid cycle. Electrons from these molecules are donated to proteins of the electron transport chain. The electrons interact with the proteins, helping them push protons into the intermembrane space. This accumulation of protons is what powers ATP synthesis via oxidative phosphorylation. When the electron reaches the final protein of the electron transport chain, it binds with oxygen to produce water.

Pyruvate decarboxylation convert pyruvate to acetyl CoA before the citric acid cycle. The TCA cycle is another name for the citric acid cycle.

The electron transport chain is primarily used to send protons across the membrane into the intermembrane space. This create a proton-motive force, which will drive ATP synthase in the final step of cellular respiration to create ATP from ADP and a phosphate group. This final process is known as oxidative phosphorylation.

The electron is passed through proteins in the electron transport chain. With each subsequent protein, more electrons are transferred to the intermembrane space of the mitochondrion. Though oxygen is the final electron acceptor, generating water from oxygen is not the primary function of the electron transport chain.

As electrons move down the electron transport chain, protein pumps are provided the energy to pump protons into the intermembrane space of the mitochondrion.

The result is a high concentration of protons in the intermembrane space and a low concentration in the mitochondrial matrix. The difference in concentration and charge balance results in an electrochemical gradient, pulling protons into the mitochondrial matrix. ATP synthase is able to use this pulling force to activate enzymatic activity and generate ATP.

This question primarily tests your knowledge of the Electron Transport Chain, which is responsible for the majority of ATP synthesis. The Electron Transport Chain relies on the delivery of electrons from electron carriers. ATP is formed through the movement of protons down their gradient through a ATP synthase protein. The formation of the proton gradient is accomplished via a double membrane in the mitochondria. Lactic Acid is not present in aerobic respiration, instead it is a derivative of fermentation. 

Molecular oxygen——is the final electron accepter in cellular respiration and acts as an oxidizing agent.