Although the citric acid cycle does synthesize two ATP per round, its main purpose is to produce NADH for the electron transport chain that makes ATP much more efficiently. Since the electron transport chain is located in the inner mitochondrial membrane, it is most efficient for the cell to produce the NADH in the mitochondrial matrix where it can be used immediately for its purpose, rather than having to use time and resources to transport it there.

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. 

Photosynthesis is the biological process by which plants convert sunlight energy into chemical energy. The main product is the simple sugar glucose. The bonds in glucose can be broken and used for energy by the plants. The formula for photosynthesis is:

The reactants are carbon dioxide, water, and light energy. The products are glucose and oxygen.

Carbohydrates are formed from carbon chains with a single oxygen group (either an aldehyde or a ketone). Carbohydrates have the general empirical formula . This formula can be used to designate a nonspecific carbohydrate molecule.

The process described is the conversion of light energy to chemical energy through photosynthesis. Chlorophyll serves as a functional pigment in this process, but is not involved in the net reaction. Carbonated water is formed from dissolved carbon dioxide in water, and is actually a solution of the two compounds. Carbon hydroxide is a misnomer, and does not refer to a real compound.

Contrary to their assigned numbers, the electron in photosystem II is excited before the electron in photosystem I. Their numbers are a reflection of the order in which they were discovered.