Oxygen acts as the terminal electron acceptor in the electron transport chain (ETC). This accounts for the reason as to why, when cells are starved of oxygen, the ETC "backs up" and the cell will divert to using anaerobic respiration, such as fermentation. At the end of the electron transport chain, the electron and a proton are passed to an oxygen molecule to produce water.

The citric acid cycle depends on oxygen in an indirect sense. The main purpose of the cycle is to produce electron donors for the electron transport chain. If the chain is not functional (due to lack of oxygen), the citric acid cycle also stops functioning. Glycolysis is not dependent on oxygen, and can function in anaerobic environments.

In ATP synthesis, the proton gradient is an interconvertible form of energy in electron transport. 2,4-dinitrophenol is an inhibitor of ATP production in cells with mitochondria. Its mechanism of action involves carrying protons across the mitochondrial membrane, which leads to the consumption of energy without ATP production.

The other answer choices are not directly related to the generation of the proton gradient.

Cellular respiration is only about 38% efficient, with the rest of the energy in glucose lost as heat.

Water and carbon dioxide are not used to store energy. Fats can be synthesized from acetyl CoA and glycerol, but are not generally created in large quantities during cellular respiration. Starches are generally used for energy storage in plants, but can be synthesized from glucose; however, starches are not a standard product of cellular respiration.

Most of the reactions in cellular respiration are exothermic, in order to support spontaneous reaction. The result is release of heat energy with most steps.

The events of the electron transport chain take place on the inner membrane of the mitochondria. The transmembrane proteins used to shuttle electrons through the electron transport chain are embedded on the inner membrane. Electrons are donated to these proteins and used to transfer protons into the intermembrane space from the matrix. After reaching the final inner membrane protein in the chain, the electron is transferred to oxygen to form water.

The mitochondrial matrix is where the ATP eventually is eventually synthesized, as well as the site of the citric acid cycle. The cytoplasm is the site of glycolysis. The outer mitochondrial membrane is not directly involved in cellular respiration.

NADH and FADH₂ are electron carriers that have the important function of actually bringing electrons to the electron transport chain. Proteins embedded in the inner membrane of the mitochondria oxidize these molecules. The proteins then transfer the electrons through a series of processes in order to pump protons into the intermembrane space, creating an electrochemical gradient. The final protein in the chain passes the electron to an oxygen molecule to generate water, and the protons in the intermembrane space can then be used to drive the function of ATP synthase to create ATP/

NADH and FADH₂ are not directly involved in ATP synthesis and oxygen is the ultimate electron acceptor in the electron transport chain.

ATP synthase is an enzyme that facilitates the generation of energy (ATP) in cells. It uses the proton gradient created by the electron transport chain to create ATP through oxidative phosphorylation. ATP synthase is an integral membrane protein in the inner membrane of mitochondria. Recall that all membranes are mostly made up of phospholipids (a type of lipid).

When electrons go through the electron transport chain, the protons in the matrix of the mitochondrion are pumped into the intermembrane space (the space between inner and outer membranes). This creates a proton gradient that is used by ATP synthase to create ATP through oxidative phosphorylation, not substrate-level phosphorylation. Remember that substrate-level phosphorylation is used by glycolysis and the Krebs cycle to generate ATP.

When electrons travel down the series of molecules in the electron transport chain they go from molecules of low electron affinity to molecules high electron affinity. The next molecule in the series must have higher affinity so that it can pull the electron away from its predecessor.

Electrons from electron carriers, such as NADH and FADH₂, go through the electron transport chain, which involves a series of molecules that accept and donate electrons. Transfer to the electron through these proteins results in the net movement of protons across the inner mitochondrial membrane and into the intermembrane space, generating the proton gradient that will drive ATP synthase.

The final molecule in the electron transport chain is oxygen. The oxygen molecule accepts the electron from the final protein in the chain and becomes water, one of the final products of metabolism. Remember that each subsequent molecule in the electron transport chain has a higher affinity for electrons than the molecule before it; therefore, the final electron acceptor will have the highest affinity for electrons. Oxygen has a very high electronegativity, making it a good electron acceptor.

The direct purpose of moving electrons down the electron transport chain is to pump protons (hydrogen ions) into the intermembrane space. This creates a chemiosmotic gradient that the cell uses to generate ATP by selectively allowing hydrogen ions to move back into the mitochondrial matrix.

Energy is not directly captured as electrons travel down the electron transport chain to synthesize ATP. GTP is a product of the Krebs cycle and can be used to generate cellular energy, but is not involved in synthesizing ATP or the electron transport chain. Other metabolic processes are often used to regulate glucose concentrations in the blood, indirectly influencing the rate of glycolysis and cellular respiration, but these processes do not directly provide energy for the electron transport chain.

ATP synthase, the enzyme responsible for ATP production on the inner mitochondrial membrane, depends on the proton gradient produced by the electron transport chain (ETC). If the proton gradient is disrupted, not as many ATP can be produced.

NADH and FADH₂ are essential to the function of the electron transport chain as electron donors, and are produced during glycolysis and the Krebs cycle to facilitate this process. Electron donation from these compounds is what fuels the formation of the proton gradient, while decreases in these compounds can cause uncoupling.