The final electron acceptor in electron transport chain is oxygen. The electrons traverse along the electron transport chain and ultimately end up being taken up by an oxygen atom. Upon accepting electrons, the oxygen atom becomes negatively charged and attracts protons (hydrogen atoms). An oxygen atom binds to two hydrogen atoms and forms water.

Energy production in ETC is facilitated by the generation of the proton gradient by the proton pump. This gradient is utilized by ATP synthase to produce ATP. Oxygen does not play a role in this process. Anaerobic respiration is a type of respiration that occurs in the absence of oxygen. Examples of anaerobic respiration include glycolysis and fermentation.

Cellular respiration has three main processes: glycolysis, Krebs cycle, and electron transport chain. Glycolysis is an anaerobic process whereas Krebs cycle and ETC are aerobic processes. A molecule of glucose enters the cell and undergoes glycolysis in the cytosol. Some of the products of glycolysis are transported to mitochondria where they undergo Krebs cycle and then, eventually, the electron transport chain; therefore, anaerobic respiration occurs in the cytosol and aerobic respiration occurs in the mitochondria.

Anaerobic respiration produces very little ATP and the ATP produced is not sufficient to power all of the active processes in a cell. Aerobic respiration, particularly ETC, produces a lot of ATP. This is why mitochondria is called the “powerhouse” of the cell. Note that the products of glycolysis can undergo another type of anaerobic respiration called fermentation. This also produces very little ATP.

Electron transport chain is a series of electron carriers located on the inner membrane of the mitochondria. Electrons traverse across these electron carriers and this motion allows for the proton pump to generate a proton gradient across the inner membrane. This gradient is generated by pumping protons from the inside of the mitochondria to the periplasmic space (space between inner and outer mitochondrial membranes). The excess protons in the periplasmic space are transported back into the mitochondria and this movement facilitates the generation of ATP by the ATP synthase. Note that ATP is generated by ATP synthase, not the proton pump.

 and  are electron carriers that carry electrons from glycolysis and Krebs cycle. These carriers enter the ETC and donate their electrons to the carriers in ETC. This occurs because ETC carriers have higher affinity for electrons. Remember that each subsequent carrier in ETC has a higher affinity for electrons, so that it is able to snatch the electron from the previous carrier. As mentioned, the ATP generation is facilitated by the proton gradient, not the sodium gradient.

The correct answer is proton gradient.  and  donate electrons to electron transport chain complexes and pass them along the membrane, causing protons  to move from the mitochondrial matrix to the intermembrane space. The high proton concentration gradient causes protons to travel through the inner mitochondrial transmembrane ATP synthase to equilibrate the gradient. Passing of protons through ATP synthase promotes synthesis of ATP from ADP from inorganic phosphate. 

Cytochrome C is responsible for accepting the electrons generated in the bc1 complex (complex III) and transferring them over to complex IV. In other words, cytochrome C oxidizes complex III and is oxidized by complex IV. This is required to then add those electrons to molecular oxygen, which forms water and contributes to the proton gradient required for ATP production. 

The final electron acceptor in the electron transport chain is oxygen gas. one oxygen gas molecule will accept 4 electrons and combine with 4 protons in order to create 2 water molecules. This reaction is the reason water is a byproduct of aerobic cellular respiration.

Glycolysis, the first step in metabolism of carbohydrates, occurs in the cytoplasm. The products of glycolysis (pyruvate and NADH) are transported to the mitochondrial matrix. The products undergo series of reactions called the Krebs cycle. The products of Krebs cycle and NADH from glycolysis enter the inner mitochondrial membrane and go through the electron transport chain (ETC). During ETC, oxidative phosphorylation generates most of the ATP used by the cells.

Reduction is the process of gaining electrons. In electron transport chain (ETC), electron carriers such as NADH and  donate electrons to the electron carriers in the ETC. These electrons are transported to subsequent molecules. The final acceptor of electrons in ETC is oxygen, which accepts electrons and gets converted into water. Since electrons are being lost from them, NADH and  are oxidized in the ETC. On the other hand, oxygen accepts electrons and is reduced in the electron transport chain.

The question states that oligomycin inhibits ATP synthase. Recall that ATP synthase (found on the inner mitochondrial membrane) generates ATP by transporting protons from the intermembrane space (space between inner and outer mitochondrial membrane) into the mitochondria. Inhibiting this will prevent the transport of protons and will, subsequently, lead to a buildup of protons in the intermembrane space.

Proton pumps are also found on the inner mitochondrial membrane. They function to pump out protons from the inside of mitochondria to the intermembrane space, thereby providing the proton gradient for ATP synthase to generate ATP. Halting ATP synthase will cause proton pump to stop pumping protons into the intermembrane space (due to the increase in protons in intermembrane space).

ATP synthase is the major generator of ATP; therefore, halting ATP synthase via oligomycin will decrease the amount of ATP generated.

The F0 subunit of ATP synthase is where protons flow through to create ATP. This mechanism involves a rotation of the subunit, producing ATP with each turn. ATP synthase is part of oxidative phosphorylation, the greatest ATP producing segment of cellular respiration.