In mitochondrial electron transport, what is the direct role of O2?

A. To oxidize NADH and FADH2 from glycolysis, acetyl CoA formation, and the citric acid cycle

B. To provide the driving force for the synthesis of ATP from ADP and Pi

C. To function as the final electron acceptor in the electron transport chain

D. To provide the driving force for the production of a proton gradient

The correct answer is C: To function as the final electron acceptor in the electron transport chain.

Oxygen is necessary for aerobic cellular respiration to occur. The direct role of oxygen in the electron transport chain is as the final electron acceptor. Therefore, oxygen has to be taken into the cell in order for the process of aerobic respiration to occur.

The oxygen is taken into cells via diffusion through the cell membrane. Animals and plants have thus evolved various mechanisms to take oxygen into the tissues for respiration.

Electron transport chain
Electron transport chain showing oxygen

Oxygen in the electron transport chain also acts to indirectly drive the proton gradient. A proton gradient is a concentration of hydrogen ions that is established across the inner mitochondrial membrane.

This proton gradient is how ATP is generated. In fact, protons move across the membrane through protein channels that are lined with an enzyme ATP synthase.

ATP is formed due to this movement of the protons through the channels. Electrons move down the chain attracted to the oxygen at the end.

This happens because oxygen is very electronegative, which means it attracts atoms to it. At the end of the chain, hydrogen combines with the oxygen to produce water.

Electron transport chain

Glycolysis is the first stage of aerobic respiration, followed by the citric acid cycle. The final stage of respiration is the electron transport chain. This is the stage where oxygen becomes very important.

The electron transport chain occurs across the inner mitochondrial membrane known as the cristae. Two molecules carry electrons from the previous reactions of cellular respiration.

These molecules that carry the electrons are NADH2 and FADH2. These two molecules are both electron carriers that are formed during the earlier stages of respiration, mainly during the citric acid cycle stage of aerobic respiration.

The oxygen molecule occurs at the base of the chain of molecules that make up the electron transport chain. This oxygen acts in an indirect fashion to drive the electrons down the chain of molecules. Furthermore, the overall process is referred to as oxidative phosphorylation by chemiosmotic coupling.

Oxidative phosphorylation

The process is named oxidative phosphorylation since it involves the process of phosphorylation. In phosphorylation, a phosphate group is added to ADP to form ATP.

Protons (hydrogen ions) move through the membrane and trigger ATP synthesis as they do so. Thus the movement of protons across the membrane is coupled with ATP synthesis. This is why the process is called chemiosmotic coupling.

Electrons and protons are carried into the electron transport chain on the molecules of NADH2 and the FADH2. Molecules along the mitochondrial membrane include various cytochromes that act to accept the electrons. In addition, the oxygen molecule at the end of the chain attracts the electrons, and therefore they tend to move along the chain.

As the electrons move along the chain they provide energy which functions to establish the proton gradient. The proton gradient is established because there are different concentrations of hydrogen ions found on either side of the membrane.

In fact, the protons are actually pumped across the membrane into the space that exists between the inner membrane (cristae) and outer membrane of the mitochondrion.

Formation of ATP

The proton gradient is formed because there is a higher concentration of hydrogen ions (protons) in the intermembrane space compared with the concentration of ions in the matrix.

The intermembrane space is the space between the inner and outer mitochondrial membrane. The matrix is the region on the inside of the mitochondrion on the inner side of the cristae.

Protons tend to want to diffuse into the matrix since there are more of them in the intermembrane space than in the matrix. This is because diffusion occurs from a high concentration of a substance to a low concentration of a substance.

The protons diffuse across the membrane through specific channels from a high concentration to a low concentration.

The channels are proteins that are lined with the enzyme called ATP synthase. Other molecules known as factors are also involved in the process.

The movement of the protons through the channel activates the enzyme ATP synthase and therefore activates a reaction. The reaction that is activated is the formation of ATP. The way it works it the enzyme ATP synthase works to combine an inorganic phosphate ion with an ADP molecule.


ADP is adenosine diphosphate which contains two phosphate groups. When a phosphate is added to the ADP it becomes ATP, adenosine triphosphate (3 phosphate groups).

The energy is carried in the phosphate bonds. This means then that the ATP has more energy than the ADP. The phosphate bonds are high energy, which means that adding a phosphate is transferring energy.

This also means that ATP is carrying more energy than ADP since it has three phosphate bonds instead of two phosphate bonds.

ATP is the energy currency of the cell and is needed and used for many reactions. Thus, the formation of ATP is very important in living cells since it provides the energy for numerous reactions.


Oxygen is a very highly electronegative molecule. The degree of electronegativity is a measure of how strongly atoms attract other atoms. As a result, the oxygen tends to attract the electrons that are moving down the electron transport chain.

Oxygen is the final electron acceptor in the electron transport chain, with water being formed at the end of the process. The system is very effective and produces a great deal of energy in the form of ATP.

A total of three ATP molecules are formed for every one pair of electrons that oxygen accepts. Therefore the electron transport system produces a lot of energy.

Without oxygen, cells may need to switch to anaerobic respiration or fermentation. This process does not produce enough ATP, which is the energy molecule of the cell.

However, respiration in the presence of oxygen produces a large quantity of ATP and thus energy. In fact, only a few organisms can respire anaerobically for a long time.

After a while, most organisms will die if they do not have oxygen. This is because most organisms can only respire anaerobically for a short while.


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  2. Editors of Encyclopedia Britannica (2018). Aerobic respiration. Retrieved from
  3. Editors of Encyclopedia Britannica (2018). Adenosine triphosphate. Retrieved from
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