# Fundamentals of Human Nutrition/Electron transport chain

The electron transport chain is one of the final stages of energy metabolism. It is an aerobic, or oxygen dependent, process of energy metabolism (Whitney & Rolfes, 2015). It is composed of mitochondrial enzymes, or carriers, that move electrons from one complex to the next, resulting in the production of ATP (“Electron Transport Chain Definition,” 2004). The electron transport chain gets the hydrogen ions (protons) and high energy electrons from the coenzymes NADH and FADH2 that are end products of Glycolysis, Lipolysis, The Citric Acid (TCA) Cycle, and the process of converting pyruvate to Acetyl-CoA. NADH and FADH2 act as electron donors which give up an electron, as well as their hydrogen ions (H+), to power the synthesis of Adenosine Triphosphate (ATP) (Alberts et al., 2002; Whitney & Rolfes, 2015).

When the coenzyme approaches the inner membrane from the inside, it is oxidized and donates an electron to the electron carrier (“Electron Transport Chain,” 2007). When the molecule is oxidized, a hydrogen ion that is removed from the molecule can be dissociated into proton and electron, allowing them to be passed separately through the chain. The electron carrier then picks up a H+ which is then passed to the outside compartment of the mitochondria as the electron moves to the next carrier. The passing of electrons through the carriers provides energy that is used to create an electrochemical proton gradient which pumps the H+ across the inner membrane (Alberts et al., 2002). This process continues to happen until the electron reaches oxygen.

The oxygen then accepts the electron and combines with H+ from within the inner compartment to form water. This can be seen with the following equation:

2H+ + 2e- + ${\displaystyle {\tfrac {1}{2}}}$ O2 → H2O

The reason that the electrons and hydrogen ions are not donated directly from NADH and FADH2 to the oxygen molecules is because there would be too much of a free energy drop where the reaction would release nearly all of the energy as heat. To prevent this, the cells gradually move the electrons across electron carriers in the inner membrane. By having the production of water be more gradual, more of the energy can be stored rather than released into the environment as heat (Alberts et al., 2002).

With the inner concentration of H+ decreasing with the formation of H2O, the H+ ions in the outer compartment are then pumped through an ATP Synthase complex (Whitney & Rolfes, 2015).. The H+ ions are moving from an area of greater concentration to an area of lesser concentration (“Electron Transport Chain,” 2007). This process powers the synthesis of ATP as it adds energy to Adenosine Diphosphate (ADP) and a phosphate group (Pi), a process that is called phosphorylation. This is represented by the formula below:

The energy in creating these bonds is then captured and the ATP leaves the mitochondria to go to the cytoplasm, where the energy is then used. The electron transport chain produces the most ATP out of all of the steps in energy metabolism with a net production of 32 (“Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways,” n.d.). NADH produces more ATP as each NADH contains more energy than each FADH2. Due to the lower energy, FADH2 does not contribute to electrochemical proton gradient as much as NADH does and therefore does not cause as many hydrogen ions to be pumped across the gradient as NADH does. Both of these coenzymes, however contribute to the production of the 32 ATP produced by the electron transport chain (“Electron Transport Chain,” n.d.).

The electron transport chain (ETC) is found within the mitochondria of living cells, more specifically between the inner mitochondrial membrane and the mitochondrial matrix. The goal of the ETC is to produce and electrochemical membrane potential which in turn drives the formation of ATP. To achieve a membrane potential the mitochondria is built with membrane channel pumps, which include 4 complexes. The first three complexes in the series take on the job of facilitating redox reactions and moving positively charged H+ ions into the inner mitochondrial membrane. NADH and FADH that is produced in Glycolysis, the Krebs Cycle, and in the formation of acetyl-CoA are used as electron carries and in essence fuel the ETC (Powers and Howley, 2015). Considered the chemiosmosis hypothesis, what happens next is simply to achieve oxidative phosphorylation. Redox reactions, that are spontaneously driven by gibbs free energy, begin to occur along the ETC transferring electrons from electron donors (NADH and FADH) to electron acceptors (O2). Hydrogen ions accepted by O2 to form water as other positively charge H+ ions flow into the intermembrane space of the mitochondrial. Along with the movement of positively charged ions into the intermembrane space and the neutralization of H+ ions in the formation of H2O comes the establishing of a difference in charge between the matrix of the mitochondria and the innermembrane. Typically because of this difference in membrane potential, when giving the opportunity to establish neutrality, H+ ions will flush back into the mitochondrial matrix and this is harnessed as energy for the phosphorylation of ADP into ATP (Chen, 1988).

The ETC is the largest and most efficient source of ATP and the main source of ATP for aerobic exercises or activities. It can only run in the presence of Oxygen because of the role that O2 takes on in the redox reactions that occur in the ETC complexes. The first 3 complexes of the ETC have one purpose and that’s to drive redox reactions and move H+ ions into the innermembrane. Although the three pumps share the same purpose they don’t quite share the same efficiency and processing. For every transport molecule of NADH the mitochondria can synthesis 2.5 molecules of ATP while FADH only allows for the synthesis of 1.5 ATP, this is specifically do to where each of the transport molecules begin their journey in the ETC. NADH starts reactions in complex 1 which moves 4 hydrogen ions into the innermembrane yet FADH skips completely over complex 1 and starts its journey in complex 2. In general one molecule of ATP can be synthesized from the energy released from 4 hydrogen ions as the move down the electrochemical gradient that’s established between the mitochondrial matrix and the inner membrane. The first pump flushes 4 H+ ions, the second pump 4 as well and the final pump 2 H+ ions. Following the 3rd pumps is a transmembrane channel that allows the flow of H+ ions into the mitochondrial matrix and couples the energy released from this with ATP synthase (Miles, 2003).

Proton Transport with Respect to the Electron Transport Chain

Protons are known for their movement across membranes. They travel across a protein pump which is found inside a lipid bilayer. Protein transport coincides with respect to the electron transport chain. A molecule will become reduced by obtaining an electron; with that it will bring along its negative charge. In order to neutralize the charge, a proton is added due to its positive charge. The net effect is to move the whole Hydrogen atom, H+ + e- . Protons are easily transferred due to the fact that electrons are transferred through a membrane using the electron transport chain. The carriers are strategically placed so when it picks up a proton from one side of the membrane it can accept an electron and release the proton onto the other side of the membrane. The cycle continues until all of the protons are transferred across the membrane by the electrons (Alberts, 2002).

The diagram above represents the electron carrier B grabbing a hydrogen molecule on one side of the membrane and transferring it to electron carrier C. As electron carrier B receives the electron (e-) from A, the proton was released as the electron was received by electron carrier C. Proton motive force is the total amount of energy that is created and gained by the electron carriers. There are only three electron carriers that are able to transport proteins across a membrane. When an area becomes condensed with protons, the intermembrane is created allowing protons to move from a high to low concentration. ADP is turned into ATP by phosphorylation (“Electron Transport Chain”, 2013). Peter Mitchell was the founder of a process called chemiosmosis. Chemiosmosis was the process of the electron transport pumps carrying protons across the inner mitochondrial membrane from the inner matrix to an inner space which would create a high hydrogen concentration gradient. They found this yielded electrical potential and pH potential across membranes (“Electron Transport- Energy of Cell”). Electron Carriers As discussed, it takes electron carriers in order to move a particle across a membrane when dealing with the electron transport membrane. What are these electron carriers exactly? There are eight of them. They include: NADH- ubiquinone oxidioreductase, succinate- ubiquinone oxidioreductase, ubiquinone/ ubiquinol, ubiquinol- cytochrome c oxidioreductase, cytochrome c oxidase, ATP synthase, electron-transferring flavoprotein (ETF), and glycerol-3- phosphate dehydrogenase. The significance of the electron transport chain is that each electron carrier has a higher standard reduction rate than the carrier located before it. The standard reduction potential is the capability of donating or receiving an electron (“Electron Transport Chain”, 2013).

The Electron Transport Chain

Citations: 1. Chen, L. (1988). Mitochondrial Membrane Potential In Living Cells. Annual Review of Cell and Developmental Biology, 155-181.

2. Powers, S., & Howley, E. (2015). Exercise physiology: Theory and application to fitness and performance (9th ed.). Boston: McGraw-Hill.

3. Miles, B. (2003). The Electron Transport Chain (pp. 4–8).

4. Alberts, B. (n.d.). Retrieved November 30, 2015, from http://www.ncbi.nlm.nih.gov/books/NBK26904/

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7. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Electron-Transport Chains and Their Proton Pumps. Retrieved November 30, 2015, from http://www.ncbi.nlm.nih.gov/books/NBK26904/

8. Electron Transport Chain. (n.d.). Retrieved November 30, 2015, from http://www.austincc.edu/emeyerth/electrontrans2.htm

9. Electron Transport Chain. (2007). Retrieved November 30, 2015, from http://media.pearsoncmg.com/bc/bc_0media_hk/animations/electron_transport/electron_transport.html

10. Glycolysis, Krebs Cycle, and other Energy-Releasing Pathways. (n.d.). Retrieved November 30, 2015, from http://www.uic.edu/classes/bios/bios100/lectures/respiration.htm

11. Whitney, E., & Rolfes, S. (2015). Chapter 7: Energy Metabolism. In Understanding Nutrition (14th ed.). Cengage Learning.

12. Electron Transport Chain Definition. (2004). Retrieved November 30, 2015, from http://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-E/electron_transport_chain.html