What is the Electron Transport Chain? 

The electron transport chain is a series of redox reactions that generate energy to produce ATP. The molecules we have synthesized throughout the TCA and glycolysis (NADH, FADH2) provide the electrons needed to power this system and allow for the oxidation/reduction reactions to occur. The goal is to use this energy to pump H+ ions against their concentration gradient to then spin a strucutre called ATP synthase, which will generate ATP. 

As a reminder, we have generated 6 NADH and 2 FADH2 from one molecule of glucose. 

This process relys on the concept of "reduction potential", specifically, the electrons referenced above are constantly looking for the carriers with the highest reduction potential, and climbing up the ladder the next time they see one that has more. 

So, what makes a molecule have a “higher reduction potential?” 

• Each electron carrier has a specific potential which is characterized mostly by charge. A more POSITIVE charge → higher affinity; a more NEGATIVE charge → lower affinity. 

  • Therefore, electrons are going to flow from negative to positive potentials. This causes them to release energy! 

To exemplify this process, lets look at the reduction potentials following the path of electrons as they move through the ETC. 

1. NADH→ NAD+ has a reduction potential of -0.33V

2. CoQ cycles between being oxidized and reduced, and has the next step up for reduction potential at +0.03V

3. Following this, cytochrome C has a reduction potential of +0.22V

4. Finally, O2 to H20 has a reduction potential of +0.82V, the highest and last stop. 

To review the concept of proton pumping as mentioned earlier, lets look at the complexes of the ETC: 

1. Complexes I, III, and IV (one, three, and four) pump protons 

2. Complex II (two) does NOT pump protons

3. Note From complex IV to O2, there is a large amount of energy released, allowing for a lot of extra proton pumping.