Sarah is your average student. Homework, studying, and socializing is her everyday routine. Usually this everyday routine goes as fast and easy as it can be! But one day, while struggling with her biology homework, her everyday routine was suddenly changed.

"Ugh! The electron transport chain? Chemiosmosis? What does this all mean!" Sarah says in frustration as her mother walks into the room. "What's with all this yelling?" her mother asks concerned. "I don't understand this process! Can you help me?" asks Sarah. "I have the perfect idea! I've been waiting for this moment! Sarah, we're gonna go on a little trip." explains her mother with excitement. 

"We're entering the mitochondria! Specifically the inner mitochondrial membrane.This is the site of the electron transport chain. To understand this process we must first understand it's function and it's goal. This process moves hydrogen ions across the membrane to produce large amounts of ATP (energy). This process can produce up to 34! The most of all the processes in cellular respiration! 

"But there are so many reactions and steps!" exclaimed Sarah. "Well, all these series of reactions take place and travel throughout 4 complexes! The electrons are transferred from NADH and FADH2 (from the process before this one, the Krebs cycle) to O2 (oxygen) by moving through 4 protein complexes! We can also refer to these as proton and hydrogen pumps! These complexes are numbered in the order in which they occur!

1. Complex I 2. Complex II 3. Complex III 4. Complex IV 

Complex I: NADH Hydrogenase

Complex II: Succinate Dehydrogenase

Complex III: Cytochrome Complex

Complex IV: Cytochrome Oxidase

"But how are these electrons transferred?" asks Sarah. "Well, to understand the transfer of electrons first, we need to understand the two ways they flow!

1. The first way is Ubiquinone (UQ): which transfers electrons through *Complex I and Complex II to Complex III*

2. The second way is Cytochrome C (cyt C): which transfers electrons through *Complex III to Complex IV*" explains the mother

Complex

I

Complex II

   UQ               Complex III            Cyt C        Complex IV

"So what happens in the first complex?" Sarah asks intrigued. "Well, an important thing to know about the first complex is that it's enzyme is a large protein called NADH hydrogenase. This is because this complex has received 2 electrons from NADH, which reduces it to NAD. One hydrogen ion per electron is pumped across the membrane from the matrix to the intermembrane space!" explains the mother, "so that means that there are two hydrogen molecules?" "yes! and a hydrogen ion gradient is made due to this!".

Complex I

      H+

                                 H+  

NADH                                NAD

"So what happens now?" asks Sarah. "Now we reach Complex II" says the mother "in this complex, the FADH2 is directly received since it could not pass through Complex I. The ubiquinone (also known as UQ) is now used since it connects the first and second complex to the third complex. The FADH2 is oxidized to FAD and also releases the hydrogen ions into the intermembrane space and passes off the electrons".

        Q

Complex II

 FADH2                                  FAD