# Electron Transport Chain --- The mitochondrial [[electron transport chain]], or **ETC**, is the driving force of [[phosphorylation]] and the the last stage of [[cellular respiration]]. The ETC is the name for the group of molecules found in inner membrane of the [[mitochondria]] that [[redox|oxidize]] the [[electron carrier|electron carriers]] that have been built up during the previous three steps of cellular respiration and use them to power massive [[adenosine triphosphate]] generation. The ETC uses "electron transport" to shuffle electrons down the line, each step harnessing its energy to pump [[hydrogen ion|protons]] into the intermembrane space, creating a [[concentration gradient]] that produces a [[osmosis|chemiosmotic]] force to power the final step, [[ATP synthase|ATP synthesis]]. Oxygen is an absolutely vital component for the ETC in order to accept the electron at the end of it. For each single molecule of glucose, 6 O$_2$ molecules are requried at the end of the ETC. ## Free Energy in NADH and FADH2: Powering the ETC Up to this point, the reactions of cellular respiration have really been keen on producing the high energy molecules [[NAD|NADH]] and [[FAD|FADH2]]. They have massive amounts of free energy in these forms, and it's that free energy that powers the ETC. ![[free energy in cellular respiration.png]] ## Structure Divided into 4 protein complexes. These complexes go back and forth from being oxidized and being reduced. At the end of the day, that electron has to go somewhere, and if it has no where to go (i.e. there is no oxygen around), then it gets backed up. If it gets paced up the electrons can't flow through the system, and the protons can't be pumped into the intermembrane space. ![[ETC (mitochondria).png]] ### Complexes #### Complexes as Protein Pumps Three out of the four complexes pump protons into the [[mitochondria|mitochondrial intermembrane space]] in order to build up a concentration gradient to power the [[ATP synthase]]. #### Complexes are Redox Centers Proton pumping requires energy. The protein complexes gets this energy from a series of internal [[redox]] reactions using the electrons that were harvested from the earlier steps of respiration. Once the electron has passed through a particular complex, it is passed out of the complex either by a membrane [[#electron carriers]] or by shoving it into a final electron acceptor, O$_2$ #### Complex I **Complex I** pumps protons into the intermembrane space. It gets its electron from [[NAD|NADH]] and passes it to [[coenzyme Q]] so it can be then passed over to [[#complex III]]. #### Complex II **Complex II** does not pump protons directly, but assists complexes [[#Complex III|III]] and [[#Complex IV|IV]] to do so. It gets its electron from [[FAD]] and passes it to [[coenzyme Q]] so it can be then passed over to [[#complex III]]. #### Complex III **Complex III**, also known as "cytochrome c reductase" pumps protons into the intermembrane space. It gets its electron from [[coenzyme Q]] and passes it to [[#cytochrome c]] so it can then be passed to [[#complex IV]]. #### Complex IV **Complex IV**, also known as "cytochrome c oxidase", pumps protons into the intermembrane space. It gets its electron from [[#cytochrome c]] and passes it off to the final acceptor, O$_2$. ### Electron Carriers The **ETC electron carriers** are [[membrane proteins]] that ferry the electrons from one complex to the next. They're very specific to what type of complex they can bind to. **[[Coenzyme Q]]** is an electron carrier that ferries electrons all over the place. **Cytochrome *c*** is a membrane protein that ferries electrons from [[#complex III]] to [[#complex IV]] ### ATP synthase [[ATP synthase]] is the protein complex that harnesses the [[proton motive force]] from the concentration gradient from the proton pumps to generate massive amounts of [[adenosine triphosphate]]. ## Function In each complex [[atom|electrons]] pass from a molecule with a lower [[redox]] potential to one with a higher redox potential. As electrons move down through the chain, they are held more and more tightly in the newly formed bonds, and the potential energy in each successive bond lessens. The protein complexes use the energy given off in these reactions to pump protons into the inner membrane space to create that much-needed electrochemical gradient. Ultimately, the goal of the chain is to create a electro-chemical [[concentration gradient|gradient]] with many protons in the intermembrane space to drive the synthesis of [[adenosine triphosphate]] through [[osmosis|chemiosmosis]] that drives [[ATP synthase]]. NADH and FADH2 kick a bunch of protons into the intermembrane space of the mitochondria through specialized protein channels, then lets them back in through [[ATP synthase]], which is a spinning wheel proton that just churns out the ATP. It does this by squeezing some [[functional group#Phosphate|inorganic phosphate]] and [[adenosine triphosphate|ADPs]] together, which is an important distinction from [[phosphorylation]], which we have seen in the previous steps of [[cellular respiration]]. ___