Re: What two electron entry points in electron transport chain?

In cellular respiration, why is it that there are two separate 
entry points into the electron transport chain (one for 
electrons from NAD and one for electrons from FAD)? 

In oxidative phosphorylation, electrons are transferred from 
NADH to oxygen through a chain of large protein complexes 
(NADH-Q reductase, cytochrome reductase, and cytochrome 
oxidase).  Electrons move from NADH-Q reductase to cytochrome 
reductase by a reduced form of ubiquinone (QH2).  Ubiquinone 
also carries electrons from FADH2 to cytochrome reductase.  

So why are there two different entry points?  Electrons from 
FADH2 enter the electron-transport chain through QH2, rather 
than through the first proton pump (NADH-Q reductase) because 
FADH2 is a less powerful electron donor than NADH.  Another 
way to say it is NADH has a more negative reduction potential 
than FADH2 .  FADH2 has less reduction potential, therefore it 
can reduce fewer proteins than NADH.  As you move down the 
electron transport chain, the proteins have higher and higher 
affinity for electrons.  That makes them easy to reduce.  The 
first protein NADH-Q reductase, is not that easy to reduce.  
Therefore, wimpy FADH2 jumps in further down, where the 
proteins are easier to reduce. Otherwise, it doesn't have 
enough reduction potential "steam" to make it all the way 
through the chain of proteins to the final acceptor oxygen.

Why use FAD at all?  The use of FAD enables electrons to be 
transported into mitochondria against a NADH concentration 
gradient. This allows the mitochondria to get more electrons 
into the oxidative phosphorylation chain, even when the 
concentration of NADH is very high.  This is important in 
tissues, such as insect flight muscle, which rapidly uses a 
lot of ATP.