What exactly is protein complex II used for in the electron transport chain in cellular respiration. ...

HI, in chemiosmosis, right?

The process begins with NADH giving up its two electrons to the first protein complex in the ETC, NADH dehydrogenase. The mobile electron carriers, Q and cytochrome c, shuttle the electrons from one protein complex to the next until they reach the final protein complex in the chain, the cytochrome oxidase complex. Finally, the enzyme cytochrome oxidase, which is part of this complex, catalyzes the reaction between the electrons, protons, and molecular oxygen to form water.

Furthermore, it is important to distinguish between NADH and FADH2 in terms of their relationship with the electron transport system. NADH and FADH2 do not transfer their electrons to the electron transport chain in the same way. NADH passes its electrons on to the first protein complex, NADH dehydrogenase, and FADH2 transfers its electrons to Q, the second component of the chain. Thus, the free energy released by the oxidation of FADH2 is used to pump two protons into the intermembrane space, while NADH oxidation pumps three. The result is that two ATP are formed per FADH2 and three ATP molecules are formed per NADH.

The primary biochemical role of FADH2 in eukaryotes is to carry high-energy electrons used for oxidative phosphorylation. Its hydrogens remain in the mitochondrial matrix, whilst FAD is tightly bound to a dehydrogenase enzyme i.e. the second protein complex in the oxidative phosphorylation chain