Flavin adenine dinucleotide (FAD) is the cofactor of cytochrome-b5 reductase, an enzyme that maintains hemoglobin in its normal functional reduced state, & for glutathione reductase, an enzyme that also protects erythrocytes from the oxidative damage.
It is responsible for transporting electrons generated in glycolysis and kreb cycle to the electron transport chain.
It also has a role in cellular respiration. It acts as an electron carrier & temporarily stores energy during cellular respiration.
The inter-membrane space which separates 2 mitochondrial membranes is called perimitochondrial space.
In the electron transport chain (ETC) FADH2 gets oxidised to FAD, and the electrons are passed to cytochrome proteins on the other hand hydrogen ions are often released into perimitochondrial space.