Question

What is the difference between NADH and $FADH_2$?

Answer 1

NADH is a high-energy electron carrier that transports electrons from the Krebs Cycle and Glycolysis to the Electron Transport Chain. $FADH_2$ is a high-energy electron carrier that transports electrons from the Krebs Cycle and Glycolysis to the Electron Transport Chain.

Complete answer:
The fundamental distinction between NADH and $FADH_2$ is that during oxidative phosphorylation, each NADH molecule makes three ATP molecules, whereas each $FADH_2$ molecule produces two. Furthermore, NADH distributes electrons to Cytochrome Complex I, whereas $FADH_2$ does so to Cytochrome Complex II.
NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are the reduced forms of coenzymes, NADH and $FADH_2$, respectively. They are essential for the creation of cellular energy. NADH is the reduced form of NAD (nicotinamide adenine dinucleotide), a key coenzyme involved in the energy transfer between metabolic activities in the cell. NAD is made up of two nucleotides: adenine and nicotinamide, which are linked together by their phosphate groups. NAD+ is the oxidised form of DNA. NAD's primary job is to aid in oxidation-reduction activities within the cell by acting as a coenzyme for enzymes like dehydrogenases, reductases, and hydroxylases in major metabolic processes like glycolysis, Krebs cycle, fatty acid synthesis, and steroid production. $FADH_2$ is a reduced version of FAD (flavin adenine dinucleotide), which is made up of two nucleotides linked together: adenine and flavin mononucleotide.
The four redox forms of FAD are flavin-N (5)-oxide, quinone, semiquinone, and hydroquinone. Quinone is the fully oxidised form, whereas hydroquinone, also known as $FADH_2$, is the fully reduced form. NADH is the reduced form of the ubiquitous coenzyme NAD, which is made up of two nucleotides: adenine and nicotinamide, whereas $FADH_2$ is the reduced form of the coenzyme FAD, which is made up primarily of riboflavin.

Note :-
The reduced form of NAD, NADH, creates three ATP molecules during oxidative phosphorylation, whereas $FADH_2$ produces two ATP molecules during oxidative phosphorylation. Other oxidation-reduction events that take place in the cell require both NADH and $FADH_2$. The number of ATP molecules produced by oxidative metabolism differs significantly between NADH and $FADH_2$.