Question

What is NAD, NADH, FAD and $FADH_2$?

Answer 1

Glycolysis literally means the breakdown of sugar (Glyc = sugar or sweet and Lysis = to cut or loosen). Glycolysis occurs in the cytoplasm of the cell. Glycolysis converts one glucose molecule with six carbons into two pyruvate molecules with three carbons each. $NAD^+$ captures electrons and hydrogen atoms during this process. Any energy released will be converted to heat or stored as ATP or NADH.

Complete answer:
The coenzymes nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) play a role in reversible oxidation and reduction reactions. Because they receive electrons (get reduced) during catabolic processes in the breakdown of organic molecules like carbohydrates and lipids, these substances are often referred to as electron carriers. The reduced coenzymes can then contribute these electrons to another metabolic reaction that is generally involved in an anabolic process (like the synthesis of ATP).
Nicotinamide Adenine Dinucleotide in its oxidized state is called $NAD^+$, after being reduced (or accepting electrons), it is referred to as NADH. This chemical is made from the vitamin Niacin (also known as B3). Niacin offers the organic ring structure necessary for the direct transfer of a hydrogen atom and two electrons. $NAD^+$ is frequently present in the presence of a "dehydrogenase" enzyme. Two hydrogen atoms are removed in a dehydrogenase reaction: one as a hydride (:$H^-$) (a hydride is a hydrogen atom with two electrons) and the other as a hydrogen cation ($H^+$) (and of course, a hydrogen cation has no electrons). The hydride binds to $NAD^+$ and forms a reduced Nicotinamide Adenine Dinucleotide molecule (NADH). In solution, the second hydrogen atom ($H^+$) is liberated.
Flavin adenine dinucleotide in its oxidized state is called FAD. After being reduced, it is called $FADH_2$. This chemical is made from the vitamin riboflavin (or B2). Riboflavin supplies the ring structures required for the direct transfer of two hydrogen atoms (each with one electron this time). FAD, like NAD, functions in tandem with a "dehydrogenase" enzyme. Two hydrogen atoms are removed in this process, each of which is a proton with one electron. FAD binds to both hydrogen atoms. Unlike the reduction of NAD, this reaction does not release an $H^+$ into solution. Flavin adenine dinucleotide in the oxidized form (FAD) accepts two hydrogen atoms (each with one electron) and becomes $FADH_2$.

Note:-
It's worth noting that when there's an abundance of glucose, part of it will be turned to glycogen. When blood sugar levels drop, glycogen can be broken down into monomers of glucose, which can then be used in glycolysis. Glycogen synthesis is a crucial mechanism that allows us to "store" sugar for usage when we aren't eating but still need to keep our blood sugar levels in check.