redox reactions and electron carriers (NAD+/FAD)
/ REE-dox; N-A-D plus / F-A-D /
Imagine passing a hot coal from hand to hand. Whoever holds it carries a bit of energy, and handing it off changes who has the heat. In a cell, the "hot coals" are electrons, and a great deal of energy travels with them. Reactions that pass electrons from one molecule to another are called redox reactions, and they are how cells move energy around.
Redox is short for reduction and oxidation, which always happen together. A molecule that loses electrons is oxidized; the one that gains them is reduced. Electrons rich in energy tend to flow from molecules that hold them loosely to molecules that hold them tightly, releasing energy along the way. Because electrons cannot just float around, the cell uses dedicated electron carriers that pick them up at one place and drop them off at another. The two main carriers are NAD+ and FAD: when NAD+ grabs two electrons (with a proton) it becomes NADH, and when FAD grabs them it becomes FADH2. The loaded forms, NADH and FADH2, are like full batteries carrying energy to be cashed in later.
This electron shuttling is the backbone of how cells extract energy from food. Catabolism strips electrons from fuel molecules and loads them onto NAD+ and FAD; later, the electron transport chain cashes those loaded carriers in to make ATP. A common confusion is the plus sign and the bookkeeping of protons: just remember NAD+ is the empty carrier and NADH is the loaded one, ready to deliver its energetic electrons.
During glucose breakdown, electrons stripped from sugar are loaded onto NAD+ to form NADH, which later delivers them to the electron transport chain to be turned into ATP.
NAD+ and FAD pick electrons up (becoming NADH and FADH2) and carry their energy to where ATP is made.
A handy mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). NAD+ is the empty carrier; NADH is the loaded one.