有机分子的氧化和还原

Oxidation is the removal of electrons from an atom, while reduction is the addition of electrons to an atom. In cells, these processes occur together as redox reactions. When a molecule gains an electron, it is reduced; when it loses electrons, it is oxidized. Because electrons are conserved, oxidation and reduction always occur in pairs during cellular metabolism.

Cells use electron-carrying coenzymes derived from B vitamins to shuttle high-energy electrons between compounds. These electron carriers, such as NAD+ and FAD, bind and carry electrons through biochemical pathways. This incremental transfer allows cells to extract and use energy in small packages rather than in a single, destructive burst, enabling efficient energy management.

During the dehydrogenation of succinate to fumarate, electrons and protons are transferred to the coenzyme FAD, reducing it to FADH2. This reduced form then transfers electrons through the electron transport chain and is oxidized back to FAD. This process demonstrates how organic molecules are broken down to generate energy through redox reactions.

Hydrogenation is the addition of hydrogen to a molecule, which is considered reduction because hydrogen atoms contain electrons. When a molecule gains a hydrogen atom, it also gains electrons, making it reduced. Conversely, dehydrogenation—the removal of hydrogens—is oxidation because the molecule loses electrons in the process.

NAD+ is the oxidized form of nicotinamide adenine dinucleotide, derived from vitamin B3 niacin. When NAD+ accepts two electrons and a proton, it becomes NADH, the reduced form. NADH then carries these high-energy electrons through endergonic and exergonic reactions in the cell, transferring energy between compounds in metabolic pathways.

Most energy stored in atoms and used to fuel cell functions exists as high-energy electrons. When electrons transfer from one compound to another, the oxidized compound loses potential energy while the reduced compound gains it. This electron transfer removes energy from one molecule and increases it in another, allowing cells to store and release energy efficiently.

Oxidation and reduction reactions occur together because the number of electrons in a reaction is conserved. When one molecule loses electrons through oxidation, another molecule must gain those same electrons through reduction. This pairing ensures electron balance and allows cells to transfer energy systematically through coupled redox reactions in metabolic pathways.