Reduction-oxidation (redox) reactions are coupled (occur together, simulateously) chemical reactions that occur when a transfer of electrons, hydrogens or oxygen takes place.

Oxidation is the loss of electrons, hydrogen, or both, or the gain of oxygen.

Reduction is the gain of electrons, hydrogen, or both, or the loss of oxygen.

Enzymes called oxidoreductases facilitate these reactions, which occur in both catabolic and anabolic pathways.  In some key energy-release and storage reactions, hydrogen atoms are removed from organic molecules through a process called dehydrogenation.

Since electrons can do great damage to the molecular structure of the cell, they must be retained and held by special receptor substances called electron-transport molecules.  These molecules are easily reduced and oxidized.  Some are derived from vitamins and are key components that complete the structure of enzymes called coenzymes, such as nicotinamide adenine dinucleotide (NAD+), nicotinamide adenine dinucleotide phosphate (NADP+) and flavine adenine dinucleotide (FAD+).  Others, such as flavoproteins and cytochromes are embedded in the cell membrane of bacteria or inner membrane of eukaryotic organelles such as mitochondria and chloroplasts.

The accumulation and transfer of electrons is essential to the synthesis of new energy-storage molecules such as ATP.  Electron accumulation  builds reducing power, while electron transfer releases energy in small, manageable amounts.