Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions), and they play a central role in the metabolism of a cell. In a redox reaction, one of the reacting molecules loses electrons and is said to be oxidized, while another reacting molecule gains electrons (the ones lost by the first molecule) and is said to be reduced. You can remember what oxidation and reduction mean with “LEO goes GER”: Lose Electrons, Oxidized; Gain Electrons, Reduced or “OIL RIG”: Oxidation Is Loss, Reduction Is Gain.

Therefore, they can be broken down into two half-reactions: the compound undergoing oxidation and the compound undergoing reduction. See below for an example of a reduction half-reaction and an oxidation half-reaction with its net reaction.

Electron carriers, sometimes called electron shuttles, are small organic molecules that readily cycle between oxidized and reduced forms and are used to transport electrons during metabolic reactions. There are two soluble electron carriers that play particularly important roles during cellular respiration: NAD⁺ (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide). Both NAD⁺ and FAD can serve as oxidizing agents, accepting a pair of electrons, along with one or more protons, to switch to their reduced forms. NAD⁺  accepts two electrons and one H⁺ to become NADH, while FAD accepts two electrons and two H⁺ to become FADH₂. NAD⁺ is the primary electron carrier used during cellular respiration, with FAD participating in just one (or two sometimes two) reactions. Flavoproteins are proteins that have a FAD or FMN (flavin mononucleotide) and are one of four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems during cellular respiration.

Practice Questions

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Key Points

• Redox reactions involve electron transfer and can be broken down into two half-reactions: the compound undergoing oxidation and the compound undergoing reduction.

• Two soluble electron carriers play particularly important roles during cellular respiration: NAD⁺ (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide).

• Flavoproteins are proteins that have a FAD or FMN (flavin mononucleotide).

Key Terms

redox reactions: reactions involving electron transfer (one molecule gains electrons and another molecule loses electrons)

NAD⁺: a soluble electron carrier that accepts two electrons and one H⁺ to become NADH

FAD: a soluble electron carrier that accepts two electrons and two H+ to become FADH₂

flavoproteins: proteins that have a FAD or FMN (flavin mononucleotide)

oxidation: a reaction in which an element’s atoms lose electrons and its oxidation state increases

reduction: a reaction in which electrons are gained, and oxidation state is reduced, often by the removal of oxygen or the addition of hydrogen

half-reactions: one of the two constituent parts of any redox reaction in which only oxidation or reduction is shown

electron carrier: a molecule that transports electrons during cellular respiration

flavoproteins: enzymes that require FMN or FAD as cofactors