Electron Carriers


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This illustration shows the molecular structure of N A D superscript plus sign baseline and N A D H. Both compounds are composed of an adenine nucleotide and a nicotinamide nucleotide, which bond together to form a dinucleotide. The nicotinamide nucleotide is at the 5 prime end, and the adenine nucleotide is at the 3 prime end. Nicotinamide is a nitrogenous base, meaning it has nitrogen in a six-membered carbon ring. In N A D H, one extra hydrogen is associated with this ring, which is not found in N A D superscript plus sign baseline.
The oxidized form of the electron carrier (NAD+) is shown on the left, and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD+.

Source: OpenStax Biology 2e

OpenStax Biology 2e

In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy electrons between compounds in biochemical pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) is derived from vitamin B3, niacin. NAD+ is the oxidized form of the molecule; NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). Note that if a compound has an “H” on it, it is generally reduced (e.g., NADH is the reduced form of NAD).

– What is a chemical entity that donates electrons to another compound, and is a reducing agent that, by virtue of its donating electrons, and is itself oxidized in the process?

NAD+ can accept electrons from an organic molecule according to the general equation:

When electrons are added to a compound, it is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD+ is reduced to NADH. When electrons are removed from a compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD+ is an oxidizing agent, and RH is oxidized to R.

– What is a chemical entity that accepts electrons transferred to it from another compound, and is an oxidizing agent that, by virtue of its accepting electrons, and is itself reduced in the process?

Similarly, flavin adenine dinucleotide (FAD+) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD+ and FAD+ are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis in plants.

The chemiosmotic theory explains the functioning of electron transport chains. According to this theory, the transfer of electrons down an electron transport system through a series of oxidation-reduction reactions releases energy. This energy allows certain carriers in the chain to transport hydrogen ions (H+ or protons) across a membrane.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e