Cells need a reliable way to capture and move energy released from nutrients. Four molecules are central to this process: NAD+, NADH, FAD, and FADH₂. They act as electron carriers, accepting and donating electrons during metabolic reactions and helping convert the chemical energy in food into a form the cell can use.
NAD+ and FAD are the oxidized forms of their respective carriers. NADH and FADH₂ are the reduced forms. The distinction matters because these molecules constantly switch between these states as they pick up and release electrons.
Their most important role is in cellular respiration, particularly the breakdown of carbohydrates and fats. Electrons carried by NADH and FADH₂ ultimately enter the electron transport chain, where their energy helps drive the production of ATP, the cell’s main immediately usable energy currency.
What electron carriers actually do
An electron carrier is a molecule that can accept electrons in one chemical reaction and later donate them in another. This allows cells to transfer energy in controlled steps rather than releasing all of it at once.
The key chemical idea is oxidation-reduction, or redox, chemistry. When a molecule loses electrons, it is oxidized. When it gains electrons, it is reduced. Because electrons carry energy, moving them from one molecule to another can also move usable chemical energy through a metabolic pathway.
NAD+ and FAD are electron acceptors. When they gain electrons during metabolic reactions, they become NADH and FADH₂:
NAD+ → NADH
FAD → FADH₂
The reverse occurs when NADH or FADH₂ gives up its electrons.
Although these carriers perform similar jobs, they are not interchangeable. They differ in chemical structure, the reactions in which they participate, and the way their electrons enter the electron transport chain.
NAD+ and NADH: the major soluble electron carrier pair
NAD+, short for nicotinamide adenine dinucleotide, is one of the cell’s most important redox cofactors. A cofactor is a non-protein molecule that helps an enzyme carry out a reaction.
When NAD+ accepts electrons, it is reduced to NADH. In many metabolic reactions, NAD+ accepts two electrons and one proton overall, while another proton is released into the surrounding solution. A simplified representation is:
NAD+ + 2e⁻ + H⁺ → NADH
NAD+ is particularly important in pathways that break down energy-rich molecules. During glycolysis, for example, NAD+ accepts electrons generated during the oxidation of glucose-derived molecules. NADH then carries those high-energy electrons onward.
NAD+ also functions in the pyruvate oxidation step that links glycolysis to the citric acid cycle, as well as at several points within the citric acid cycle. The result is a steady production of NADH as carbon-containing nutrients are progressively oxidized.
The relationship between NAD+ and NADH is therefore dynamic. Cells need enough NAD+ available to accept electrons so that oxidation reactions can continue. They also need mechanisms to regenerate NAD+ from NADH.
FAD and FADH₂: a related carrier with different chemistry
FAD, or flavin adenine dinucleotide, is another redox cofactor. Its reduced form is FADH₂.
Like NAD+, FAD can accept electrons during metabolic reactions. But FAD has an important chemical difference: it can accept electrons and hydrogen atoms in a way that allows it to participate in reactions that NAD+ cannot.
FAD is often tightly associated with the enzyme that uses it. Such a cofactor is sometimes described as a prosthetic group when it is bound especially firmly to the protein. In contrast, NAD+ commonly moves between enzymes, accepting electrons in one reaction and carrying them to another location.
One prominent example is succinate dehydrogenase, an enzyme of the citric acid cycle. It oxidizes succinate to fumarate while reducing its FAD cofactor to FADH₂. Succinate dehydrogenase is unusual because it also forms part of Complex II of the electron transport chain. As a result, electrons captured by FADH₂ in this reaction can be passed directly into the respiratory electron-transfer system.
FAD therefore tends to be especially useful in reactions where the chemistry of electron and hydrogen transfer is not well suited to NAD+.
The difference between NADH and FADH₂ matters for ATP production
Both NADH and FADH₂ deliver electrons to the electron transport chain, but they enter at different points.
NADH transfers its electrons to Complex I, the first major electron-entry complex of the respiratory chain. From there, electrons pass through additional components of the chain. The energy released during this electron transfer is used to pump protons across the inner mitochondrial membrane in animal and other eukaryotic cells.
FADH₂-associated electrons enter through Complex II, which does not pump protons. Because these electrons enter downstream of Complex I, they contribute to less proton pumping overall.
This is why NADH generally supports the formation of more ATP than FADH₂ during aerobic respiration.
A commonly used approximate accounting is about 2.5 ATP per NADH and 1.5 ATP per FADH₂. These are estimates rather than fixed conversion ratios. The actual ATP yield depends on how electrons and metabolites move through the cell and on the efficiency of oxidative phosphorylation.
The important principle is not the precise number: NADH delivers electrons to the respiratory chain at a higher-energy entry point than FADH₂, so its electrons generally yield more ATP.
How the carriers connect food to the electron transport chain
The cell does not extract most of the energy from glucose or fatty acids in a single reaction. Instead, metabolism breaks these molecules down through a series of oxidation reactions.
During these reactions, electrons are transferred to NAD+ and, in certain reactions, FAD. The carriers temporarily hold those electrons in reduced forms.
For glucose, the overall sequence can be viewed as a flow:
Glucose → glycolysis → pyruvate → acetyl-CoA → citric acid cycle → NADH/FADH₂ → electron transport chain → proton gradient → ATP
The first stages harvest electrons from carbon compounds. NADH and FADH₂ then transport the captured reducing power to the respiratory chain.
The electron transport chain uses that energy to establish a proton gradient across the inner mitochondrial membrane. Protons then flow back through ATP synthase, an enzyme that uses this flow to synthesize ATP from ADP and inorganic phosphate.
Oxygen is the final electron acceptor in aerobic respiration. At the end of the chain, electrons combine with oxygen and protons to form water.
In this sense, NADH and FADH₂ are not themselves the cell’s final energy source. They are intermediaries that transfer reducing power from metabolic reactions to the machinery that produces ATP.
NAD+ and NADH have roles beyond energy metabolism
NAD+ is not simply an electron shuttle for cellular respiration. It also participates in several other forms of cellular chemistry.
NAD+ is consumed in reactions involving enzymes such as PARPs and sirtuins, which are involved in processes including DNA-related responses and regulation of cellular proteins. These reactions can change the cellular NAD+/NADH balance and connect metabolism with other aspects of cell function.
NAD+ and related molecules are also part of broader cellular signaling and biosynthetic systems. Consequently, the NAD+/NADH ratio can provide information about the redox state of a cell and influence which metabolic reactions are favorable.
FAD likewise has functions extending beyond its role as an electron carrier. Flavin-containing cofactors participate in numerous oxidation-reduction reactions, including reactions involved in metabolism and other cellular processes.
Why cells need both NADH and FADH₂
It might seem unnecessary for cells to maintain two electron-carrier systems that perform similar jobs. Their differences, however, give metabolism considerable chemical flexibility.
NAD+ is particularly effective as a mobile electron acceptor and donor in many oxidation reactions. FAD, because of its different chemical properties and frequent tight association with enzymes, can participate in reactions that require a different mechanism of electron transfer.
The two carriers also capture electrons at different stages of metabolic pathways. Their eventual entry points into the electron transport chain preserve some of the energetic differences between those reactions.
Together, they allow cells to extract energy efficiently from a wide range of nutrients.
NAD+ versus NADH, and FAD versus FADH₂
The names can make these molecules seem like four unrelated substances, but they are best understood as two pairs.
| Electron carrier pair | Oxidized form | Reduced form | Main role |
|---|---|---|---|
| NAD+/NADH | NAD+ | NADH | Carries electrons from many metabolic oxidation reactions |
| FAD/FADH₂ | FAD | FADH₂ | Carries electrons in specific enzyme-catalyzed oxidation reactions |
The word oxidized means that the carrier is in the electron-accepting state. The word reduced means that it has gained electrons.
Thus, when a metabolic reaction produces NADH, the cell has not created an entirely new kind of carrier. It has converted NAD+ into its reduced form. When NADH later donates its electrons, NAD+ is regenerated.
The same principle applies to FAD and FADH₂.
What happens when oxygen is unavailable
In aerobic respiration, NADH can transfer its electrons into the mitochondrial electron transport chain, where oxygen ultimately accepts them.
But some cells and tissues must continue producing ATP when oxygen is limited. A major example is glycolysis. Glycolysis requires a supply of NAD+ because one of its oxidation reactions transfers electrons to NAD+.
If the respiratory chain cannot regenerate enough NAD+, glycolysis would eventually stop.
Cells can instead regenerate NAD+ by transferring electrons from NADH back to organic molecules. In human skeletal muscle under conditions of insufficient oxygen supply, for example, pyruvate can be reduced to lactate while NADH is oxidized back to NAD+. This allows glycolysis to continue producing a limited amount of ATP without requiring oxygen as the final electron acceptor.
This illustrates an important point: regenerating NAD+ can be just as important as producing NADH. Electron carriers must cycle between their oxidized and reduced forms for metabolism to keep running.
The central idea
NAD+, NADH, FAD, and FADH₂ are best understood as parts of a continuous electron-transfer system.
NAD+ and FAD accept electrons during oxidation reactions. Their reduced forms, NADH and FADH₂, carry those electrons to other reactions. In aerobic cells, much of this reducing power ultimately reaches the electron transport chain, where electron transfer is coupled to the production of ATP.
NADH and FADH₂ therefore connect the chemical breakdown of nutrients with the generation of usable cellular energy. Their different chemistry and different entry points into the electron transport chain allow cells to capture energy from metabolism in controlled, efficient steps.