A Glimpse at the Function of NADH and FADH2 in Cellular Respiration

Function of NADH and FADH2
Cells of all living organisms need NADH and FADH2 (naturally occurring coenzymes) for energy production. During cellular respiration, the cells use these coenzymes to turn fuel from food into energy. This Buzzle post elaborates more on the function of NADH and FADH2.
Did You Know?
Every cell has a mitochondrion―the energy factory of the cell. However, the brain cells may contain more than one mitochondrion, since they are involved in lot of processing and require more energy to perform multiple tasks.
NADH is the reduced version of nicotinamide adenine dinucleotide (NAD), which is essentially a co-enzyme form of niacin (vitamin B3), present in all living cells. The enzyme is present in all livings organisms including plants. NAD+, the oxidized version of niacin, gains two electrons (2e-) and a hydrogen ion (H+) to form a NADH molecule. These redox (reduction-oxidation) reactions play a crucial role in energy generation.
Similar to NADH, FADH2 is the reduced form of FAD (flavin adenine dinucleotide), a co-enzyme. This oxidized form FAD, accepts two electrons and two hydrogen atoms to form FADH2. These conversions also assist in cellular energy production.
Function of NADH and FADH2
NADH and FADH in our body plays a crucial role in cellular energy production. The food that is consumed cannot be directly used as a source of energy. Metabolism that involves a series of chemical reactions, help to convert energy from food into energy that can be easily used by our body. This readily-available energy is stored in ATP (adenosine triphosphate)―a nucleotide. Also referred to as energy currency of the cell, the ATP molecule serves as the main storage of energy in cells.
NADH and FADH2 in Cellular Respiration
ATP production is an important part of cellular respiration (the process of generating energy from food) and both NADH and FADH2 that are involved in this process help in making more ATP. It is observed that during cellular respiration, every NADH molecule produces 3 ATP molecules, whereas each FADH2 molecule generates 2 ATP molecules.

Cellular respiration is essentially a 4-step process that includes glycolysis, acetyl CoA formation, Krebs cycle, and electron transport chain. In glycolysis, sugar is broken down to generate the end product, pyruvate. Pyruvate is a 3-carbon molecule, which gets converted into acetyl coenzyme-A (CoA). In the Krebs cycle, acetyl CoA is oxidized, which releases high energy electrons. These electrons and hydrogen atoms combine with NAD+ and FAD molecules to form NADH and FADH2, respectively.

NADH and FADH2 that act as electron carriers give away their electrons to the electron transport chain. The electron transport chain refers to a group of chemical reactions in which electrons from high energy molecules like NADH and FADH2 are shifted to low energy molecules (energy acceptors) such as oxygen. The electron transport chain is the primary means by which energy is derived in cellular respiration as well as in other processes like photosynthesis.

The electron transport chain occurs in the mitochondrion, the energy centers of the cell. The electrons that are shifted from NADH and FADH2, are essentially high-energy electrons. The energy that is released while transferring these electrons is used for making ATP. The electron transport chain is the last stage in the cellular respiration that is marked by formation of ATP in the inner mitochondrial membrane.
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