FADH2 & NADH: Definition & Overview

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  • 0:01 Overview of Cellular…
  • 0:40 What Are FADH & NADH?
  • 1:16 Functions
  • 2:56 Lesson Summary
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Lesson Transcript
Jeremy Battista

Jeremy has a master of science degree in education.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

In this lesson, we take a closer look at some of the materials and mechanisms used in cellular respiration. As this is a very complex process, we will specifically focus on FADH2 and NADH.

Overview of Cellular Respiration

Cellular respiration is the process of utilizing oxygen and food molecules to create energy, carbon dioxide, water, and waste products. Basically, respiration is how we convert food into energy using water and oxygen. Respiration consists of three separate metabolic phases: glycolysis, the Krebs cycle, and the electron transport chain. We will not be going into any great detail on these three phases here. Instead, we'll be focusing on two compounds, FADH2 and NADH, and how they are incorporated into respiration.

What Are FADH2 and NADH?

Flavin adenine dinucleotide, or FADH2, is a redox cofactor that is created during the Krebs cycle and utilized during the last part of respiration, the electron transport chain. Nicotinamide adenine dinucleotide, or NADH, is a similar compound used more actively in the electron transport chain as well. In fact, more NADH is produced and used than FADH2 in the process of creating energy. There are actually six NADH produced and only two FADH2 molecules.


FADH2 and NADH are created from FAD and NAD+ through reduction-oxidation reactions in the Krebs cycle during respiration as seen below:

This cycle gives off small amounts of energy in the form of adenosine triphosphate, or ATP, and produces these compounds, FADH2 and NADH. The Krebs cycle is like a wheel. Every time it makes one full rotation, energy is created and released. As you can see from the diagram, the NAD+ and FAD are brought in at key points throughout the cycle and are attached to other electrons resulting in the formation of NADH and FADH2.

This energy is then shuttled off to be used by the cell, mostly for the continuation of cellular respiration.

As they are shuttled away, these two compounds are used to move electrons into the electron transport chain, the final stage of respiration. It is in this stage that most of the energy is created and released from the mitochondria (powerhouse of the cell).

Basically, the NADH and FADH2 molecules are affixed with electrons and are transferred to the inner membrane of the mitochondria. They travel down the electron transport chain, releasing the electrons that they once had. The end result is loads of energy, approximately 34 ATP (energy molecule).

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Oxidation-Reduction (Redox) Reactions

Let's begin with a review of redox reactions. All redox reactions involve the transfer of one or more electrons from one substance to another. The substance that gives up electrons is oxidized and the one that receives the electrons is reduced. Oxidation and reduction are always coupled. Because electrons have energy, energy is transferred from the oxidized substance to the one that is reduced.

Cellular respiration is essentially a series of redox reactions that transfer electrons, often as part of H atoms, from glucose to other molecules. These redox reactions involve the intermediate electron carriers NAD+ and FAD, which are temporarily reduced to NADH and FADH2. Ultimately, the reactions of cellular respiration produce carbon dioxide (CO2) and water (H2O). Carbon dioxide is the waste product of reactions of the citric acid cycle. Water is formed in the last step of the electron transport chain when electrons from glucose reach oxygen (O2).

Citric Acid Cycle

Examine the diagram of the citric acid cycle. Don't be overwhelmed by the many steps and molecules. The questions will guide you through the most important features and help you to understand the redox reactions of cellular respiration.

The citric acid cycle begins with the joining of a 2-carbon molecule to molecule X. Each complete turn of the cycle regenerates molecule X.

  1. How many carbon (C) atoms are present in molecule X? How many C atoms are present in molecule I?
  2. How many hydrogen atoms (H) are present in molecule X? How many H atoms are present in molecule I? (Note: The acids of the citric acid cycle are shown in their ionized form. Wherever you see a COO-, add one more H because the non-ionized form is COOH.)
  3. How many reactions of the citric acid cycle are redox reactions? Hint: Look for the electron carriers NAD+ and FAD.
  4. At which steps is a molecule of CO2 produced and released? What do you notice about the product that continues in the cycle after CO2 is released?
  5. After one round of the citric acid cycle (starting from molecule I), how many molecules of NADH have been produced? How many molecules of FADH2 have been produced?

Hopefully, as you answered the questions above, you noticed that Molecule I begins with more C and more H atoms than what are present in molecule X. As the citric acid cycle proceeds, the C atoms are lost as CO2 and the H atoms are transferred to one of the electron carriers, NAD+ or FAD, to form NADH or FADH2.


  1. four; six
  2. four; eight
  3. four; three involving NAD+ and one involving FAD
  4. steps 4 and 5; the product of each reaction has one less C atom than the previous product, resulting in conversion of a 6C molecule to a 5C molecule to a 4C molecule
  5. three NADH and one FADH2

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