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This lesson is on the role of nicotinamide adenine dinucleotide + hydrogen (NADH) in cellular respiration. In this lesson, we’ll learn about NADH and cellular respiration, and we’ll discuss how the two work together to make energy for the cell.

What Is Cellular Respiration?

As we sit here reading and writing, our brain cells are hard at work trying to understand the world around us.

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They receive information from our bodies, process it in a series of signals and then send more information back to the body. Millions of cells are involved in this process, which goes on in the blink of an eye.What on Earth powers this unparalleled processing ability? The answer is cellular energy, or adenosine triphosphate (ATP). Every one of your cells needs thousands of ATP molecules each day, some more than others.

Your cells use a process called cellular respiration, which make the energy we need. It uses glucose and oxygen and makes ATP and a waste product, carbon dioxide (the same carbon dioxide we exhale). Today, we’ll learn about a key player in this process called nicotinamide adenine dinucleotide + hydrogen, or NADH for short.

What Is NADH?

NADH is a crucial coenzyme in making ATP. It exists in two forms in the cell: NAD+ and NADH.

The first form, NAD+, is called the oxidized form. When a molecule is in an oxidized state, it means it can accept electrons, tiny negatively charged particles, from another molecule. After it gets the electrons, it has a negative charge, so it also picks up a hydrogen atom from the surrounding environment, since hydrogen atoms are positively charged. Now, we have the reduced form, or NADH.The molecule acts as a shuttle for electrons during cellular respiration. At various chemical reactions, the NAD+ picks up an electron from glucose, at which point it becomes NADH. Then NADH, along with another molecule flavin adenine dinucleotide (FADH2) will ultimately transport the electrons to the mitochondria, where the cell can harvest energy stored in the electrons.

Think of the NADH as a cargo truck, transporting electrons like trucks transport goods to a factory. At the factory, the workers, or in our case proteins in the mitochondria, take the raw goods and make something they can sell for money, or ATP. In fact, many biologists refer to ATP as the ‘energy currency of life.’There are three main steps of cellular respiration:

  1. Glycolysis
  2. The citric acid cycle, which makes the most NADH
  3. Oxidative phosphorylation, which makes the most ATP from electrons carried by NADH.

Let’s look at where NADH is made at each step and how it’s converted to ATP.


During glycolysis, glucose enters the cell. The cell moves it through a series of chemical reactions and ultimately makes two pyruvate, which are needed for the next step. It also creates two ATP and two NADH, which are carried to the mitochondria.

Citric Acid Cycle

The two pyruvate are converted to another molecule called acetyl-CoA where they enter the mitochondria for the citric acid cycle. During the citric acid cycle, six electrons are harvested as NADH, and acetyl-CoA is regenerated, hence the ‘cycle’ part of the citric acid cycle. FADH2 is also made.

FADH2 carries an extra electron, allowing it to make more energy per molecule than NADH.

Oxidative Phosphorylation

Oxidative phosphorylation is the end step for NADH and FADH2. These molecules all hurry to shuttle their electrons to the membrane of the mitochondria. The membrane is made up of two layers, with a little space in between called the intermembrane space.When they get there, the NADH and FADH2 give their electrons to proteins in the electron transport chain, in which electrons are passed among molecules and release energy. These proteins are arranged by electronegativity, which refers to how much they like to hold electrons.

As the chain proceeds, each protein wants the electrons more than the last protein, so the electrons keep getting passed down. As each protein gets the electrons, they pump hydrogen ions into the intermembrane space. This creates a chemical gradient where there are more hydrogen atoms in the intermembrane space than inside the mitochondria.

At the very end of the chain, oxygen sits as the ultimate electron acceptor. Oxygen wants the electrons more than any of the proteins, so the electrons keep getting passed down. At the end, oxygen gets two electrons and grabs two hydrogen ions to make water.

Now, the real energy is made. All those hydrogen ions in the intermembrane space have built up quite a bit of energy, like water being stored at the top of a dam. The hydrogen ions flow through a protein called ATP synthase, which is like the dam itself, producing energy as water passes through. The cell uses this protein to make ATP as the hydrogen ions flow.This step makes the most ATP, between 32 and 34 molecules. All of these ATP are created using the electron carrying powers of NADH and FADH2.

At the end, since they’ve given away their electrons, NADH and FADH2 are converted to their oxidized forms, NAD+ and FAD+. Now, they are ready to go back to the starting point and be used again.

Lesson Summary

In summary, cellular respiration is the process of making energy from glucose and oxygen. It produces ATP and carbon dioxide. Cellular respiration has three steps, each designed to generate NADH, which carries electrons to the electron transport chain. In glycolysis, two NADH and two ATP are produced, as are two pyruvate.

The pyruvate are carried to the citric acid cycle, where they are converted to acetyl-CoA, and six NADH and one FADH2 are made. Lastly, the electron carriers proceed to the electron transport chain to do oxidative phosphorylation. NADH and FADH2 give their electrons to proteins in the electron transport chain, which ultimately pump hydrogen ions into the intermembrane space. This chemical gradient is used to create ATP using ATP synthase.

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