Recall that glycolysis takes place in the cytosol and that pyruvate decarboxylation and the TCA cycle occurs in the matrix of the mitochondria. The final step in cellular respiration that I will discuss here is oxidative phosphorylation or the electron transport chain (ETC). It occurs in the inner membrane of the mitochondria.


The purpose the the ETC is to take the electrons from NADH and FADH2 and use them to make the high energy molecule, ATP. The ETC consists of key protein complexes which can be simplified as protein complex I, II, II and IV. There full names are:

  1. Complex I: NADH dehydrogenase
  2. Complex II: Succinate dehydrogenase
  3. Complex III: Cytochrome oxidoreductase
  4. Complex IV: Cytochrome oxidase
The Electron Transport Chain. Image source.

High energy electrons from NADH are accepted by Complex I, regenerating NAD+ for glycolysis and the TCA cycle. High energy electrons from FADH2 are accepted by Complex II, regenerating FAD for the TCA cycle. The electrons from complex I and II are picked up and transported over to Complex III by a carrier molecule called coenzyme Q (CoQ) or ubiquinone. Another carrier molecule called cytochrome c is responsible for taking electrons from Complex III to Complex IV. Complex IV carries electrons into the matrix where diatomic oxygen acts as the final acceptor of the electrons. The process produces water.

The movement of electrons generates a proton pump that pushes protons (hydrogen) across protein complex I, III and IV in the order, 4, 4, and 2 protons. Therefore, there is always a higher concentration of protons in the intermembrane space compared to the matrix. The final protein complex at the end of the ETC called ATP synthase uses this electrochemical gradient to pump protons back into the matrix. For every four protons pumped in, one ADP molecule is phosphorylated to ATP. Hopefully at this point you can see why the process is called oxidative phosphorylation. The process is dependent on oxygen and involves a phosphorylation step to make ATP.

For every NADH molecule that enters the ETC, 10 protons are pumped out, and hence 2.5 ATP molecules are produced (10/4 = 2.5 ATP)

For every FADH2 molecule that enters the ETC, 6 protons are pumped out, and hence 1.5 ATP molecules are produced (6/4 = 1.5 ATP).

Calculating the Number of ATP Made in Cellular Respiration

Since we now know how much ATP is generated from NADH and FADH2, we can now calculate the amount of ATP produced in cellular respiration. Check out the following table.  

Respiration StepCalculated ATPs
Glycolysis2 ATP = 2 ATP, 2 NADH x 2.5 = 5 ATP
Pyruvate Decarboxylation2 NADH x 2.5 = 5 ATP
TCA Cycle 2 ATP = 2 ATP, 6 NADH x 2.5 = 15 ATP, 2 FADH2 x 1.5 = 3 ATP

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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