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17.3.3: Electron Transport Chain (keep!)

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    The Electron Transport Chain and Oxidative Phosphorylation Produce the Majority of ATP

    We reviewed the three Energy Systems in section 9.2., where you learned about the ATP-PCr energy pathway, the glycolytic pathway, as well as the oxidative system that involves the Krebs Cycle, but also the electron transport chain (ETC) to covert the harvested energy molecules into ATP. Please recall from figure Figure 9.2.3.1 that the production of ATP starts during glycolysis or glycogenolysis with the formation of pyruvate, which is further broken down into acetyl coenzyme A to enter the Krebs cycle. The hydrogen ions released in this reaction are carried to the ETC where a large amounts of ATP molecules are formed.

    The Electron Transport Chain as major ATP generator

    The electron transport chain is composed of a series of protein complexes located in the inner mitochondrial membrane that function as electron carriers (Figure 9.3.2.1). The high-energy electrons delivered to the electron transport chain by NADH + H and FADH2 are passed from one protein complex to the next. As the electrons are passed along the chain, hydrogen ions are pumped out of the mitochondrial matrix into the intermembrane space. The hydrogen ions accumulate, creating a high concentration gradient that forces them back across the mitochondrial membrane into the matrix. There, the enzyme ATP synthase uses the energy generated by the concentration gradient to add a phosphate to ADP, forming ATP through a process called oxidative phosphorylation. At the same time, oxygen, electrons, and hydrogen ions combine to form water. The ATP produced flows into the cytoplasm to be used by the body.1

    Definition: Electron Transport Chain - final stage of energy metabolism in which NADH and FADH2 transport high-energy electrons to the protein complexes resulting in the formation of ATP and water.

    Figure 6.261.png

    Figure 9.3.2.1 The pathways involved in aerobic respiration2

    The electron transport chain contains a number of electron carriers. These carriers take the electrons from NADH and FADH2, pass them down the chain of complexes and electron carriers, and ultimately produce ATP. More specifically, the electron transport chain takes the energy from the electrons on NADH and FADH2 to pump protons (H+) into the intermembrane space. This creates a proton gradient between the intermembrane space (high) and the matrix (low) of the mitochondria. ATP synthase uses the energy from this gradient to synthesize ATP. Oxygen is required for this process because it serves as the final electron acceptor, forming water. Collectively this process is known as oxidative phosphorylation. The following figure and animation do a nice job of illustrating how the electron transport chain functions.

    Figure 6.262.png

    Figure 9.3.2.2 Location of the electron transport chain in the mitochondria3

    Energy Harvesting Results

    In the ETC, 2.5 ATP/NADH and 1.5 ATP/FADH2 are produced (some resources will say 3 ATP/NADH and 2 ATP/FADH2).

    Notice that the vast majority of ATP is generated by the electron transport chain. If we do the math, 28/32 X 100 = 87.5% of the ATP from a molecule of glucose is generated by the electron transport chain. Remember that this is aerobic and requires oxygen to be the final electron acceptor. If 3 ATP/NADH and 2 ATP/FADH2 are used instead of 2.5 ATP/NADH and 1.5 ATP/FADH2 that were used above, total ATP and percentage of ATP produced by the electron transport chain would be different. But the takeaway message remains the same. The electron transport chain by far produces the most ATP from one molecule of glucose.

    Glycolysis: 2 NADH

    Transition Reaction: 2 NADH

    Krebs Cycle: 6 NADH, 2 FADH2

    Total 10 NADH, 2 FADH2

    Multiply that by the amount of ATP per NADH or FADH2 to yield:

    10 NADH X 2.5 ATP/NADH = 25 ATP

    2 FADH2 X 1.5 ATP/FADH2 = 3 ATP

    Total 28 ATP

    Therefore under consideration of the preceding pathways, one molecule of glucose produces:

    Metabolic Pathway

    ATP Generated

    NADH Generated

    FADH2 Generated

    Glycolysis

    		 2 2  
    Transition Reaction   2  
    Krebs Acid Cycle 2 6 2
    Electron Transport Chain 28
    Total ' 32

    Putting it together video

    If you are an audio-visual learner, please watch this 13 minute Crash Course on the Krebs Cycle and the Electron Transport Chain

    For additional videos, look at

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    References & Links

    1. Blake, J. S., Munoz, K. D., & Volpe, S. (2019). Nutrition: From Science to You (4th ed.). Pearson.

    2. http://en.Wikipedia.org/wiki/File:CellRespiration.svg

    3. en.Wikipedia.org/wiki/File:Mi...%80%94Etc4.svg

    4. Gropper SS, Smith JL, Groff JL. (2016) Advanced nutrition and human metabolism (7th ed.). Cengage.

    17.3.3: Electron Transport Chain (keep!) is shared under a CC BY license and was authored, remixed, and/or curated by LibreTexts.