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Electron Transport

The third and final stage of cellular respiration; accounts for most of the ATP.

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Electron Transport

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Ways to transport. To make ATP, energy must be ‘‘transported’’ - first from glucose to NADH, and then somehow passed to ATP. How is this done? With an electron transport chain.

Cellular Respiration Stage III: Electron Transport

Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH2, which result from the Krebs cycle, is transferred to ATP. Can you predict how this happens? (Hint: How does electron transport occur in photosynthesis?)

Transporting Electrons

High-energy electrons are released from NADH and FADH2, and they move along electron transport chains, like those used in photosynthesis. The electron transport chains are on the inner membrane of the mitochondrion. As the high-energy electrons are transported along the chains, some of their energy is captured. This energy is used to pump hydrogen ions (from NADH and FADH2) across the inner membrane, from the matrix into the intermembrane space. Electron transport in a mitochondrion is shown in Figure below.

Electron transport chains are the last step of cellular respiration

Electron-transport chains on the inner membrane of the mitochondrion carry out the last stage of cellular respiration.

Making ATP

The pumping of hydrogen ions across the inner membrane creates a greater concentration of the ions in the intermembrane space than in the matrix. This chemiosmotic gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. ATP synthase acts as a channel protein, helping the hydrogen ions cross the membrane. It also acts as an enzyme, forming ATP from ADP and inorganic phosphate. After passing through the electron-transport chain, the “spent” electrons combine with oxygen to form water. This is why oxygen is needed; in the absence of oxygen, this process cannot occur.

How much ATP is produced? The two NADH produced in the cytoplasm produces 2 to 3 ATP each (4 to 6 total) by the electron transport system, the 8 NADH produced in the mitochondria produces three ATP each (24 total), and the 2 FADH2 adds its electrons to the electron transport system at a lower level than NADH, so they produce two ATP each (4 total). This results in the formation of 34 ATP during the electron transport stage.


  • Electron transport is the final stage of aerobic respiration. In this stage, energy from NADH and FADH2 is transferred to ATP.
  • During electron transport, energy is used to pump hydrogen ions across the mitochondrial inner membrane, from the matrix into the intermembrane space.
  • A chemiosmotic gradient causes hydrogen ions to flow back across the mitochondrial membrane into the matrix, through ATP synthase, producing ATP.


  1. Summarize the overall task of Stage III of aerobic respiration.
  2. Explain the chemiosmotic gradient.
  3. What is the maximum number of ATP molecules that can be produced during the electron transport stage of aerobic respiration?

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ATP synthase An ion channel and enzyme that uses the energy of hydrogen ions to chemically bond a phosphate group to ADP, producing ATP.
chemiosmotic gradient An electrochemical gradient across the thylakoid membrane and the inner membrane of the mitochondrion; due to concentration differences of both positive charges and hydrogen ions.
electron transport Third stage of aerobic respiration in which the majority of ATP is produced.
electron transport chain Series of electron-transport molecules that pass high-energy electrons from molecule to molecule and capture their energy.

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