2.20: Electron Transport

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Train, truck, boat or plane?

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 glycolysis and 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.

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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.

Summary

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.

Review

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

Image	Reference	Attributions
	[Figure 1]	Credit: Andrew Ryzhkov Source: commons.wikimedia.org/wiki/File:OH_orb5.jpg License: CC BY-NC
	[Figure 2]	Credit: Mariana Ruiz Villarreal (LadyofHats) for the CK-12 Foundation Source: CK-12 Foundation License: CC BY-NC 3.0