Electron Transport and Oxidative Phosphorylation

A Reminder: Why Do Cellular Respiration?

• The goal: “staying alive”
• How do we continue “staying alive”?
  • Convert energy from a form unusable by the cell to a form which is usable.
  • That is, convert glucose to ATP.

Via Domenic Denicola's mad Photoshopping skills
(original image from Amazon.com)

The Story Thus Far…

• We have seen how one molecule of glucose has been catabolized into the following molecules:
  • Glycolysis has generated:
    • 2 ATP
    • 2 NADH
  • The bridge has generated:
    • 2 NADH
  • The Krebs cycle has generated:
    • 2 ATP
    • 6 NADH
    • 2 FADH₂
• The problem:
  • One glucose has only produced 4 ATPs.
  • We need more ATPs: a lot more, if we want to keep “staying alive.”

Where the Action Is Occuring

• The Krebs cycle occurs inside the matrices of mitochondria.
• Therefore, the matrices of mitochondria are full of the energy-rich molecules NADH and FADH₂.

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The Solution To Our ATP Needs: Electron Transport and Oxidative Phosphorylation

• Converts the energy from NADH and FADH2 into ATP.
• How?
  • By using the energy from NADH and FADH₂ to pump hydrogen ions (H⁺) from the mitochondrial matrix into the intermembrane space.
  • Then, by allowing the hydrogen ions (H⁺) to flow back into the matrix in a controlled way, ATP is synthesized.

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The Two-Step Process

1. Electron Transport

• A group of molecules, called the electron transport chain, moves electrons from one to another.
  • The members of the chain are collected into complexes as shown below.
    • The electrons come from NADH and FADH₂.
    • The final electron acceptor is oxygen.
  • As the electrons are transferred along the chain, energy is transferred as well: this movement of electrons is an exergonic process.
  • This exergonic process allows work to be done.
• The work done is moving hydrogen ions (H⁺) out of the mitochondrial matrix and into the intermembrane space.
  • The energy is now stored as potential energy—like water behind a dam.
• This is why we need to breathe oxygen: if we ever stop breathing oxygen, this process stops, and we can't make all this ATP!
• The final product is water, which is why we breathe out water vapor.
• Key idea: the transfer of electrons is an exergonic process that provides the energy to pump the hydrogen ions out of the matrix.

Figure 9.13, page 158, and 9.15, page 160, Campbell's Biology, 5th Edition

2. ATP Synthesis

• The enzyme ATP synthase, embedded in the inner mitochondrial membrane, allows the hydrogen ions (H⁺) to flow back into the matrix through a hole in its center.
• It uses the ions' motion to convert ADP to ATP.
• This process is called oxidative phosphorylation because the ADP is phosphorylized, and via oxidation several oxygen derivatives can be formed (see Wikipedia for more information on this).

Figure 9.14, page 159, Campbell's Biology, 5th Edition

Energy (ATP) Count

• ATP from NADH and FADH₂
  • 10 NADH × 3 ATP/NADH = 30 ATP
  • 2 FADH₂ × 2 ATP/FADH₂ = 4 ATP
  • Note: these conversion factors are the theoretical maximums. Wikipedia gives 2.5 and 1.5 as realistic values.
• Other sources of ATP
  • Glycolysis: 2 ATP
  • Krebs cycle: 2 ATP
• Total: 38 ATP (maximum)
  • 1 ATP “toll” is exacted for each NADH moved into the mitochondrion; this lowers the total to 36 ATP in some cells.
  • The above note about conversion factors can lower this more.

Pictoral Summary of Cellular Respiration

Figure 9.16, page 161, Campbell's Biology, 5th Edition; Unknown source