Electron Transport, Oxidative Phosphorylation, and Oxygen Metabolism

Chapter 14: Electron Transport, Oxidative Phosphorylation, and Oxygen Metabolism

The Mitochondrion: Scene of the Action

The mitochondrion is the primary site of aerobic energy metabolism in eukaryotic cells. It houses the machinery for the citric acid cycle, electron transport chain, and oxidative phosphorylation, which are essential for ATP production.

• Mitochondria are double-membraned organelles with distinct compartments: the outer membrane, intermembrane space, inner membrane (with cristae), and matrix.

• The inner mitochondrial membrane contains the protein complexes responsible for electron transport and ATP synthesis.

• The matrix is the site of the citric acid cycle and contains enzymes for the oxidation of metabolic fuels.

Overview of Oxidative Energy Generation

Oxidative energy generation is the process by which cells extract energy from organic molecules through a series of metabolic pathways. This process occurs in three main stages:

1. Stage 1: Carbon from metabolic fuels (such as glucose, fatty acids, and amino acids) is converted into acetyl-CoA.

2. Stage 2: The citric acid cycle (Krebs cycle) oxidizes acetyl-CoA to produce CO2, reduced electron carriers (NADH and FADH2), and a small amount of ATP (or GTP).

3. Stage 3: The reduced electron carriers are reoxidized in the electron transport chain, providing energy for the synthesis of additional ATP via oxidative phosphorylation.

• Relatively little ATP is generated in stages 1 and 2; most ATP is produced during stage 3.

• Stages 2 and 3, as well as part of stage 1, occur in the mitochondria.

Key Terms and Concepts

• Acetyl-CoA: A central metabolic intermediate formed from carbohydrates, fats, and proteins.

• Citric Acid Cycle: A series of enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA to CO2 and generate NADH and FADH2.

• Electron Transport Chain (ETC): A sequence of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, coupled to proton pumping and ATP synthesis.

• Oxidative Phosphorylation: The process by which ATP is synthesized as a result of electron transport-driven proton gradients across the inner mitochondrial membrane.

Example: ATP Yield from Glucose Oxidation

• Glycolysis and the citric acid cycle produce 10 NADH and 2 FADH2 per glucose molecule.

• These reduced electron carriers are reoxidized in the ETC, driving the synthesis of most cellular ATP.

Additional info: The process of oxidative phosphorylation is tightly regulated and is the primary source of ATP in aerobic organisms. Defects in mitochondrial function can lead to a variety of metabolic diseases.