Oxidative Phosphorylation

Study Guide on Aerobic ATP Production

• Aerobic Metabolism: The process that provides most of the energy required for long-duration physical activity.

• Also Known As: Oxidative phosphorylation.

• Three Stages of Aerobic ATP Production:

  1. Glycolysis: The breakdown of glucose for energy.

  2. Krebs Cycle (Citric Acid Cycle): A series of enzymatic reactions that produce energy carriers.

  3. Electron Transport Chain (ETC): The final stage where ATP is produced using energy from electrons.

• Oxygen Use: Uses oxygen to convert nutrients (primarily carbohydrates, fats, and some protein) into ATP.

• Start-up Time: Takes longer to activate due to the requirement for adequate O2 uptake.

• Exercise Fueling: Primarily supports longer, less intense bouts of exercise.

Oxidative (Aerobic System)

• Primary Goal of the Krebs Cycle: The oxidation (removal of hydrogen) of carbohydrates (CHO), fats, or proteins using NAD and FAD.

• Oxygen Usage: Oxygen is not necessary at this stage, but it is the final hydrogen acceptor before the Electron Transport Chain (ETC).

Krebs Cycle - Oxidative System

• Primary Function: The Krebs Cycle, also known as the Citric Acid Cycle, is crucial for the oxidation (removal of hydrogen) of carbohydrates (CHO), fats, or proteins. This process utilizes coenzymes NAD and FAD to facilitate energy production.

• Process Overview: The cycle takes place in the mitochondrial matrix and involves a series of enzymatic reactions that convert acetyl-CoA derived from carbohydrates, fats, and proteins into energy carriers like NADH and FADH2.

• Oxygen Role: While oxygen is not directly needed for the Krebs Cycle to occur, it plays a vital role as the final hydrogen acceptor in the Electron Transport Chain (ETC) that follows the Krebs Cycle. This ensures continuous operation of the Krebs Cycle by maintaining an adequate supply of NAD and FAD for ongoing reactions.

Electron Transport Chain (ETC)

• Transport Molecules: NADH and FADH2 molecules transport H+ to the Electron Transport Chain.

• Energy Movement: Electrons move from high to low energy, creating a proton gradient.

• ATP Production: The proton gradient powers production of ATP from ADP.

• ATP Yield: The ETC yields 36-38 ATP molecules per 1 glucose molecule.

Oxidative (Aerobic System)

• Oxygen:

  • Primary source of ATP at rest and during low-intensity activities.

  • Only occurs in mitochondria.

  • Uses carbohydrates first, then fats, and then proteins.

  • Takes over as primary source of energy after approximately 2 minutes of exercise.

• Glucose and Glycogen Oxidation:

  • Metabolism of blood glucose and muscle glycogen begins with glycolysis and leads to the Krebs cycle.

Oxidative Phosphorylation

• Location: Takes place in mitochondria.

• 3 Stage Process:

  1. Generation of Acetyl-CoA: Prepares substrates for the Krebs cycle from carbohydrates, fats, and proteins.

  2. Krebs Cycle: Oxidizes acetyl-CoA to produce energy carriers (NADH and FADH2).

  3. Electron Transport Chain: Utilizes energy from NADH and FADH2 to produce ATP through a proton gradient.

The Krebs Cycle

• Glycolysis: Results in two pyruvates.

• Conversion to Acetyl-CoA: Pyruvate is broken down to form acetyl-CoA in the presence of O2.

• Cycle Turns: Each molecule of glucose results in two pyruvate, leading to two acetyl-CoA and thus two turns of the Krebs cycle.

The Krebs Cycle

• NADH and FADH2 Processing: Once NADH and FADH2 go through the Electron Transport Chain (ETC), they contribute to ATP production.

• ATP Yield:

  • One molecule of NADH results in approximately 2.5 ATP.

  • One molecule of FADH2 results in approximately 1.5 ATP.

Krebs Cycle - Summary

• Conversion of Pyruvate to Acetyl-CoA: Results in 1 NADH.

• Krebs Cycle Results:

  • 1 ATP

  • 3 NADH

  • 1 FADH2

• Multiplication by 2: For every glucose (GLU) molecule, there are 2 pyruvates moving through the system.

• Total from Krebs Cycle:

  • 2 ATP

  • ~8 NADH

  • 2 FADH2

NADH and FADH₂ are critical coenzymes generated during the Krebs cycle. They transport high-energy electrons to the Electron Transport Chain (ETC) in the mitochondria. Once there, these molecules facilitate the creation of a proton gradient through a series of reactions, which ultimately powers the production of ATP. Specifically, one molecule of NADH can yield approximately 2.5 ATP, while one molecule of FADH₂ can yield around 1.5 ATP, enabling the aerobic system to produce a significant amount of energy.

The Krebs Cycle

• Triglyceride Breakdown: Triglycerides can be broken down to form free fatty acids and glycerol.

• Beta-Oxidation: These free fatty acids can undergo beta-oxidation to form acetyl-CoA, allowing them to enter the Krebs cycle.
  
  

The Krebs Cycle

• Completes the oxidation of carbohydrates (CHO), fats, and proteins.

• Produces carbon dioxide (CO2) as a byproduct.

• Supplies electrons to be passed on to the next phase of oxidative phosphorylation, which is the Electron Transport Chain (ETC).

Molecules in the Electron Transport Chain (ETC)

• Molecules in the ETC pump H+ ions from the mitochondrial matrix into the intermembrane space.

• This action results in an increased concentration of H+ in the intermembrane space, creating a large H+ gradient between the intermembrane space and the matrix.

• The movement of H+ ions through ATP synthase provides the energy needed to produce ATP.

Oxygen is crucial for aerobic ATP production because it serves as the ultimate electron acceptor in the Electron Transport Chain (ETC). If the final cytochrome cannot accept additional electrons, the entire ETC would cease to function. Consequently, organisms rely on breathing oxygen to ensure a continuous flow of electrons during aerobic metabolism.

The Electron Transport Chain (ETC) results in the formation of H2O and ATP. For each molecule of NADH, approximately 2.5 ATP are produced, while each FADH2 yields about 1.5 ATP. However, it is important to note that some texts may state that 1 NADH corresponds to 3 ATP and 1 FADH2 corresponds to 2 ATP.