Krebs Cycle and Oxidative Phosphorylation

The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions that complete the breakdown of glucose. It occurs in the mitochondrial matrix and is preceded by a preparatory step.

Vocabulary: Oksidatif Fosforilasyon (Oxidative Phosphorylation) is the process by which ATP is produced using the energy from electron transfer in the electron transport chain.

The Krebs cycle preparation involves:

1. Conversion of pyruvic acid (3C) to Acetyl-CoA (2C)
2. Release of CO₂ and production of NADH+H⁺

The Krebs cycle itself involves a series of reactions that produce:

• 2 ATP (via substrate-level phosphorylation)
• 6 NADH+H⁺
• 2 FADH₂
• 4 CO₂

Example: For every glucose molecule, the Krebs cycle runs twice, as two Acetyl-CoA molecules are produced from one glucose.

The electron transport chain (ETS) is the final stage of aerobic respiration. It occurs in the mitochondrial cristae and is where the majority of ATP is produced through oxidative phosphorylation.

Highlight: Oksidatif fosforilasyon nerede gerçekleşir (Where does oxidative phosphorylation occur): It takes place in the inner mitochondrial membrane.

During ETS:

• NADH+H⁺ produces approximately 2.5 ATP
• FADH₂ produces approximately 1.5 ATP
• Oxygen acts as the final electron acceptor, forming water

ATP Production and Energy Yield

The total ATP production from one glucose molecule through aerobic respiration can be calculated as follows:

1. Substrate-level phosphorylation:

   • Glycolysis: 2 ATP
   • Krebs cycle: 2 ATP

2. Oxidative phosphorylation:

   • From glycolysis: 2 NADH+H⁺ → 5 ATP
   • From Krebs cycle preparation: 2 NADH+H⁺ → 5 ATP
   • From Krebs cycle: 6 NADH+H⁺ → 15 ATP
   • From Krebs cycle: 2 FADH₂ → 3 ATP

Highlight: The total ATP yield from one glucose molecule through aerobic respiration is approximately 32 ATP.

The chemiosmotic hypothesis explains how the electron transport chain generates a proton gradient across the inner mitochondrial membrane, which is then used by ATP synthase to produce ATP.

Definition: Substrat düzeyinde fosforilasyon (Substrate-level phosphorylation) is the direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP without the involvement of the electron transport chain.

Lactic Acid Fermentation

Lactic acid fermentation occurs in muscle cells and certain bacteria.

Example: This process is important in yogurt production and muscle function during intense exercise.

Highlight: Unlike ethanol fermentation, lactic acid fermentation is reversible, allowing the conversion back to pyruvate when oxygen becomes available.