Cellular respiration is a fundamental process that converts nutrients into adenosine triphosphate (ATP), the cell’s primary energy currency. This process extracts chemical energy stored in food molecules. The Krebs cycle, also known as the citric acid cycle, is a central component of this energy production system.
What is the Krebs Cycle?
The Krebs cycle is a series of chemical reactions central to cellular metabolism. In eukaryotic cells, these reactions occur within the mitochondrial matrix. For prokaryotic cells, the Krebs cycle takes place in the cytoplasm.
The main input for the Krebs cycle is acetyl-CoA, a two-carbon compound from the breakdown of carbohydrates, fats, and proteins. Acetyl-CoA combines with oxaloacetate, a four-carbon molecule, to initiate each turn of the cycle. Outputs include carbon dioxide (CO2), a small amount of ATP, and the electron carriers NADH and FADH2. These carriers transport high-energy electrons for a subsequent stage of energy production.
The Bigger Picture: Cellular Respiration
Cellular respiration is a multi-stage process that extracts energy from glucose. It begins with glycolysis, an anaerobic process occurring in the cytoplasm. Glycolysis breaks down glucose into two pyruvate molecules, generating ATP and NADH.
If oxygen is present after glycolysis, pyruvate converts to acetyl-CoA, entering the Krebs cycle. The Krebs cycle then produces more electron carriers (NADH and FADH2) and carbon dioxide. The final stage of aerobic cellular respiration is the electron transport chain (ETC) and oxidative phosphorylation. This stage occurs on the inner mitochondrial membrane in eukaryotes, where protein complexes transfer electrons from NADH and FADH2. Oxygen plays an important role here, serving as the final electron acceptor to form water, which drives significant ATP production.
Oxygen’s Indirect Connection to the Krebs Cycle
While the Krebs cycle does not directly consume oxygen, it is considered an aerobic process. This classification stems from its dependence on the electron transport chain, which requires oxygen. The Krebs cycle produces NADH and FADH2, which are reduced electron carriers. For the cycle to continue, these carriers must be recycled into their oxidized forms, NAD+ and FAD.
This regeneration of NAD+ and FAD occurs through the electron transport chain. If oxygen is unavailable, the electron transport chain cannot function because there is no final electron acceptor. Consequently, NADH and FADH2 accumulate, and the supply of NAD+ and FAD diminishes. This buildup inhibits key enzymes within the Krebs cycle, bringing the cycle to a halt. Therefore, for the Krebs cycle to operate continuously, oxygen must be present to regenerate NAD+ and FAD via the electron transport chain, allowing the aerobic respiration pathway to proceed.