Living organisms convert nutrients into energy through cellular respiration, a process that generates adenosine triphosphate (ATP), the primary energy currency for cellular activities. This article explores the citric acid cycle, a key stage of cellular respiration, and its relationship with oxygen.
Cellular Respiration Overview
Cellular respiration is a set of metabolic reactions that break down glucose and other organic molecules to produce ATP. The initial stage, glycolysis, takes place in the cytoplasm.
Following glycolysis, subsequent stages of cellular respiration occur within mitochondria in eukaryotic cells. These stages include the citric acid cycle and oxidative phosphorylation.
The Citric Acid Cycle’s Function
The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway located within the mitochondrial matrix. Its primary function involves the oxidation of acetyl-CoA, a two-carbon molecule derived from the breakdown of carbohydrates, fats, and proteins. As acetyl-CoA is processed, the cycle releases carbon dioxide as a waste product.
Throughout its eight steps, the cycle generates high-energy electron carriers, specifically nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). A small amount of ATP or guanosine triphosphate (GTP) is also directly produced during this cyclical process. The regeneration of the starting molecule, oxaloacetate, allows the cycle to continue processing incoming acetyl-CoA.
Oxygen’s Indirect Dependency
While the citric acid cycle does not directly consume oxygen within its own reactions, its operation is entirely dependent on the presence of oxygen for downstream processes. The NADH and FADH2 molecules produced by the citric acid cycle are crucial for the next stage of cellular respiration, the electron transport chain (ETC). In the ETC, these electron carriers donate their electrons, powering the synthesis of the majority of cellular ATP.
Oxygen serves as the final electron acceptor in the electron transport chain, combining with electrons and hydrogen ions to form water. Without oxygen to accept these electrons, the electron transport chain cannot function. This halt in the ETC leads to a buildup of reduced NADH and FADH2, which then cannot be re-oxidized back to NAD+ and FAD. Consequently, the lack of available NAD+ and FAD inhibits key enzymes within the citric acid cycle, causing the cycle to slow down or stop completely.