Cellular respiration is the process cells use to convert chemical energy stored in nutrient molecules, primarily glucose, into adenosine triphosphate (ATP), the cell’s main energy currency. This energy conversion occurs through a series of metabolic pathways split into distinct phases. These stages are categorized as anaerobic (independent of oxygen) or aerobic (relying on oxygen). The overall efficiency and high energy yield of cellular respiration depend on the seamless transition from initial anaerobic steps to subsequent aerobic ones.
The Anaerobic Starting Line (Glycolysis)
The journey of energy extraction begins with glycolysis, a metabolic pathway that is entirely anaerobic, requiring no oxygen. This initial stage takes place in the cytosol, the fluid-filled space within the cell. Here, a single six-carbon glucose molecule is broken down through a sequence of ten enzyme-catalyzed reactions. Glycolysis splits glucose into two three-carbon molecules called pyruvate, resulting in the net production of two ATP and two NADH molecules. The fate of this pyruvate is determined by oxygen availability, which dictates whether the cell proceeds to the highly efficient aerobic stages.
The Mitochondrial Matrix Stages (Pyruvate Oxidation and Krebs Cycle)
When oxygen is present, pyruvate transitions from the cytosol into the mitochondrial matrix, marking the shift to aerobic respiration. The first aerobic step is pyruvate oxidation, which prepares the molecules for the main cycle. During this process, each three-carbon pyruvate molecule is converted into Acetyl-CoA, a two-carbon compound attached to Coenzyme A. This reaction releases carbon dioxide and creates an \(\text{NADH}\) molecule.
The Acetyl-CoA then enters the Krebs cycle, also known as the citric acid cycle, a continuous series of eight enzyme-catalyzed reactions. The cycle completes the breakdown of glucose by fully oxidizing the acetyl group, releasing carbon dioxide and generating a small amount of \(\text{ATP}\). The most significant output is the production of high-energy electron carriers: three \(\text{NADH}\) and one \(\text{FADH}_2\) for each acetyl group.
Although oxygen is not a direct participant in the cycle’s reactions, the Krebs cycle is considered aerobic. This is because the electron carriers \(\text{NADH}\) and \(\text{FADH}_2\) must be recycled back to their oxidized forms, a process that requires oxygen in the subsequent stage.
The Essential Role of Oxygen (Electron Transport Chain)
The final and most productive stage of aerobic respiration is oxidative phosphorylation, which consists of the Electron Transport Chain (ETC) and chemiosmosis. This phase takes place on the inner mitochondrial membrane, utilizing the high-energy electrons carried by \(\text{NADH}\) and \(\text{FADH}_2\). The ETC is a series of protein complexes embedded in the membrane that accept these electrons.
As electrons are passed along the ETC in redox reactions, they gradually release energy. This energy is used by the complexes to pump protons from the mitochondrial matrix into the intermembrane space. This constant pumping establishes an electrochemical gradient across the inner membrane.
The potential energy stored in this proton gradient is harnessed by \(\text{ATP}\) synthase. Protons flow back into the matrix through \(\text{ATP}\) synthase, driving the synthesis of large amounts of \(\text{ATP}\) from \(\text{ADP}\) and inorganic phosphate. This process of \(\text{ATP}\) generation is known as chemiosmosis.
Oxygen serves as the final electron acceptor at the very end of the ETC. Oxygen accepts the spent, low-energy electrons and combines with hydrogen ions to form water. This continuous acceptance of electrons keeps the entire chain moving forward. Without oxygen, the chain would back up, the proton gradient would collapse, and the entire aerobic process would immediately halt.