Aerobic respiration is a biological process that allows organisms to efficiently generate energy. This pathway converts glucose and oxygen into usable energy in the form of adenosine triphosphate (ATP), along with carbon dioxide and water as byproducts. ATP is often referred to as the “energy currency” of the cell, powering virtually all cellular functions, from muscle contraction to nerve impulse transmission and molecular synthesis.
Glycolysis
Glycolysis represents the initial stage of aerobic respiration, taking place within the cytoplasm, or cytosol, of the cell. This process does not require oxygen, making it a universal pathway found in nearly all organisms. During glycolysis, a single six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvate.
This breakdown yields a net gain of two ATP molecules, providing a small, immediate energy supply for the cell. Additionally, two molecules of NADH, an electron carrier, are produced. These NADH molecules transport high-energy electrons.
The Krebs Cycle
Following glycolysis, if oxygen is available, the two pyruvate molecules are transported into the mitochondrial matrix to enter the Krebs cycle, also known as the Citric Acid Cycle. Before entering the cycle, each pyruvate molecule is converted into acetyl-CoA, a two-carbon compound. This conversion also generates additional NADH and releases carbon dioxide.
The Krebs cycle then begins as acetyl-CoA combines with a four-carbon molecule, oxaloacetate, to form citrate. Through a series of enzymatic reactions, citrate is systematically oxidized, leading to the release of carbon dioxide. The primary function of this cycle is to generate a significant number of electron carriers.
For each turn of the cycle, molecules of NADH and FADH2 (another electron carrier) are produced. A small amount of ATP, or an equivalent molecule called GTP, is also generated directly within the cycle.
The Electron Transport Chain
The Electron Transport Chain (ETC) is the final and most energy-rich stage of aerobic respiration, occurring on the inner membrane of the mitochondria. The NADH and FADH2 molecules generated during glycolysis and the Krebs cycle deliver their high-energy electrons to the ETC.
As electrons move through a series of protein complexes embedded in the inner mitochondrial membrane, energy is released. This energy is utilized to pump hydrogen ions, or protons, from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
Oxygen’s role in aerobic respiration becomes evident in the ETC. Oxygen acts as the final electron acceptor at the end of the chain, combining with electrons and protons to form water molecules. Without oxygen to accept these electrons, the entire chain would halt, stopping ATP production. The flow of protons back into the matrix through a specialized enzyme called ATP synthase drives the synthesis of large quantities of ATP, a process known as oxidative phosphorylation.