When a cell breaks down glucose to begin extracting energy, the initial process is called glycolysis. This process takes place in the cell’s cytoplasm and does not require oxygen. Glycolysis breaks the six-carbon glucose molecule into two three-carbon molecules called pyruvate, generating a small net gain of two ATP. The presence of oxygen is the decisive factor that allows the cell to transition to a much more complex and productive system. With oxygen available, the cell directs the pyruvate into the mitochondria to unlock the remaining energy reserves.
Preparing for the Main Cycle
The two pyruvate molecules created in the cytoplasm must first be moved into the inner compartment of the mitochondria, called the matrix. This transport is a necessary gateway before the primary energy-releasing reactions can begin. Once inside the matrix, each three-carbon pyruvate molecule undergoes a rapid, three-step conversion process.
The first step involves the removal of one carbon atom, which is released as carbon dioxide (\(\text{CO}_2\)), a waste product the body will eventually exhale. This leaves a two-carbon fragment that is then oxidized, removing high-energy electrons and packaging them onto a carrier molecule, \(\text{NAD}^+\), turning it into \(\text{NADH}\). The final step attaches this two-carbon fragment, now an acetyl group, to Coenzyme A, forming Acetyl-CoA. Acetyl-CoA is the essential entry molecule that feeds the two-carbon unit into the subsequent metabolic loop.
The Citric Acid Cycle
With the production of Acetyl-CoA, the energy-releasing process moves into the Citric Acid Cycle, which occurs entirely within the mitochondrial matrix. This cycle begins when the two-carbon acetyl group combines with a four-carbon starting molecule, oxaloacetate, to form citrate. The cycle then systematically dismantles the citrate molecule through a series of eight chemical reactions.
The function of this pathway is to harvest high-energy electrons. As the carbon atoms are oxidized and rearranged, their electrons are stripped away and loaded onto electron carrier molecules. For every acetyl group that enters, three molecules of \(\text{NADH}\) and one molecule of \(\text{FADH}_2\) are generated, representing the cell’s energy currency for the final stage. During this process, the remaining carbon atoms are fully oxidized, with two more \(\text{CO}_2\) molecules released for each acetyl group. The cycle is considered aerobic because the production of these carriers relies on the final stage, which requires oxygen.
The Massive Energy Payoff
The vast majority of the cell’s energy is produced in the final stage, a two-part process called Oxidative Phosphorylation, which involves the Electron Transport Chain (ETC) and Chemiosmosis. This complex machinery is embedded within the inner membrane of the mitochondria, a highly folded structure that creates a large surface area for these reactions. The electron carriers, \(\text{NADH}\) and \(\text{FADH}_2\), arrive at this membrane and donate their high-energy electrons to a series of protein complexes that form the ETC.
These electrons move sequentially from one protein complex to the next, steadily releasing small amounts of energy at each step. This released energy is used by the protein complexes to pump positively charged hydrogen ions, or protons, from the inner matrix space into the narrow space between the inner and outer mitochondrial membranes. This constant pumping action creates a high concentration of protons in the outer compartment, establishing an electrochemical gradient.
The second part of the final stage, Chemiosmosis, harnesses the potential energy stored in this proton gradient. The hydrogen ions, driven by the strong concentration difference, rush back into the inner matrix. Their only route is through a specialized enzyme complex called ATP synthase. The flow of protons through this enzyme causes it to rotate, and this mechanical energy is used to attach a phosphate group to \(\text{ADP}\), generating the large quantities of \(\text{ATP}\).
Oxygen is necessary for this entire system to operate. At the end of the ETC, oxygen acts as the final electron acceptor, collecting the electrons after they have completed their journey. Oxygen then combines with these electrons and hydrogen ions from the matrix to form water, the final product of aerobic respiration. Without oxygen to remove the spent electrons, the entire chain backs up, the proton gradient cannot be maintained, and \(\text{ATP}\) synthesis stops.