Cellular respiration is the fundamental process by which living cells convert the chemical energy stored in food molecules, primarily glucose, into a readily usable form of energy called adenosine triphosphate (ATP). This complex metabolic pathway allows organisms to power virtually all cellular activities, from muscle contraction to the creation of new proteins. Cellular respiration is divided into three distinct and sequential stages: glycolysis, the citric acid cycle, and the electron transport chain. The products of one stage serve as the raw materials for the next, maximizing the energy yield from a single glucose molecule.
Glycolysis: The Initial Breakdown
The first stage, known as glycolysis, occurs in the cytoplasm, outside of the mitochondria. Glycolysis begins with a single six-carbon glucose molecule. This process is considered anaerobic because it does not require oxygen to proceed, making it an ancient and universal method of energy production.
The glucose molecule is systematically broken down into two three-carbon molecules called pyruvate. During this conversion, energy is captured directly through substrate-level phosphorylation, resulting in a net gain of two ATP molecules. Additionally, the process yields two molecules of the electron carrier NADH, which will be utilized later in the final stage. The two pyruvate molecules generated then move into the mitochondria if oxygen is available.
The Citric Acid Cycle
The second stage is the citric acid cycle, also known as the Krebs cycle, and takes place within the mitochondrial matrix. Before the cycle can begin, the pyruvate from glycolysis must first be converted into a two-carbon molecule called acetyl-CoA. This preparatory step releases carbon dioxide as a waste product and generates another molecule of NADH.
The acetyl-CoA then enters the cycle by combining with a four-carbon molecule, and through a series of eight chemical reactions, the cycle systematically extracts the remaining energy from the carbon atoms. Although the cycle does not use oxygen directly, it is considered aerobic because it relies on the third stage, which does require oxygen. The primary function of this cycle is to strip electrons and hydrogen atoms from the intermediate molecules.
For every two molecules of acetyl-CoA that enter, the cycle produces six NADH and two FADH2 molecules. It also releases the remaining carbon atoms as four molecules of carbon dioxide and generates only two ATP (or GTP) molecules through substrate-level phosphorylation. These newly formed electron carriers are loaded with energy, setting the stage for the final step.
The Electron Transport Chain
The final and most productive stage is the electron transport chain (ETC), which occurs across the inner mitochondrial membrane. The NADH and FADH2 molecules generated in the previous stages deliver their high-energy electrons to a series of protein complexes embedded within it. As electrons move down this chain, they gradually release their energy.
The energy released is used to actively pump hydrogen ions, or protons, from the inner matrix into the intermembrane space. This pumping action creates a high concentration of protons, establishing a strong electrochemical gradient, often referred to as the proton-motive force.
The protons then flow back into the matrix, moving down their concentration gradient through a specialized enzyme complex called ATP synthase. This flow of ions powers the ATP synthase, which harnesses the energy to phosphorylate ADP, synthesizing massive amounts of ATP in a process known as chemiosmosis or oxidative phosphorylation. At the very end of the chain, molecular oxygen acts as the final electron acceptor, combining with the spent electrons and hydrogen ions to form water, removing the waste products and keeping the entire process running.
ATP Yield and Overall Summary
Glycolysis and the citric acid cycle produce a minimal amount of ATP directly, only four molecules per glucose, through substrate-level phosphorylation. The vast majority of the energy is harvested indirectly through the electron transport chain, which utilizes the electron carriers to generate a large yield.
The final output from oxidative phosphorylation produces approximately 26 to 28 ATP molecules, bringing the total net ATP yield from the complete aerobic oxidation of one glucose molecule to a range of 30 to 32 ATP molecules. Overall, cellular respiration can be summarized by the equation: Glucose and Oxygen are consumed to produce Carbon Dioxide, Water, and usable energy in the form of ATP.