Cellular respiration is a fundamental biological process that converts nutrients into adenosine triphosphate (ATP), the cell’s primary energy currency. This series of metabolic reactions fuels nearly all life functions, from muscle contraction to the synthesis of complex molecules. As a catabolic process, cellular respiration breaks down larger molecules, such as glucose, to release stored chemical energy. This energy-generating mechanism occurs in most organisms, highlighting its importance for sustaining life.
Glycolysis: The Starting Point
Cellular respiration begins with glycolysis, the initial stage where a single glucose molecule is broken down. This process occurs in the cytoplasm of the cell. During glycolysis, a six-carbon glucose molecule undergoes a series of reactions, ultimately yielding two molecules of a three-carbon compound called pyruvate.
Glycolysis requires an initial input of two ATP molecules to prepare the glucose for breakdown. The process then enters an energy-yielding phase, producing four ATP molecules and two molecules of NADH. This results in a net gain of two ATP and two NADH molecules per glucose molecule. Glycolysis is an anaerobic process, meaning it does not require oxygen.
Pyruvate Oxidation: Preparing for the Cycle
Following glycolysis, the two pyruvate molecules enter the next stage, known as pyruvate oxidation. This process serves as a crucial link, preparing the products of glycolysis for entry into the subsequent cycle. In eukaryotic cells, pyruvate molecules are transported from the cytoplasm into the mitochondrial matrix, the innermost compartment.
Once inside the mitochondrial matrix, each three-carbon pyruvate molecule undergoes a transformation. A carboxyl group is removed, releasing one molecule of carbon dioxide. The remaining two-carbon molecule is then oxidized, and the high-energy electrons released are captured by NAD+ to form NADH. This two-carbon unit, an acetyl group, combines with coenzyme A to form acetyl-CoA.
The Krebs Cycle: Energy Extraction
The Krebs cycle, also known as the citric acid cycle, is the third stage of cellular respiration and operates in the mitochondrial matrix. This cycle serves as a central hub for energy extraction, accepting the acetyl-CoA generated from pyruvate oxidation. The acetyl-CoA combines with a four-carbon molecule, oxaloacetate, to initiate the cycle by forming a six-carbon citrate molecule.
The cycle then proceeds through a series of eight enzyme-mediated reactions, regenerating oxaloacetate to continue the process. Its primary function is to generate high-energy electron carriers, NADH and FADH2, which will be utilized in the final stage of ATP production. For each turn of the cycle, one molecule of acetyl-CoA yields three NADH molecules, one FADH2 molecule, and one ATP (or GTP) molecule, along with the release of two carbon dioxide molecules.
Since each glucose molecule from glycolysis yields two pyruvates, and thus two acetyl-CoA molecules, the Krebs cycle completes two full turns per original glucose molecule. The carbon dioxide released during this cycle represents the complete oxidation of the original glucose molecule.
Oxidative Phosphorylation: The Major ATP Producer
The final and most productive stage of cellular respiration is oxidative phosphorylation, which generates the vast majority of cellular ATP. This complex process occurs on the inner mitochondrial membrane and involves two interconnected components: the electron transport chain (ETC) and chemiosmosis.
The electron carriers, NADH and FADH2, produced in earlier stages, donate their high-energy electrons to the electron transport chain. As electrons move through a series of protein complexes embedded in the inner mitochondrial membrane, energy is released. This energy is used to pump protons (hydrogen ions) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient.
This proton gradient represents a significant store of potential energy. Protons then flow back into the mitochondrial matrix through a specialized enzyme complex called ATP synthase, a process known as chemiosmosis. The movement of protons through ATP synthase drives the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate. Oxygen serves as the final electron acceptor at the end of the electron transport chain, combining with electrons and protons to form water.