Cellular respiration is a fundamental biological process where cells convert chemical energy from glucose and other organic molecules into adenosine triphosphate (ATP). ATP is the primary energy currency for various cellular activities. This process is essential for sustaining life in most organisms and involves multiple stages, each occurring in specific cellular compartments.
Glycolysis: The Cytoplasmic Beginning
The initial stage of cellular respiration, glycolysis, takes place in the cytoplasm. This anaerobic process, meaning it does not require oxygen, breaks down a single six-carbon glucose molecule into two three-carbon pyruvate molecules. During this transformation, a small amount of ATP is directly produced, along with NADH molecules. Glycolysis serves as a foundational step, providing the pyruvate molecules that will proceed to subsequent stages of cellular respiration if oxygen is available.
Pyruvate Oxidation and the Krebs Cycle: Inside the Mitochondrial Matrix
Following glycolysis, if oxygen is present, pyruvate molecules are transported into the mitochondria. Inside the mitochondrial matrix, the innermost compartment of the mitochondrion, pyruvate undergoes pyruvate oxidation. Each pyruvate molecule is converted into a two-carbon compound called acetyl-CoA, releasing a molecule of carbon dioxide and producing more NADH. This acetyl-CoA then enters the Krebs cycle, also known as the citric acid cycle.
The Krebs cycle is a cyclical series of reactions also occurring within the mitochondrial matrix. Acetyl-CoA combines with a four-carbon molecule and is completely oxidized, releasing two more carbon dioxide molecules per turn. The primary output of the Krebs cycle is the generation of a number of electron carriers, specifically NADH and FADH2. These electron carriers are important for the final stage of cellular respiration.
Oxidative Phosphorylation: The Inner Mitochondrial Membrane
The final stage of cellular respiration is oxidative phosphorylation, which occurs on the inner mitochondrial membrane. This process consists of two interconnected components: the electron transport chain and chemiosmosis. The NADH and FADH2 molecules generated in previous stages deliver their electrons to the electron transport chain, a series of protein complexes embedded within this membrane. As electrons move along the chain, energy is released to pump protons (hydrogen ions) from the mitochondrial matrix into the intermembrane space, creating a proton gradient across the inner membrane.
This proton gradient represents stored potential energy. Protons then flow back into the mitochondrial matrix through an enzyme called ATP synthase, also located on the inner mitochondrial membrane. The movement of protons through ATP synthase drives the synthesis of ATP from ADP and inorganic phosphate, a process known as chemiosmosis. Oxygen serves as the final electron acceptor at the end of the electron transport chain, forming water. This stage is dependent on oxygen. Oxidative phosphorylation is responsible for producing the majority of ATP generated during cellular respiration.