Cellular respiration is the set of metabolic reactions and processes that take place within the cells of organisms to convert chemical energy from nutrients (like glucose) into adenosine triphosphate (ATP), the main energy currency of the cell. The process is typically broken down into three main stages: **Glycolysis**, where glucose is broken down in the cytoplasm; the **Citric Acid Cycle (or Krebs Cycle)**, which takes place in the mitochondria and completes the breakdown; and **Oxidative Phosphorylation**, where the electron transport chain in the mitochondria uses oxygen to produce the vast majority of ATP.
The foundational understanding of cellular respiration was built over centuries. In the 1780s, Antoine Lavoisier demonstrated that respiration was a slow form of combustion. Louis Pasteur's work on fermentation in the 1850s further illuminated metabolic processes. The detailed biochemical pathways were largely uncovered in the early 20th century, with landmark discoveries by Gustav Embden and Otto Meyerhof for glycolysis, and by Hans Krebs, who elucidated the citric acid cycle in 1937, earning him a Nobel Prize. The mechanisms of oxidative phosphorylation were pieced together later by researchers like Peter D. Mitchell.
Cellular respiration is a fundamental process essential for the life of nearly all eukaryotic organisms, including humans, as well as many prokaryotes. It is the primary way organisms generate the energy needed for all cellular activities, such as growth, repair, and movement. A deep understanding of this process is critical in medicine for studying metabolic diseases (like diabetes), understanding the effects of poisons (like cyanide, which inhibits the electron transport chain), and developing therapies. It is also central to exercise physiology, ecology (in understanding energy flow through ecosystems), and biotechnology.