In 1784, French chemist Antoine Lavoisier demonstrated that the body's heat is connected to the inhalation of oxygen and exhalation of carbon dioxide in a chemical process he called respiration. Later, in 1929, German biochemist Karl Lohmann discovered adenosine triphosphate (ATP), the energy carrier in cells. In 1946, German-American biochemist Fritz Lipmann uncovered coenzyme A, which is essential for the citric acid cycle. By 1948, American biochemists Eugene Kennedy and Albert Lehninger identified mitochondria as the site of respiration reactions.
The biochemical process of respiration occurs in all living cells. With the aid of oxygen, it extracts energy from food to fuel other vital chemical processes. The term "respiration" was introduced in the 18th century by chemists exploring gases. They observed that animals and plants emitted more carbon dioxide and less oxygen than they absorbed, hypothesizing that glucose served as the fuel for this gas exchange. In the early 1930s, scientists discovered how glucose was broken down into pyruvate through glycolysis, a process releasing energy without oxygen. This ancient process, known as fermentation, was utilized by early organisms before Earth's atmosphere became oxygen-rich.
The Citric Acid Cycle
In 1937, Hans Krebs, a German chemist, detailed the steps by which pyruvate is oxidized in respiration—a process involving electron loss and energy release, which can then be used by other cellular molecules. By experimenting with pigeon muscle and liver tissues, Krebs deduced that organic compounds with four or six carbon atoms could be produced through the gradual oxidation of pyruvate. These observations led Krebs to propose a metabolic pathway in a loop that starts and ends with citric acid, hence called the citric acid cycle.
The citric acid cycle (CAC) is fundamental to cellular respiration. Pyruvate is converted into acetyl-CoA, where CoA is coenzyme A that reduces pyruvate into an acetyl group and carbon dioxide. This acetyl group enters the cycle and reacts with oxaloacetate to form citrate. Through a series of oxidation reactions, electrons and energy are released, captured in reduction reactions by other molecules. After eight steps, the cycle regenerates oxaloacetate, transforming molecules from six carbon atoms to four, and releasing carbon dioxide. The reactions' energy is stored in high-energy intermediates, akin to a charged battery, for future cellular needs. Enzymes regulate the cycle to match the cell’s energy requirements, highlighting Krebs’s contribution to understanding metabolism and energy.
Hans Adolf Krebs
Hans Adolf Krebs, born in 1900 in Hildesheim, Germany, became a doctor by 25 and pursued a biochemistry career in Berlin. In 1932, he discovered the urea formation pathway, boosting his scientific reputation. Due to his Jewish ancestry, Krebs fled Nazi Germany in 1933, securing a position at the University of Sheffield, UK, where he discovered the citric acid cycle. Despite initial recognition challenges, he was elected to Britain's Royal Society in 1947 and awarded the Nobel Prize in Physiology or Medicine in 1953 alongside Fritz Lipmann. Later, collaborating with British-American biochemist Hans Kornberg, Krebs identified the glyoxylate cycle in 1957. He passed away in Oxford, UK, in 1981.
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