THE BIOLOGY BOOK

Metabolism

1614

For thirty years, Santorio Sanctorius, the Italian physiologist, physician, and inventor of the medical thermometer, meticulously weighed himself prior to and after engaging in all manner of life’s activities—eating, drinking, fasting, eliminating, sleeping, and engaging in sex. These he published in 1614, in Ars de statica medicina, in which he described the first controlled experiment that introduced quantification to medical practice. Sanctorius noted that the weight of his feces and urine was less than the food he had ingested and attributed the difference to “insensible perspiration.” So began the study of metabolism.

BUILDING UP AND BREAKING DOWN. One of the fundamental characteristics of all living beings is their use of energy to perform their activities. Metabolism, a word from the Greek meaning to “change” or “overthrow,” refers to all chemical reactions in living beings that generate or utilize energy, and these are divided into anabolic and catabolic reactions. Anabolic reactions use energy for the biosynthesis (manufacture) of larger organic molecules as well as for the growth and differentiation of cells. Conversely, catabolic reactions involve the breakdown of these molecules to produce energy. Chemical reactions are organized into metabolic pathways in which one chemical is converted to another in a sequence, and these reactions are catalyzed by enzymes. The metabolic pathways involve carbohydrates, fats, proteins, and nucleic acids, and the nature of the chemicals in these pathways is very similar across many different species, ranging from microbes to humans.

Studies performed by Hans Krebs during the 1930s provided the basis for our fundamental understanding of metabolic pathways. Krebs was a German-born physician-biochemist who discovered the urea cycle, which described the pathway by which organisms remove ammonia formed in the body to the less toxic urea. After a Nazi order forbidding him as a Jew to practice medicine in Germany, he emigrated to England. There he made his most important discovery, the 1937 identification of the citric acid cycle (Krebs cycle), which described a series of chemical reactions used by all aerobic organisms to generate energy from carbohydrates, proteins, and fats. In recognition, he was awarded the 1953 Nobel Prize in Physiology or Medicine.

SEE ALSO: Plant Defenses against Herbivores (c. 400 Million BCE), Enzymes (1878), Inborn Errors of Metabolism (1923).

Adenosine triphosphate (ATP) is the “molecular unit of currency” for the transfer of energy within cells and represents the major source of energy for the metabolic reactions of most organisms. This image shows a three-dimensional representation of ATP.