Mitchell, Peter Dennis (1920-1992) was a British chemist who won the 1978 Nobel Prize in chemistry for his theory of chemiosmosis. That theory explains how energy is transported and stored in the living cell. He spent more than 20 years doing research in his own private research institute to get the evidence that convinced other chemists.

Mitchell was born in Mitcham, Surrey, England. He attended Jesus College at Cambridge University, where he studied physics, chemistry, physiology, mathematics, and biochemistry. He graduated in 1943. He then accepted a research post in the Department of Biochemistry at Cambridge, and earned his Ph.D. degree there in 1950 for his work on penicillin. From 1950 to 1955 he was a demonstrator at the university's department of biochemistry. In 1955, he set up and directed the Chemical Biology Unit at Edinburgh University. However, poor health, brought on by acute gastric ulcers, forced him to resign in 1963.

For the next two years Mitchell took a leave from research altogether. In that time he focused on the restoration of an 18th-century mansion in Bodmin, Cornwall, that he called Glynn House. Besides family living quarters, Mitchell also adapted and furnished a major part of the house for use as a research laboratory. With a former colleague, Jennifer Moyle, Mitchell established Glynn Research Ltd., a charitable foundation designed to promote biological research and to finance the work at the laboratory, called Glynn Research Institute. Mitchell and his older brother donated the original endowment for the institute, and Mitchell served as director of the institute until 1986.

In 1965, Mitchell and Moyle undertook a program of biochemical research. They were interested in how plant cell structures called mitochondria—which are the various round or long organelles surrounding the nucleus—store energy. The prevailing theory at that time was that energy, which originated from sunlight, was transported along the cell's respiratory chain by an unknown high-energy intermediate compound formed during oxidation. The energy was then stored in a “universal energy currency”—a substance called adenosine triphosphate (ATP). Energy is then released when a group of phosphorus and oxygen atoms splits off from the ATP, leaving a molecule of adenosine diphosphate (ADP) or adenosine monophosphate (AMP). The released energy produces chemical reactions in the organism, such as those involved in muscle contraction or in the production of chemical substances. Energy reserves in the cell are then refilled by regenerating (renewing) the ATP molecule. The plant does this by using energy from sunlight or oxidation to rejoin one or more phosphorus group or groups to the ADP and AMP molecules.

The intermediate compound was presumed to be a number of enzymes, but discovering those enzymes remained elusive. Mitchell developed a completely new and unorthodox approach to the problem, that the process was an electrical one, not chemical, and involved the movement of positive hydrogen ions (protons). His hypothesis, called chemiosmosis, proposed that protons are expelled from the outer surface of the inner mitochondrial membranes and accumulate outside them. This process then creates an electric potential (voltage) that powers the energy storage process. Mitchell compared the process to the way electricity moves from a high concentration to one low enough to power an electric appliance.

Mitchell's theory at first met with great skepticism, but it since has been established as the fundamental principle in the science of bioenergetics. His discovery opened up a new research field that has been the basis for explaining many biological processes. For example, it has also been demonstrated that other cellular processes that require energy use chemiosmosis, and that the equilibrium in the phosphorylation (the combining with the phosphorus atoms) of ADP to ATP is highly sensitive to the concentration of ions, which is controlled by the mitochondrial membrane.

Mitchell and Moyle conducted other experiments, including one that identified the membrane protons that provide a link to the movement of other molecules across the cell membrane. They also demonstrated that the membrane serves to halt the movement of other molecules.

Mitchell was elected a fellow of the Royal Society in 1974, and was awarded the Copley Medal in 1981. Mitchell died at Glynn House in 1992.