Chapter I
Chemistry or Life?
In the early nineteenth century many chemists believed that the substances of living things could only be made by a vital force. In 1828 Friedrich Wöhler made urea, a substance from urine, by heating an ordinary salt. He wrote to his mentor Berzelius that he could make urea without needing a kidney. It did not end vitalism at once. But over the century chemists made more and more natural compounds in the laboratory, and the special status of living chemistry faded.
Fermentation was the test case. Pasteur showed that it needs living yeast. Justus von Liebig insisted it was ordinary chemistry. In 1897 Eduard Buchner settled it by grinding yeast with sand and filtering out every cell. The juice still turned sugar into alcohol. The work was done by substances in the cell, enzymes, and both men had been partly right.
Chapter II
Enzymes
What were enzymes? In 1913 Leonor Michaelis and Maud Menten described how fast they work, treating each enzyme as binding its target before transforming it. In 1926 James Sumner crystallised the enzyme urease and showed it was a protein. The eminent chemist Richard Willstätter disputed it for years, but by the 1930s it was clear: enzymes are proteins, each shaped to catalyse one reaction.
Biochemists then traced how cells break down food. Sugar is split step by step, and in 1937 Hans Krebs, who had fled Nazi Germany, found the cycle of reactions in which the fragments are burned to carbon dioxide. Fritz Lipmann identified ATP as the molecule in which the energy is captured and spent. Every living thing, from bacteria to people, turned out to use the same core pathways.
Chapter III
A Closer Look: Your Weight in ATP
A resting adult uses around 2,000 kilocalories a day, about 8.4 million joules. Most of that energy passes through ATP. Food is oxidised, the energy is captured by making ATP, and ATP is broken down to power everything from muscle contraction to pumping ions across nerve membranes.
How much ATP is that? Under cell conditions, breaking one mole of ATP releases about 50 kilojoules. Only part of the energy in food ends up in ATP, perhaps 40–60%, the rest being lost as body heat. Taking about half:
A mole of ATP weighs about 507 grams, so that is roughly 40 kilograms of ATP made and broken down every day, and more during exercise. Estimates of this kind put daily ATP turnover at around a person's own body weight.
Yet the body contains only about 250 grams of ATP at any moment. So each ATP molecule must be rebuilt from its breakdown products, about times a day, roughly every nine minutes. The machine that rebuilds it is ATP synthase, a rotary motor in the mitochondrial membrane. Protons flowing through it turn a rotor about a hundred times a second, and each full turn makes three ATP molecules. That protons drive it is what Peter Mitchell proposed in 1961. The rotary motor itself was worked out later, by Paul Boyer and John Walker.
Chapter IV
Energy From a Membrane
Mitchell's chemiosmotic theory was the last great surprise of classical biochemistry. For years biochemists had searched for a chemical intermediate that links burning food to making ATP. Mitchell, working from a private laboratory in a restored Cornish manor house, argued that there was none. Respiration pumps protons across a membrane, and their flow back drives ATP synthesis. It was ridiculed for a decade and then confirmed. Bacteria, mitochondria and chloroplasts all make ATP this way, a sign of their shared ancestry.
By the 1950s biochemistry had also merged with genetics. The one gene–one enzyme idea tied genes to proteins, and molecular biology found how genes encode them. The structures of cell parts and how enzymes are organised in space became cell biology.