<P> By the end of the 19th century all of the major pathways of drug metabolism had been discovered, along with the outlines of protein and fatty acid metabolism and urea synthesis . In the early decades of the 20th century, the minor components of foods in human nutrition, the vitamins, began to be isolated and synthesized . Improved laboratory techniques such as chromatography and electrophoresis led to rapid advances in physiological chemistry, which--as biochemistry--began to achieve independence from its medical origins . In the 1920s and 1930s, biochemists--led by Hans Krebs and Carl and Gerty Cori--began to work out many of the central metabolic pathways of life: the citric acid cycle, glycogenesis and glycolysis, and the synthesis of steroids and porphyrins . Between the 1930s and 1950s, Fritz Lipmann and others established the role of ATP as the universal carrier of energy in the cell, and mitochondria as the powerhouse of the cell . Such traditionally biochemical work continued to be very actively pursued throughout the 20th century and into the 21st . </P> <P> Following the rise of classical genetics, many biologists--including a new wave of physical scientists in biology--pursued the question of the gene and its physical nature . Warren Weaver--head of the science division of the Rockefeller Foundation--issued grants to promote research that applied the methods of physics and chemistry to basic biological problems, coining the term molecular biology for this approach in 1938; many of the significant biological breakthroughs of the 1930s and 1940s were funded by the Rockefeller Foundation . </P> <P> Like biochemistry, the overlapping disciplines of bacteriology and virology (later combined as microbiology), situated between science and medicine, developed rapidly in the early 20th century . Félix d'Herelle's isolation of bacteriophage during World War I initiated a long line of research focused on phage viruses and the bacteria they infect . </P> <P> The development of standard, genetically uniform organisms that could produce repeatable experimental results was essential for the development of molecular genetics . After early work with Drosophila and maize, the adoption of simpler model systems like the bread mold Neurospora crassa made it possible to connect genetics to biochemistry, most importantly with Beadle and Tatum's one gene - one enzyme hypothesis in 1941 . Genetics experiments on even simpler systems like tobacco mosaic virus and bacteriophage, aided by the new technologies of electron microscopy and ultracentrifugation, forced scientists to re-evaluate the literal meaning of life; virus heredity and reproducing nucleoprotein cell structures outside the nucleus ("plasmagenes") complicated the accepted Mendelian - chromosome theory . </P>

Which came first in the scientific study of living things