point of view. The discovery in 1837 that yeast was a living organism suggested that fermentation itself was a living process. Only a few scientists rejected this vitalistic protoplasm theory, notably Traube. He was the first to define enzymes as specific protein-like compounds and to formulate the necessity of direct molecular contact between enzyme and substrate for fermentation to occur. He classified enzymes by reaction type, much as is done today. Long before Eduard Buchner discovered non-cellular fermentation in 1897, Traube isolated an enzyme from potatoes which could turn guaiacum blue, thus demonstrating the continued efficacy of plant enzymes after they had been extracted from the cell. Until recently, biochemical history has not noted that Traube began to investigate the kinetics of reactions and also demonstrated a reciprocal relationship between reaction time and quantity of enzyme. To defend his theory, Traube had to argue vigorously against Louis Pasteur and Felix Hoppe-Seyler. He contradicted Pasteur's assertion that fermentation could not occur without vital activity. In the context of these experiments Traube became the first to describe a process for making pure yeast. His differences with Hoppe-Seyler had to do with the mechanism by which oxygen was activated in fermentation reactions. Traube's experiments were designed to demonstrate activation via enzymes as catalysts and refute Hoppe-Seyler's hypothesis of oxygen activation by nascent hydrogen produced by enzymes. Physiology of plants and the invention of artificial semipermeable membranes In 1864 Traube was the first to produce artificial semipermeable membranes, recognizing them as molecular sieves and using them in developing the first physical-chemical theory of plant cell growth. The artificial cells were created by putting droplets of glue in tannic acid; these grew under infusion of water. (Other membranes were created with tannic acid plus verdigris or potassium ferrocyanide plus copper chloride). These membranes laid the foundation for research into osmotic pressure in solutions (Wilhelm Pfeffer and Jacobus Henricus van 't Hoff used them), and Traube himself did research on the manifestations of diffusion and osmosis. Pathophysiology, bacteriology and hygienics Traube also made an important contribution to the study of the etiology of disease. Together with Gscheidlen, an assistant of Rudolf Heidenhain he was the first to demonstrate via animal experiments that the organism has the ability to eliminate putrefactive bacteria. In evaluating the results, he distinguished chemical poisoning from infection with microorganisms on the one hand, and pathogenic from putrefactive bacteria on the other. Further,