Enzymes conform to all of these properties of catalysts, and are commonly defined as the "catalysts of living matter." They are almost universally present in living organs of every kind, and perform exceedingly important functions, both in the building-up of synthetic materials and in the rendering soluble of the food of both plants and animals, so that it can be translocated from place to place through the tissues of the organism. Enzymes differ from inorganic catalysts in being destroyed by heat, in not always carrying the reaction to the same stage as does the inorganic catalyst which may accelerate the same reaction, and in producing different changes in the same substance by different enzymes. The name "enzyme" comes from Greek words meaning "in yeast," as the nature and effect of the enzyme involved in the alcoholic fermentation of sugars by yeast were those which were first recognized and understood. It was at first thought, by Pasteur and his students, that fermentation is the direct result of the life activities of the yeast plant. Later, it was found that water extracts from sprouted barley, from almond seeds, and from the stomach, pancreas, etc., were able to bring about the decomposition of starch, of amygdalin, and of proteins, respectively, in a way which seemed to be quite comparable to the fermentative action of yeasts. Hence, it was thought that there were two varieties of active agents of this kind, one composed of living cells and the other non-living chemical compounds, and these were called the "organized ferments" and the "unorganized ferments," respectively. However, in 1897, Büchner found that by grinding yeast cells with sharp sand until they were completely disintegrated and then submitting the mass to hydraulic pressure, he could obtain a clear liquid, entirely free from living cells, which was just as active in producing fermentation as was the yeast itself. This discovery paved the way for a long series of investigations, which have conclusively demonstrated that there is no distinction between "organized" and "unorganized" ferments, that all living organisms perform their characteristic functions by means of the enzymes which they contain, and that these enzymes can bring about their characteristic catalytic effects outside the cell, or tissue which elaborates them, just as well as within it, provided only that the conditions of temperature, acidity or alkalinity of the medium, etc., are suitable for the particular enzyme action which is under consideration. GENERAL PROPERTIES OF ENZYMES