Organisms are composed of a great variety of chemicals, some simple and some complex. But these chemicals do not of themselves possess the properties we recognize as life. This scientific fact strongly suggests that some kind of order is imposed on them, that they are not simply dispersed in random fashion in an aqueous medium. The components of living matter are indeed elaborately organized, as demonstrated when tissues are viewed under a compound microscope. The fundamental unit of organization is the cell. In the tradi¬tional view and using a compound microscope, cells consist of a substance called protoplasm, which is subdivided into two major areas: the nucleus, which is the control center of the cell, and the cytoplasm, which includes all the rest of the cell. What sort of substance is protoplasm? The standard definition, “living substance,” conveys remarkably little.

The Cell Theory

The idea that all living things are composed of cells-the cell theory ¬is commonly credited to two German investigators, the botanist Matthias Jakob Schleiden and the zoologist Theodor Schwann, who published their conclusions in 1838 and 1839 respectively. Although this idea, one of the most important generalizations of modern biology, had been expressed earlier, Schleiden and Schwann stated the principle with particular clarity, and helped it gain general acceptance.

An important extension of the cell theory, proposed in 1858 by the German physician Rudolf Virchow, was that all living cells arise from pre-existing living cells, that there is no spontaneous creation of cells from nonliving matter. The theory of biogenesis, life from life, con¬tradicted the belief in spontaneous generation, and then widely held not only by the general public but by scientists as well. It was Louis Pas¬teur in France who, a few years later (1862), supplied proof for Vir¬chow’s theory in a series of now classic experiments. Pasteur showed that the microorganisms that cause wine or milk in an open container to go bad is spread by the air. They do not arise spontaneously from the nutrient media.

The principle of biogenesis has needed some modification in recent years and the advent of the compound microscope has also helped. Current theory holds that spontaneous generation of life from nonliving matter, while it does not occur under present conditions, probably did occur under the con¬ditions existing on the primitive earth when life first arose.

The two components of the cell theory - that all living things are composed of cells and that all cells arise from other cells - give us the basis for a working definition of living things: Living things are chemical organizations composed of cells and capable of reproducing themselves as can be seen under the compound microscope.