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IN THEIR EFFORTS to learn about life and living beings, biologists have developed many specialties, for although they know but a tiny fraction of what they need to know in order to understand fully what life is and how it functions, the sum total of biological knowledge is already far too extensive and complex for any one man to grasp it all. And so, some biologists have concerned themselves primarily with the identification and recognition of the different kinds of organisms, others with the manifold structures which these oragnisms have developed; still others have sought to analyze their activities, how they nourish themselves, grow and reproduce, how they behave in their native surroundings, and under controlled conditions. Most of these and other specialties, however, attack the problem of the nature of life only indirectly, by studying the varied manifestations of life, the results of life activity. It is only by a study of that part of a living organism which is itself alive (the so-called protoplasm) that one can hope to gain an understanding of what life is in essence, what lies at the bottom of and is responsible for the phenomena which together make up what we call life. To achieve this goal, one must turn to the cell, for protoplasm exists, with very few exceptions, not in large masses, but in minute units which are called cells. Cytology is the study of the cell, and it is, therefore, of all fields the one which comes closest to the heart of the major quest of biology the understanding of life in its essence. The cell is a marvelous microcosm, extremely small, yet unbelievably complex. Although protoplasm is incomparably the most complex system known, it is organized into units whose size, or lack of size, is difficult for the average man to grasp. As the late Professor Sponsler (1940) has pointed out, the average-sized cell has a volume about onemillionth that of the average raindrop. It might seem that a unit of matter so small would be incapable of containing a substance as complicated as protoplasm. The complexities of protoplasm, however, are at the molecular level. It is an organized system of molescules of myriad kinds, some simple, some ranging up to the most complex molecules known, each kind of molecule having its own chemical properties, the sum total of all these properties adding up, when the molecules are organized in just the right way, to what we call life. The huge complexity of protoplasm does not require great mass or bulk, for even the largest molecules are minute in comparison with the size of the cell, and the space necessary to accommodate the total organization of these molecules is easily provided by the average-sized cell. That there is plenty of space for a system as complicated as protoplasm in the cell is shown by a comparison of the size of an average cell with that of the various molecules which make up protoplasm. Protoplasm is composed of a skeletal framework made up of protein molecules, to which molecules of other kinds are attached, the whole suspended or dissolved in water. One of the substances found in some abundance in the cell is sugar. As Sponsler has pointed out, there is room in an average-sized cell for 64 trillion molecules of glucose (grape sugar), each with a molecular weight of 180 (i.e., it is 180 times as heavy as a hydrogen atom). It would take a person, counting at the rate of one a second, over 2 million years to count 64 trillion. The protein molecules which form the structural framework of the protoplasm are much larger than glucose molecules, averaging about 36,000 molecular weight. A cell with a volume a millionth that of the average raindrop is large enough to accommodate over 60 billion such protein molecules of average size (about 25 times as many as there are people on the face of the earth) . Some of the protein molecules in the cell, however, may have molecular weights as high as 6 million. There would be room for as many as 500 million molecules of this size in such a cell. It is evident, therefore, that a cell could even be much smaller. than a millionth the size of an average raindrop and still be abundantly able to enclose an enormous number of molecules of all sizes and sorts, and to permit a molecular organization so complex that its properties would total that of life itself. The cell has been known for a long time, but its nature, its organization and the ways in which it functions have only recently begun to be elucidated. Back in 1665, Robert Hooke, an Englishman, constructed one of the first "microscopes," and with it saw many things never before seen by the eyes of man-among them, the box-like structure of cork. The compartments which he found to compose the structure of cork he called "cells'-he thought that they were empty compartments, as indeed, in cork they were, since cork cells die soon after they have been formed and hence lose their contents. It was not until nearly 200 years after Hooke that it became evident that living cells are not walls surrounding empty spaces, but are masses of material surrounded by walls or membranes. The gradual revelation of the structural characteristics of this material, the discovery that it is endowed with the power of movement and able to react and respond in various ways to various stimuli, was the work of 19th century investigators. The 1870's and 1880's were especially significant in the history of cytol |
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