. Principles of modern biology. Biology. 90 - The Cell G C T G A. Fig. 4-20. Molecular structure of a single strand of nucleic acid (DNA). Black circles = sugar (deoxyri- bose); white circles = phosphate; A, adenine; C, cyto- sine; G, guanine; and T, thymine (Fig. 4-19). The chain, when broken up, yields either nucleotides (phosphate- sugar-base units) or nucleosides (slightly smaller sugar- base units, devoid of phosphate). In the living cell the various compounds may be united into larger complexes: pro- teins with salts, carbohydrates, and lipids; steroids with other lipids; carbohydrates w
. Principles of modern biology. Biology. 90 - The Cell G C T G A. Fig. 4-20. Molecular structure of a single strand of nucleic acid (DNA). Black circles = sugar (deoxyri- bose); white circles = phosphate; A, adenine; C, cyto- sine; G, guanine; and T, thymine (Fig. 4-19). The chain, when broken up, yields either nucleotides (phosphate- sugar-base units) or nucleosides (slightly smaller sugar- base units, devoid of phosphate). In the living cell the various compounds may be united into larger complexes: pro- teins with salts, carbohydrates, and lipids; steroids with other lipids; carbohydrates with inorganic salts, etc. Water associates with virtually all compounds dissolved in it; and in the case of such supermolecular aggre- gates it is not possible to distinguish clearly between chemical and physical combinations. Such linkages tend to be disrupted by the very methods usually employed to extract the protoplasmic substances from the cell, and much still remains to be learned about the chemistry of protoplasm. However, our present knowledge makes it plain that the complexity of the composition of protoplasm is sufficiently great to account for the amaz- ingly intricate behavior of living cells. PHYSICAL STRUCTURE OF PROTOPLASM Protoplasm is not just a mixture of chemi- cal components; if such were the case, one should be able to create a living cell by merely adding the proper ingredients to a test tube. This, however, would be like ex- pecting an accurately timed watch to emerge from a randomly piled assortment of cog- wheels, screws, springs, etc. Obviously the creation of any functioning mechanism de- pends not only upon the materials of which it is composed, but also upon how these ma- terials are formed and fitted into a func- tional relationship. In the last analysis, protoplasm, like other matter, is particulate, that is, composed of particles. In protoplasm, however, the par- ticles display a tremendous variability of size and shape: from single molecules and
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