One of the most dramatic results of modern scientific technology 

 and of the development of the electron microscope has been the 

 revival of interest in cytology. The increased resolution of the elec- 

 tron microscope has revealed structures of amazing complexitv 

 where none was known to exist and, indeed, virtually has brought 

 form and function together at the level of macromolecules and their 

 aggregates. From our point of view, these results are particularly 

 interesting; from them we can hypothesize that the membrane sys- 

 tems of which cells are largely composed may have been the in- 

 evitable result of the mixture of large and complex molecules, such 

 as lipids and proteins, before the origin of life itself, as suggested 

 in Chap. 1. These macromolecular structures, originating bv chance, 

 may be similar to the membranes seen to be combined in cells in a 

 variety of ways. The basic cellular constituents are common to plants 

 and animals, providing a structural ground plan for all life except 

 in the most highly specialized cells or organisms. A brief review of 

 cell structure is given below to provide background for the later 

 discussion of the evolution of genetic mechanisms and systems. 



Whatever may have been the origin of cells, both plant and animal 

 cells show such striking similarity in structure as to suggest that 

 either there is a common ancestral type or, with life as we know it, 

 only one basic type of structure (Fig. 2.1) is compatible with func- 

 tion. The chemical composition of cells is relatively easy to deter- 

 mine, and many physical properties of cells and their constituents 

 can be measured. However, it is in the organization of these chem- 

 icals that the unique property of life and the cellular state is 

 achieved. The chief structural units of cells are large molecules of 

 proteins, carbohydrates, and lipids. Interspersed among, and some- 

 times actually associated with, the physical framework that results 

 from the aggregation of these molecules are the myriad types of 

 smaller molecules: soluble proteins, amino acids, vitamins, inorganic 

 constituents, etc. 



STRUCTURE OF CELLS 



Both plant and animal cells appear to be bounded by a membrane, 

 called the plasma membrane, which has the important propcrtv of 

 being differentially permeable. Physical, chemical and biological 

 studies of this membrane indicate that it is a complex structure com- 

 posed of protein and lipid molecules associated in layers. The bipolar 

 lipid molecules are arranged in two layers, with their hydrophobic 



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