Cell Chemistry and Cell Structure

Abstract

In recent years developments in the rapidly expanding areas of molecular biology, biochemical evolution, biochemical genetics, and enzymology have made it increasingly clear that the beginning student of biology needs to have some knowledge of the relationships between chemical constituents and structural components of cells in order to appreciate and understand the living organism. This trend toward a greater emphasis on molecular and cellular biology was recognized in the final report (1967) of the Commission on Undergraduate Education in the Biological Sciences (CUEBS) Panel on Undergraduate Major Curricula (2). The authors of the Biological Sciences Curriculum Study (BS CS) text and laboratory materials have stressed that the student can best gain this type of understanding through his experiences in the laboratory, but it is difficult to find laboratory investigations which really deal with the chemical components of a living organism. Most of the existing laboratory exercises emphasize that cells are composed of proteins, carbohydrates, lipids, and nucleic acids, and that there are appropriate color tests to distinguish each of these major groups of compounds. The student is then led, cookbook style, through a series of analyses for starch, reducing sugars, protein, and possibly nucleic acids, using materials such as bread, flour, honey, gelatin, and egg albumen. Tests performed in this manner fail to provide the student with the excitement of working with live material. Furthermore, there often seems to be too much emphasis on the color change per se and not enough on the part of the living cell with which the protein, carbohydrate, lipid, or nucleic acid is associated; the functional implications of its presence; or the reasons for the specificity of the test. In searching for a more stimulating way to give the student some insight into the techniques used by the cell biologist and a meaningful picture of the roles of the various chemical compounds in the cell, we hit on the rather obvious approach of examining intact cells, as well as parts of cells, obtained from living tissue for the major categories of organic compounds mentioned above. In the investigation we have developed, the student considers types of compounds as they are related to different structures and functions of the cell. He sees for himself that cell membranes contain substantial amounts of lipid, that DNA is localized in the nucleus, that RNA is associated with the cytoplasmic and membranous material, and that proteins are widely distributed substances. After he has made these observations, both chemically and microscopically in the laboratory, he is better able to take the next step-that of studying why the chemical characteristics of each group of compounds make it possible for them to play their particular roles. In designing this kind of laboratory investigation a number of factors had to be considered: a) The material that was to be chosen for analysis had to be easily and inexpensively obtained and had to be practically colorless if the color changes associated with the chemical analyses were to be accurately observed. b) The enrollments in beginning classes are often large. c) The schedules of most colleges (and high schools) do not allow for more than two hours of regularly scheduled laboratory work a week in beginning classes. d) The chemical analyses and staining procedures had to be simple techniques. Bean seedlings, grown in the dark, were selected