All cells are composed of water, proteins, carbohydrates, lipids, nucleic acids, salts, and minute quantities of a diversity of organic compounds such as vitamins and intermediates of cellular metabolism.
The proteins, carbohydrates, lipids, and nucleic acids are compounds called biomolecules, and the larger of these molecules such as the proteins and nucleic acids are also called macromolecules.
Water, ionized salts, and certain vitamins may complex with the macromolecules of the cell or they may remain free.
The quantities of each of these classes of compounds vary widely from one type of cell to another and from one organism to another (Table 3-1).
Water is the most common molecule, whereas carbohydrates and proteins are the most prevalent organic substances. In this article, some fundamentals of cellular chemistry and the properties of a number of common cellular constituents are considered.
Standard Units of Measurement:
One of the special concerns of any scientific endeavor is the effort to acquire accurate measurements in a form that can be understood and verified by others. To this end, cell biologists, like scientists in other disciplines, rely on the use of a number of standard units of measure.
For many years this function has been fulfilled by the metric system. In the 1960s, in an effort to establish uniformity among all fields of science, an international conference on weights and measures recommended certain modifications of the metric system and gave it a new name. It is now called the Systeme Internationale d’Unites (i.e., International System of Units) and is abbreviated S.I.
The S.I. system recognizes seven basic units (mass, length, time, electric current, temperature, luminous intensity and quantity of a substance). Table 3-2 lists these units and gives the symbol used for each. The metric and English equivalents of the S.I. are given in Table 3-3.
Of special note for the cell biologist and biochemist is the recommended use of the joule (J) rather than the calorie (cal) as the standard unit of energy. (One joule equals 1 kg m2/s2.) In view of the fact that energy values expressed in calories are still to be found in much of the biological literature, the caloric energy values along with their joule equivalents will be cited in this book.
