The nucleus, which in many cells is the largest and most central body, is of special importance. It contains a number of threadlike bodies, or chromosomes, that are the carriers of the cell’s heredity-controlling system. These contain granules of a material called chromatin, which is rich in a nucleic acid, DNA (deoxyribonucleic acid). The chromosomes usually are not readily seen in the nucleus except when the cell, along with its nucleus, is dividing. When the nucleus is not dividing, a spherical body, the nucleolus, can be seen. (In some nuclei there may be more than one.) When the nucleus is dividing, the nucleolus disappears. Not all cells possess all these structures. For instance, the red cells of the blood do not have a nucleus, and in other cells the endoplasmic reticulum is at a minimum. The diagram (Figure 2) is valid for a great majority of the cells of higher organisms. The cell structures shown in Figure 3 are visible with an electron microscope. They contain the chemical components of the cell. The chief classes of these constituents are the carbohydrates (sugars), the lipids (fats), the proteins, and the nucleic acids. However, a cell also contains water (about 70% of the cell weight is due to water) and several other organic and inorganic compounds, such as vitamins and minerals. Carbohydrates serve mostly as foodstuff within the cell. They can be stored in several related forms. Further, they may serve a number of functions outside the cell, especially as structural units. In this way structure and function are correlated. Lipids in the cell occur in a great variety of types: alcohols, fats, steroids, phospholipids, and aldehydes. They are found in all fractions of the cell. Their most important functions seem to be to form membranes and to give these membranes specific permeability. They are also important as stores of chemical energy, mostly in the form of neutral fats. [Illustration: Figure 4 _Scientists using an electron microscope (left) and an optical microscope (right) in fundamental biochemical research. Both instruments are important tools in studies of life processes._] The proteins occur in many cell structures and are of many kinds: Enzymes, the catalysts for the cell’s metabolic processes, are proteins, for instance. The nucleic acids are DNA and RNA (ribonucleic acid), which function together to manufacture the cell’s proteins. Since a large share of the remaining pages will be devoted to a discussion of proteins and nucleic acids, at this point we need only emphasize that these two types of materials are interrelated in their function and that both are essential. The Two Nucleic Acids