Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
The Composition of Living Matter: Biomolecules
The main classes of biomolecules in cells are represented by very large macromolecules

Tables 3-6 list the Major Classes of Biomolecules found in the bacterium Escherichia coli, their contribution to the total Cell mass, and the approximate number of distinct biomolecule species in each Class. Water accounts for the largest fraction of the mass in E. coli, as it does in all other Cells and organisms. Inorganic salts and other mineral components make up only a minor fraction of the cellular dry weight; many of these substances are present in roughly the same proportions as in seawater. Virtually all of the dry matter in E. coli, similar to all other cells, consists of Organic compounds represented by four primary types of molecules: Proteins, Nucleic Acids, Polysaccharides, and Lipids. Proteins constitute the major portion of living matter not only in E. coli but in all other cell types as well. The term "protein" is derived from the Greek word proteios, meaning "first" or "foremost." In All living organisms, proteins are the direct products of genes and the effectors of their action. Many proteins exhibit specific catalytic activity and function as Enzymes. Other types of proteins serve as structural elements in Cells and Tissues. A number of membrane-associated proteins facilitate The transport of specific molecules into and out of The Cell. Proteins also participate in a multitude of other biological Functions, making them arguably the most versatile biomolecules in this regard. Nucleic acids, DNA and RNA, perform identical functions across all cell types, ensuring the storage, transmission, and expression of Genetic information. DNA acts as the repository of genetic information, whereas various types of RNA facilitate its expression during Protein Synthesis.

Table 3-6. Molecular Components of the E. coli cell

Fig. 3-9. A. Scanning electron micrograph of starch granules within a potato cell. B. Electron micrograph showing the layered packing of Cellulose fibers that form the structural framework of Plant Cell Walls.

Polysaccharides primarily serve two functions. Some, such as starch, act as Various Forms of reserve "fuel" that supply the cell with energy, whereas others, such as cellulose, function as extracellular structural components (Fig. 3-9). Lipids, which include fats and fat-like substances, firstly play The Role of essential structural components in membranes and, secondly, serve as an energy-rich fuel storage form.

These four major classes of biomolecules share a common characteristic: they are relatively large structures with high molecular weights and are therefore referred to as macromolecules. The molecular weights of various proteins range from 5,000 to 1 million; for certain nucleic acids, molecular weights can reach several billion; polysaccharides, such as starch, also exhibit high molecular weights on the order of millions. Individual lipid molecules are considerably smaller (molecular weight 50–1,500). However, lipid molecules typically associate with one another to form very large structures comprising thousands of molecules, functioning essentially as macromolecular systems (such systems serve, in particular, as the "backbone" of cell membranes). Thus, we may also classify such lipid assemblies as macromolecules.