Biological Molecules

The complexity of life processes requires that many of the molecules that participate in these processes be enormous. The most extreme example is DNA. Consider, for instance, the DNA molecules released from one human chromosome. The long, looped thread corresponds to just two enormous molecules, each with a molecular weight of about 20 billion. Protein molecules are generally much smaller, but they are still large, with a typical protein having a mass of 50,000 Daltons. To give an idea of the complexity of such a molecule, Figure 1.6 shows the three-dimensional structure of one protein molecule as revealed by x-ray crystallography.

All large macromolecules in the cell are polymers, made by joining prefabricated units, or monomers. The monomers of a given type of macromolecule are of limited diversity and are linked together, or polymerized, by identical mechanisms. A simple example is the carbohydrate cellulose (Figure 1.7a), a major constituent of the cell walls of plants. Cellulose is a polymer made by joining thousands of molecules of glucose, a simple sugar. In this polymer all of the chemical linkages between the monomers are identical. Covalent links between glucose units are formed by removing a water molecule between two adjoining molecules; the portion of a glucose molecule remaining in the chain is called a glucose residue.

Because cellulose is a polymer of a simple sugar, or saccharide, it is called a polysaccharide. This particular polymer is constructed from identical monomeric units, so it is called a homopolymer. In contrast, many polysaccharides and all nucleic acids and proteins are heteropolymers, polymers constructed from a number of different kinds of monomer units. Nucleic acids (Figure 1.7b) are polymers of four nucleotides, so nucleic acids are also called polynucleotides. Similarly, proteins (Figure 1.7c) are assembled from combinations of 20 different amino acids. Protein chains are called polypeptides, a term derived from the peptide bond that joins two amino acids together.

> Polysaccharides serve both as structural components and as reserves of biological energy.

> Nucleic acids participate in information storage, transmission, and expression.

> Proteins - 1. Perform structural roles in the body, 2. Act as transport substances (for example, hemoglobin, the oxygen-carrying protein of blood), 3. Defend an organism against infection (antibodies), and 4. Function as enzymes, catalyzing the thousands of chemical reactions that occur within an individual cell.

> Lipids serve as the major structural element of the membranes that surround cells and partition them into various compartments. They also are the major energy storage form of cells.

See also: Amino Acids, Sugars, Nucleotides, Glucose, Peptide Bond