At first glance, the subject matter of biochemistry seems too complicated to do anything other than blindly memorize it. Fortunately, biochemistry has a number of unifying themes, which can help you keep the varying branches in perspective.
Four classes of small molecules combine to make up most of the important bimolecular structures (see Figure ). Most of these are optically active, that is, they are found in only one of the possible stereoisomers. ( Stereoisomers are compounds that have the same kinds and numbers of atoms but have different molecular arrangements.
- Amino acids all have the common core structure shown in Figure . Generally, amino acids found in nature are the L‐stereoisomers. Amino acids are the building blocks of proteins, and have an important role in energy metabolism and in cellular signaling. They are also a small but important part of cell membranes.
- Carbohydrates are molecules of the empirical formula C n(H 2O) n where n usually ranges from 3–7. They are found in sugars and starches and make up parts of nucleotides (the energy currency of a cell, and the building blocks for genetic information). They are also present in some components of all cell membranes. They are the central components of energy‐producing pathways in biology.
- Lipids are closely related to hydrocarbons (compounds containing hydrogen and carbon atoms exclusively), although they usually have other atoms beside C and H. Characterized by limited solubility in water, lipids are essential components of membranes, and are important energy stores in plants and animals.
- Nucleosides and nucleotides contain a carbohydrate component joined to one of four carbon‐ and nitrogen‐containing ring compounds called bases. They make up the energy currency of the cell, and, when joined end‐to‐end (polymerized) into DNA or RNA chains, form the genetic information of a cell.