Chapter 25: Biomolecules

Carbohydrates function as both structural materials and energy sources and are classified by their molecular size into monosaccharides, disaccharides, and polysaccharides. Most naturally occurring sugars belong to the D configuration and exist in solution as cyclic hemiacetal structures with either six membered pyranose or five membered furanose rings. The cyclization process creates an anomeric center that generates alpha and beta stereoisomers, and this structural distinction determines whether a sugar can reduce metallic oxidizing agents. Glycosidic bond formation between sugars creates non-reducing structures while the specific orientation of these bonds in cellulose, starch, and glycogen determines their biological roles in structural support or energy storage. Proteins are polymers of alpha amino acids held together by peptide bonds, and at physiological pH these amino acids exist as zwitterionic species with defined isoelectric points. The structure of proteins operates at four hierarchical levels: the primary sequence of amino acids, secondary structures stabilized by hydrogen bonding such as alpha helices and beta sheets, the tertiary three dimensional coiling of the entire molecule, and the quaternary associations of multiple polypeptide chains. Enzymes represent a specialized category of globular proteins that accelerate biochemical reactions by reducing activation energy, and many require metal ion cofactors or organic coenzyme molecules for catalytic function. Nucleic acids store and transmit genetic information through polymers of nucleotides consisting of phosphate groups, pentose sugars, and heterocyclic nitrogenous bases. DNA exists as a complementary double helix stabilized by specific hydrogen bonding between adenine thymine and guanine cytosine base pairs, while RNA typically remains single stranded and substitutes uracil for thymine. The central dogma of molecular biology describes the sequential processes of DNA replication, transcription to messenger RNA, and translation to proteins through the codon matching activity of transfer RNA molecules. Modern techniques including dideoxy sequencing methods and polymerase chain reaction amplification using heat stable enzymes have revolutionized genetic analysis and manipulation. Additionally, certain RNA molecules termed ribozymes demonstrate enzymatic activity, expanding the known catalytic capabilities beyond proteins.