Chapter 2: The Chemical Basis of Life

The stability of matter is rooted in covalent bonds, characterized by shared electrons and high bond energy, which includes single, double, and triple bonds, as well as the concepts of electronegativity, polarity, and ionization leading to cations and anions. In contrast, weaker noncovalent bonds govern molecular interactions; these include ionic bonds (salt bridges), hydrogen bonds (essential for water’s structure and properties), and van der Waals forces that stabilize close-fitting nonpolar molecules via transient dipoles, alongside hydrophobic interactions driven by the exclusion of water (entropy maximization). Water, a highly polar molecule, exhibits life-supporting properties due to its extensive capacity for hydrogen bonding, including thermal stability and its role as an unparalleled solvent. The core of biological molecules lies in carbon chemistry, forming diverse backbones decorated by functional groups that confer specific chemical reactivity. These molecules fall into four major classes: carbohydrates, used for energy storage (e.g., glycogen and starch) and structure (e.g., cellulose and chitin), built from sugar monomers like glucose, which exist primarily in ring structures with various stereoisomers; lipids, encompassing fats (triacylglycerols, key for long-term energy), steroids (like cholesterol), and phospholipids (amphipathic components of cell membranes); and nucleic acids (DNA and RNA), polymers of nucleotides, serving primarily in information storage and transmission, with RNA also exhibiting complex structural and catalytic roles (ribozymes). The most functionally diverse class is proteins, constructed from 20 types of amino acids, linked by peptide bonds. Protein structure is hierarchical: primary (sequence), secondary (localized folding like the alpha helix and beta sheet), tertiary (overall 3D fold stabilized by side chains), and quaternary (multi-subunit complexes like hemoglobin). The amino acid sequence contains all the information necessary for the protein to achieve its final, low-energy conformation, a principle known as self-assembly, although many proteins rely on molecular chaperones (like Hsp70 and GroEL) to prevent aggregation during the folding process. Misfolding is highly relevant in human health, linked to neurodegenerative diseases like prion disorders (CJD) and Alzheimer’s disease, where misfolded proteins form aggregates like amyloid plaques. Advanced techniques like proteomics (large-scale analysis of the entire protein complement, often via mass spectrometry) and interactomics (mapping protein-protein interactions) are used to understand cellular activity and structure-based drug design. Finally, the chapter addresses the organization of larger cellular elements, discussing the self-assembly of viral particles and ribosomal subunits, and introducing the concept of phase-separated compartments—liquid droplets formed by proteins and RNA that increase local concentration to facilitate reactions.