Chapter 2: Chemical Composition of the Body and Its Relation to Physiology

Beginning with atomic organization, the chapter explains how protons, neutrons, and electrons combine to form elements and how variations in atomic structure produce isotopes and radioisotopes. The discussion then addresses ions and their physiological significance, particularly electrolytes like sodium, potassium, calcium, and chloride that regulate nerve impulses, muscle contraction, and fluid distribution across cellular compartments. The chapter explores how atoms bond together through covalent, ionic, and hydrogen interactions, emphasizing that bond type and molecular polarity directly influence a substance's solubility and physiological behavior. Water's unique properties as a polar molecule and universal solvent are highlighted as fundamental to life, with explanations of how water participates in hydrolysis and dehydration synthesis reactions and drives osmotic processes critical to cellular homeostasis. The chapter then addresses quantitative concepts including molarity, molecular weight, and pH, stressing why maintaining blood pH between 7.35 and 7.45 is necessary for enzyme function and survival. The four major classes of organic molecules are examined in detail: carbohydrates serving as immediate energy sources and structural components; lipids functioning in energy storage, insulation, and cell signaling; proteins displaying hierarchical structural organization that determines their diverse roles in virtually all physiological processes; and nucleic acids storing and transmitting genetic information. Throughout the chapter, the relationship between molecular structure and physiological outcome is reinforced, particularly through the clinical example of sickle-cell disease, where a single amino acid substitution alters hemoglobin's three-dimensional structure, triggering abnormal protein polymerization and severe pathological consequences. This chapter establishes the principle that all physiological mechanisms ultimately arise from chemical and physical laws operating at the molecular level.