This lecture discusses two vital classes of organic molecules in living organisms: proteins and nucleic acids. Proteins, built from amino acids, are emphasized for their numerous functions, ranging from structural support and communication to transportation and defense, with their unique three-dimensional shape being crucial for their activity. The formation of polypeptides through peptide bonds and the hierarchical nature of protein structure (primary to quaternary) are explained, alongside the importance of non-protein components and the sensitivity of proteins to environmental factors like temperature and pH. The lecture also briefly introduces nucleic acids, specifically DNA and RNA, as the molecules responsible for genetic information and highlights their role in various cellular processes and modern medical applications, noting that both are composed of nucleotide subunits, one of which also forms ATP, the energy currency of the cell.