Chapter 9: Fungal Physiology and Metabolism

Fungal cells contain characteristic macromolecules including proteins that function as enzymes and structural glycoproteins, nucleic acids for genetic storage and expression, carbohydrates such as chitin in cell walls and storage compounds like glycogen and trehalose, and lipids including the sterol ergosterol unique to fungal membranes. Fungal metabolism encompasses both anabolic pathways that construct cellular biomass and catabolic pathways that extract energy, with enzymatic control determining metabolic fate. Primary metabolism follows classical routes including glycolysis through the Embden-Meyerhof pathway and hexose monophosphate shunt for glucose breakdown, fermentation that regenerates cofactors under anaerobic conditions, and aerobic respiration involving the citric acid cycle and electron transport chain within mitochondria. Fungi employ distinctive biosynthetic routes such as the AAA pathway for lysine synthesis, which differs fundamentally from plant and bacterial pathways and serves as a taxonomic marker, alongside variations in tryptophan biosynthesis that reflect evolutionary relationships. Extracellular enzyme production enables fungal decomposition of complex substrates including cellulose, lignin, and protein, making fungi indispensable decomposers in ecosystems. Secondary metabolism generates bioactive compounds with significant applications and consequences, such as antibiotics including penicillin and cephalosporin, immunosuppressants like cyclosporine, toxins including aflatoxins and ergot alkaloids, and other specialized metabolites. Growth physiology progresses through spore germination, exponential hyphal expansion, branching patterns controlled by apical dominance, and developmental processes like septation and sporulation. Environmental factors including temperature, light quality, and gravitational cues regulate growth and reproductive development. Sexual reproduction involves pheromone signaling mechanisms, exemplified by sirenin in aquatic fungi, trisporic acids in zygomycetes, and mating hormones in model organisms. The chapter concludes by examining antifungal agents and their mechanisms, including polyenes that disrupt membrane sterols, azole fungicides blocking ergosterol synthesis, polyoxins inhibiting chitin synthesis, and mitotic inhibitors, demonstrating how understanding fungal physiology informs medical and agricultural applications.