Chapter 3: Microbial Metabolism

Metabolism is divided into catabolism, which breaks down organic or inorganic molecules to release energy, and anabolism, which uses that energy to synthesize cellular components. Microorganisms require water, energy sources, electron donors, and nutrients to build macromolecules and maintain cellular processes. The chapter explains the foundations of bioenergetics, including the role of free energy in chemical reactions, the coupling of exergonic and endergonic reactions, and the central role of ATP as the primary cellular energy currency. Redox reactions drive metabolism through electron transfer between electron donors and acceptors, generating reducing power and enabling energy conservation. Microorganisms are classified metabolically based on energy and carbon sources, including phototrophs, chemotrophs, chemoorganotrophs, chemolithotrophs, autotrophs, and heterotrophs. Major catabolic pathways are explored, including glycolysis, the citric acid cycle, and fermentation, which allow cells to extract energy from organic compounds. The chapter also examines respiration and electron transport chains, where electrons flow through membrane-bound carriers to generate a proton motive force used by ATP synthase to produce ATP through oxidative phosphorylation. Metabolic diversity among microorganisms includes anaerobic respiration using alternative electron acceptors, inorganic energy metabolism in chemolithotrophs, and photophosphorylation in phototrophic organisms. Carbon assimilation pathways such as the Calvin cycle enable autotrophic growth through carbon dioxide fixation. Additional metabolic processes include nitrogen fixation through nitrogenase and biosynthetic pathways responsible for the production of carbohydrates, nucleotides, amino acids, lipids, and other cellular macromolecules. Together, these processes illustrate how microbial metabolism supports growth, energy conservation, ecological diversity, and the global cycling of nutrients.