Chapter 4: Energy and Cellular Metabolism

Energy and Cellular Metabolism begins by distinguishing between kinetic and potential energy and applies the first and second laws of thermodynamics to physiological systems, explaining how the human body conserves energy while combating the natural tendency toward entropy and disorder. The text explores the mechanisms of chemical reactions, differentiating between exergonic reactions that release free energy and endergonic reactions that require energy input, illustrating how these are coupled through the hydrolysis of high-energy phosphate bonds in adenosine triphosphate (ATP). A major focus is placed on the role of enzymes as biological catalysts that accelerate reaction rates by lowering activation energy, with a detailed discussion on enzyme specificity, isozymes, and the necessity of cofactors and coenzymes derived from vitamins. The summary explains how cells regulate metabolic pathways through feedback inhibition, reversible reactions, and the compartmentalization of enzymes within organelles. Metabolic processes are categorized into catabolism and anabolism, with a specific focus on glucose metabolism. The chapter outlines the anaerobic conversion of glucose to lactate via glycolysis in the cytosol, contrasted with the high-yield aerobic pathways that occur in the mitochondria, including the conversion of pyruvate to acetyl CoA, the citric acid cycle (Krebs cycle), and the electron transport system. Concepts such as oxidative phosphorylation and the chemiosmotic theory are introduced to explain how high-energy electrons from NADH and FADH2 drive ATP synthesis. Finally, the chapter connects energy metabolism to the molecular basis of cellular function by detailing protein synthesis, tracing the genetic code from DNA transcription and mRNA processing (including alternative splicing) to translation at the ribosome and posttranslational modification.