Chapter 20: Photosynthesis and Carbohydrate Synthesis in Plants: Light Reactions, Carbon Fixation, and Starch Synthesis

Section 20.1 discusses the thermodynamic basis of metabolism, emphasizing the concepts of free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), and explains how cells couple exergonic and endergonic reactions using high-energy compounds like ATP. The importance of phosphoryl group transfers is examined in detail, highlighting the role of ATP, phosphocreatine, and other high-energy intermediates in driving biosynthetic processes. Section 20.2 introduces redox reactions as central to energy metabolism, with nicotinamide and flavin coenzymes (NAD⁺/NADH, FAD/FADH₂) facilitating electron transfer in catabolic pathways. The standard reduction potential (E°′) is explained as a predictor of electron flow, and the electrochemical basis of ATP synthesis through oxidative phosphorylation is introduced. Section 20.3 discusses the hierarchical organization of metabolic pathways, including the concepts of metabolic flux, control points, and the roles of allosteric enzymes, covalent modification, and substrate availability in regulating pathway activity. It also highlights the central role of key intermediates—such as glucose, acetyl-CoA, and pyruvate—that link major metabolic pathways like glycolysis, the TCA cycle, and fatty acid oxidation. Section 20.4 introduces compartmentalization in eukaryotic cells as a strategy to separate and regulate metabolic processes efficiently, using organelles like mitochondria, cytosol, and endoplasmic reticulum. The chapter concludes by emphasizing the dynamic and adaptive nature of metabolism, showing how signaling pathways, hormones, and nutrient availability influence metabolic priorities. This chapter sets the biochemical framework for understanding energy flow, biosynthesis, and metabolic integration across all organisms.