PART BIOENERGETICS AND METABOLISM 13 Principles of Bioenergetics 480 Although metabolism embraces hundreds of differ 14 Glycolysis, Gluconeogenesis, and the Pentose ent enzyme-catalyzed reactions, our major concern in Phosphate Pathway 521 Part II is the central metabolic pathways, which are few 15 Principles of Metabolic Regulation, Ilustrated with in number and remarkably similar in all forms of life the Metabolism of Glucose and Glycogen 560 ing organisms can be divided into two large groups according to the chemical form in which they obtain 16 The Citric Acid Cycle 601 carbon from the environment Autotrophs(such as 17 Fatty Acid Catabolism 631 photosynthetic bacteria and vascular plants)can use 18 Amino Acid Oxidation and the production carbon dioxide from the atmosphere as their sole source of carbon, from which they construct all their carbon of Urea 666 containing biomolecules(see Fig. 1-5). Some auto 19 Oxidative Phosphorylation and trophic organisms, such as cyanobacteria, can also Photophosphorylation 700 atmospheric nitrogen to generate all their nitrogenous 20 Carbohydrate Biosynthesis in Plants components. Heterotrophs cannot use atmospheric and Bacteria 761 carbon dioxide and must obtain carbon from their en vironment in the form of relatively complex organic mol 21 Lipid Biosynthesis 79 ecules such as glucose Multicellular animals and most 22 Biosynthesis of Amino Acids, Nucleotides, and microorganisms are heterotrophic. Autotrophic cells Related molecules 843 and organisms are relatively self-sufficient, whereas het 23 Integration and Hormonal Regulation of Mammalian erotrophic cells and organisms, with their requirements Metabolism 891 for carbon in more complex forms, must subsist on the products of other organisms Many autotrophic organisms are photosynthetic and obtain their energy from sunlight, whereas het erotrophic organisms obtain their energy from the etabolism is a highly coordinated cellular activity degradation of organic nutrients produced by auto in which many multienzyme systems(metabolic trophs. In our biosphere, autotrophs and heterotrophs pathways)cooperate to(1)obtain chemical energy by live together in a vast, interdependent cycle in which capturing solar energy or degrading energy-rich nutrients autotrophic organisms use atmospheric carbon dioxide from the environment;(2)convert nutrient molecules to build their organic biomolecules, some of them gen the cells own characteristic molecules, including en from water in the precursors of macromolecules;(3) polymerize mono- in turn use the organic products of autotrophs as nu meric precursors into macromolecules: proteins, nucleic trients and return carbon dioxide to the atmosphere acids, and polysaccharides; and (4)synthesize and Some of the oxidation reactions that produce carbon degrade biomolecules required for specialized cellular dioxide also he oxygen, converting it to water. functions, such as membrane lipids, intracellular mes. Thus carbon, oxygen, and water are constantly cycled sengers, and pigments between the heterotrophic and autotrophic worlds, withMetabolism is a highly coordinated cellular activity in which many multienzyme systems (metabolic pathways) cooperate to (1) obtain chemical energy by capturing solar energy or degrading energy-rich nutrients from the environment; (2) convert nutrient molecules into the cell’s own characteristic molecules, including precursors of macromolecules; (3) polymerize mono￾meric precursors into macromolecules: proteins, nucleic acids, and polysaccharides; and (4) synthesize and degrade biomolecules required for specialized cellular functions, such as membrane lipids, intracellular mes￾sengers, and pigments. Although metabolism embraces hundreds of differ￾ent enzyme-catalyzed reactions, our major concern in Part II is the central metabolic pathways, which are few in number and remarkably similar in all forms of life. Living organisms can be divided into two large groups according to the chemical form in which they obtain carbon from the environment. Autotrophs (such as photosynthetic bacteria and vascular plants) can use carbon dioxide from the atmosphere as their sole source of carbon, from which they construct all their carbon￾containing biomolecules (see Fig. 1–5). Some auto￾trophic organisms, such as cyanobacteria, can also use atmospheric nitrogen to generate all their nitrogenous components. Heterotrophs cannot use atmospheric carbon dioxide and must obtain carbon from their en￾vironment in the form of relatively complex organic mol￾ecules such as glucose. Multicellular animals and most microorganisms are heterotrophic. Autotrophic cells and organisms are relatively self-sufficient, whereas het￾erotrophic cells and organisms, with their requirements for carbon in more complex forms, must subsist on the products of other organisms. Many autotrophic organisms are photosynthetic and obtain their energy from sunlight, whereas het￾erotrophic organisms obtain their energy from the degradation of organic nutrients produced by auto￾trophs. In our biosphere, autotrophs and heterotrophs live together in a vast, interdependent cycle in which autotrophic organisms use atmospheric carbon dioxide to build their organic biomolecules, some of them gen￾erating oxygen from water in the process. Heterotrophs in turn use the organic products of autotrophs as nu￾trients and return carbon dioxide to the atmosphere. Some of the oxidation reactions that produce carbon dioxide also consume oxygen, converting it to water. Thus carbon, oxygen, and water are constantly cycled between the heterotrophic and autotrophic worlds, with PART BIOENERGETICS AND METABOLISM II 13 Principles of Bioenergetics 480 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway 521 15 Principles of Metabolic Regulation, Illustrated with the Metabolism of Glucose and Glycogen 560 16 The Citric Acid Cycle 601 17 Fatty Acid Catabolism 631 18 Amino Acid Oxidation and the Production of Urea 666 19 Oxidative Phosphorylation and Photophosphorylation 700 20 Carbohydrate Biosynthesis in Plants and Bacteria 761 21 Lipid Biosynthesis 797 22 Biosynthesis of Amino Acids, Nucleotides, and Related Molecules 843 23 Integration and Hormonal Regulation of Mammalian Metabolism 891 481