MetabolismAn Overview


18.1 • Virtually All Organisms Have the Same

Basic Set of Metabolic Pathways

18.2 • Metabolism Consists of Catabolism

(Degradative Pathways) and Anabolism (Biosynthetic Pathways)

18.3 • Experimental Methods To Reveal

Metabolic Pathways

18.4 • Nutrition Special Focus: Vitamins

Anise swallowtail butterfly (Papilio zelicans) with its pupal case. Metamorphosis of butterflies is a dramatic example of metabolic change. (© 1986 Peter Bryant/Biological Photo Service)

The word metabolism derives from the Greek word for "change." Metabolism represents the sum of the chemical changes that convert nutrients, the "raw materials" necessary to nourish living organisms, into energy and the chemically complex finished products of cells. Metabolism consists of literally hundreds of enzymatic reactions organized into discrete pathways. These pathways proceed in a stepwise fashion, transforming substrates into end products through many specific chemical intermediates. Metabolism is sometimes referred to as intermediary metabolism to reflect this aspect of the process. Metabolic maps (Figure 18.1) portray virtually all of the principal reactions of the intermediary metabolism of carbohydrates, lipids, amino acids, nucleotides,

FIGURE 18.1 • A metabolic map, indicating the reactions of intermediary metabolism and the enzymes that catalyze them. Over 500 different chemical intermediates, or metabolites, and a greater number of enzymes are represented here. (© 1997 20th edition, designed by and courtesy of D. E. Nicholson, University of Leeds, U. K, and the Sigma Chemical Co.)

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