Chapter17SUMMARY - [Summary_J Metabolism represents the sum...

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Unformatted text preview: [Summary #_J 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. Metabo- lism consists of literally hundreds of enzymatic reactions organized into discrete pathways. j7.1 Are There Similarities of Metabolism Between Organ- lSl'I‘IS? One of the great unifying principles of modern biology is that Organisms show marked similarity in their major pathways of metabo- lism. Given the almost unlimited possibilities within organic chem- istry, this generality would appear most unlikely Yet it's true, and it provides strong evidence that all life has descended from a common ancestral form. All forms of nutrition and almost all metabolic path- ways evolved in early prokaryotes prior to the appearance of eukary— otes 1 billion years ago. All organisms, even those that can synthesize their own glucose, are capable of glucose degradation and ATP syn— thesis via glycolysis. Other prominent pathways are also virtually ubiq- uitous among organisms. 17.2 How Do Anabolic and Catabolic Processes Form the Core of Metabolic Pathways? Catabolism involves the oxidative degrada- tion of complex nutrient molecules (carbohydrates, lipids, and proteins) obtained either from the environment or from cellular reserves. The breakdown of these molecules by catabolism leads to the formation ol simpler molecules such as lactic acid, ethanol, carbon dioxide, urea, or ammonia. Catabolic reactions are usually exergonic, and often the chem- ical energy released is captured in the form of ATP. Anabolz‘sm is a syn- thetic process in which the varied and complex biomolecules (proteins nucleic acids, polysaccharides, and lipids) are assembled from simple! precursors. Such biosynthesis involves the formation of new covalent 76 Chapter 17 Metabolism—An Overview mds, and an input of chemical energy is necessary to drive such ender- )nic processes. The ATP generated by catabolism provides this energy. urthermore, NADPH is an excellent donor of high-energy electrons for ie reductive reactions of anabolism. 7.3 What Experiments Can Be Used to Elucidate Metabolic athways? An important tool for elucidating the steps in the path— ay is the use of metabolic inhibitms. Adding an enzyme inhibitor to a cell- 'ee extract causes an accumulation of intermediates in the pathway rior to the point of inhibition. Each inhibitor is specific for a particular Ie in the sequence of metabolic events. Genetics provides an approach ) the identification of intermediate steps in metabolism that is some- hat analogous to inhibition. Mutation in a gene encoding an enzyme often results in an inability to synthesize the enzyme in an active form, Such a defect leads to a block in the metabolic pathway at the poim where the enzyme acts, and the enzyme’s substrate accumulates. Such genetic disorders are lethal if the end product of the pathway is essential or if the accumulated intermediates have toxic effects. In microorgam isms, however, it is often possible to manipulate the growth medium so that essential end products are provided. Then the biochemical conse. quences of the mutation can be investigated. 17.4 What Food Substances Form the Basis of Human Nutri- tion? In addition to essential fiber, the food that human beings require includes the macronutrients—protein, carbohydrate, and lipid—and the micronutrients—including vitamins and minerals. ...
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Chapter17SUMMARY - [Summary_J Metabolism represents the sum...

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