Metabolism: Basic Concepts and Design

  • 15.1 Energy Is Required to Meet Three Fundamental Needs

  • 15.2 Metabolism Is Composed of Many Interconnecting Reactions

  • 15.3 ATP Is the Universal Currency of Free Energy

  • 15.4 The Oxidation of Carbon Fuels Is an Important Source of Cellular Energy

  • 15.5 Metabolic Pathways Contain Many Recurring Motifs

  • 15.6 Metabolic Processes Are Regulated in Three Principal Ways

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An infinite number of books can be written with only a limited number of letters, 26 in the case of english. Likewise, the complex biochemistry of a cell—intermediary metabolism—is constructed from a limited number of recurring motifs, reactions, and molecules.

The generation of energy from the oxidation of food takes place in three stages (Figure 15.1). In the first stage (top panel), large molecules in food are broken down into smaller units. This process is digestion. As discussed in Chapter 14, digestion renders the macromolecules in our meals into biochemically more manageable fragments. Proteins are hydrolyzed to the 20 amino acids, polysaccharides are hydrolyzed to simple sugars such as glucose, and fats are hydrolyzed to fatty acids. This stage is strictly a preparation stage; no useful energy is captured at this point.

Figure 15.1: Stages of catabolism. The extraction of energy from fuels can be divided into three stages.

In the second stage (middle panel), these numerous small molecules are degraded to a few simple units that play a central role in metabolism. In fact, most of them—sugars, fatty acids, glycerol, and several amino acids—are converted into acetyl CoA, the activated two-carbon unit that is the fuel for the final stages of aerobic metabolism. Some adenosine triphosphate (ATP) is generated in the second stage, but the amount is small compared with that obtained in the third stage.

In the third stage (bottom panel), ATP is produced from the complete oxidation of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules. Acetyl CoA brings the breakdown products of proteins, sugars, and fats into the citric acid cycle (also called the tricarboxylic acid (TCA) cycle or Krebs cycle), where they are completely oxidized to CO2.

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Stages 2 and 3 are the topic of Sections 7 through 13 and consist of many metabolic pathways and hundreds of reactions. In this chapter, we will see the basic principles that underlie not only the biochemistry of Sections 7 through 13, but also the remainder of the text. Just as a simple alphabet can be used to generate an unlimited number of books, the basic principles of biochemistry discussed in this chapter can be manipulated to allow a cell and an organism to respond to a wide range of physiological circumstances.