HUMAN ANATOMY & PHYSIOLOGYCopyright Wiley, 2020953CHAPTER 25The food we eat is our only source of energy for running, walking, and even breathing. Many molecules needed to maintain cells...

HUMAN ANATOMY & PHYSIOLOGY Copyright W iley, 2020 953 CHAPTER 25 The food we eat is our only source of energy for running, walking, and even breathing. Many molecules needed to maintain cells and tissues can be made from simpler precursors by the body’s metabolic reactions; others—the essential amino acids, essential fatty acids, vitamins, and minerals—must be obtained from our food. As you learned in Chapter 24, carbohydrates, lipids, and proteins in food are digested by enzymes and absorbed in the gastrointestinal tract. The products of digestion that reach body cells are monosaccharides, fatty acids, glycerol, monoglycerides, and amino acids. Some minerals and many vitamins are part of enzyme systems that catalyze the breakdown and synthesis of carbohydrates, lipids, and proteins. Food molecules absorbed by the gastrointestinal (GI) tract have three main fates: 1. Most food molecules are used to supply energy for sustaining life pro- cesses, such as active transport, DNA replication, protein synthesis, muscle contraction, maintenance of body temperature, and mitosis. 2. Some food molecules serve as building blocks for the synthesis of more complex structural or functional molecules, such as muscle proteins, hormones, and enzymes. 3. Other food molecules are stored for future use. For example, glycogen is stored in liver cells, and triglycerides are stored in adipose cells. In this chapter we discuss how metabolic reactions harvest the chemical energy stored in foods; how each group of food molecules contributes to the body’s growth, repair, and energy needs; how energy balance is maintained in the body; and how body temperature is regulated. Finally, we explore some aspects of nutrition to discover why you should opt for fish instead of a burger the next time you eat out. Q Did you ever wonder how fasting and starvation affect the body? Metabolism and Nutrition Metabolism, Nutrition, and Homeostasis Metabolic reactions contribute to homeostasis by harvesting chemical energy from consumed nutrients for use in the body’s growth, repair, and normal functioning. c25MetabolismAndNutrition.indd Page 953 10/14/16 11:55 AM f-512 /208/WB01989/9781119287759/ch25/text_s 882 Copyright W iley, 2020 954 CHAPTER 25 Metabolism and Nutrition for less than a minute before being used. Thus, ATP is not a long-term storage form of currency, like gold in a vault, but rather convenient cash for moment-to-moment transactions. Recall from Chapter 2 that a molecule of ATP consists of an adenine molecule, a ribose molecule, and three phosphate groups bonded to one another (see Figure 2.26). Figure 25.1 shows how ATP links anabolic and catabolic reactions. When the terminal phosphate group is split off ATP, adenosine diphosphate (ADP) and a phosphate group (sym- bolized as P ) are formed. Some of the energy released is used to drive anabolic reactions such as the formation of glycogen from glucose. In addition, energy from complex molecules is used in catabolic reactions to combine ADP and a phosphate group to resynthesize ATP: ADP + P + energy ATP About 40% of the energy released in catabolism is used for cellular functions; the rest is converted to heat, some of which helps maintain normal body temperature. Excess heat is lost to the environment. Compared with machines, which typically convert only 10–20% of energy into work, the 40% eff iciency of the body’s metabolism is impressive. Still, the body has a continuous need to take in and process external sources of energy so that cells can synthesize enough ATP to sustain life. Checkpoint 1. What is metabolism? Distinguish between anabolism and catabolism, and give examples of each. 2. How does ATP link anabolism and catabolism? 25.1 Metabolic Reactions OBJECTIVES • Define metabolism. • Explain the role of ATP in anabolism and catabolism. Metabolism (me-TAB-ō-lizm; metabol- = change) refers to all of the chemical reactions that occur in the body. There are two types of metabolism: catabolism and anabolism. Those chemical reactions that break down complex organic molecules into simpler ones are collectively known as catabolism (ka-TAB-ō-lizm; cata- = downward). Overall, catabolic (decomposition) reactions are exergonic; they pro- duce more energy than they consume, releasing the chemical energy stored in organic molecules. Important sets of catabolic reactions occur in glycolysis, the Krebs cycle, and the electron transport chain, each of which will be discussed later in the chapter. Chemical reactions that combine simple molecules and monomers to form the body’s complex structural and functional components are collectively known as anabolism (a-NAB-ō-lizm; ana- = upward). Examples of anabolic reactions are the formation of peptide bonds between amino acids during protein synthesis, the building of fatty acids into phospholipids that form the plasma membrane bilayer, and the linkage of glucose monomers to form glycogen. Anabolic reactions are endergonic; they consume more energy than they produce. Metabolism is an energy-balancing act between catabolic (decomposition) reactions and anabolic (synthesis) reactions. The molecule that participates most oft en in energy exchanges in living cells is ATP (adenosine triphosphate), which couples energy-releasing catabolic reactions to energy-requiring anabolic reactions. The metabolic reactions that occur depend on which enzymes are active in a particular cell at a particular time, or even in a par - ti cular part of the cell. Catabolic reactions can be occurring in the mitochondria of a cell at the same time as anabolic reactions are taking place in the endoplasmic reticulum. A molecule synthesized in an anabolic reaction has a limited life- time. With few exceptions, it will eventually be broken down and its com- ponent atoms recycled into other molecules or excreted from the body. Recycling of biological molecules occurs continuously in living tissues, more rapidly in some than in others. Individual cells may be refurbished molecule by molecule, or a whole tissue may be rebuilt cell by cell. Coupling of Catabolism and Anabolism by ATP The chemical reactions of living systems depend on the eff icient transfer of manageable amounts of energy from one molecule to another. The molecule that most oft en performs this task is ATP, the “energy currency” of a living cell. Like money, it is readily available to “buy” cellular activities; it is spent and earned over and over. A typical cell has about a billion molecules of ATP, each of which typically lasts FIGURE 25.1 Role of ATP in linking anabolic and catabolic reactions. When complex molecules and polymers are split apart (catabolism, at left ), some of the energy is transferred to form ATP and the rest is given off as heat. When simple molecules and monomers are combined to form complex molecules (anabolism, at right), ATP provides the energy for synthesis, and again some energy is given off as heat. The coupling of energy-releasing and energy-requiring reactions is achieved through ATP. ATP Simple molecules such as glucose, amino acids, glycerol, and fatty acids Complex molecules such as glycogen, proteins, and triglycerides PADP + Heat released Anabolic reactions transfer energy from ATP to complex molecules Catabolic reactions transfer energy from complex molecules to ATP Heat released Q In a pancreatic cell that produces digestive enzymes, does anabolism or catabolism predominate? c25MetabolismAndNutrition.indd Page 954 10/14/16 11:55 AM f-512 /208/WB01989/9781119287759/ch25/text_s 883 johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight Copyright W iley, 2020 25.2 Energy Transfer 955 25.2 Energy Transfer OBJECTIVES • Describe oxidation–reduction reactions. • Explain the role of ATP in metabolism. Various catabolic reactions transfer energy into the “high-energy” phosphate bonds of ATP. Although the amount of energy in these bonds is not exceptionally large, it can be released quickly and easily. Before discussing metabolic pathways, it is important to understand how this transfer of energy occurs. Two important aspects of energy transfer are oxidation–reduction reactions and mechanisms of ATP generation. Oxidation–Reduction Reactions Oxidation (ok′-si-DA- -shun) is the removal of electrons from an atom or molecule; the result is a decrease in the potential energy of the atom or molecule. Because most biological oxidation reactions involve the loss of hydrogen atoms, they are called dehydrogenation reactions. An example of an oxidation reaction is the conversion of lactic acid into pyruvic acid: Pyruvic acidLactic acid COOH C O CH3 H COOH | ‖C || OH | | | CH3 Oxidation )HRemove 2 H ( H++ − In the preceding reaction, 2H (H+ + H−) means that two neutral hydrogen atoms (2H) are removed as one hydrogen ion (H+) plus one hydride ion (H−). Reduction (rē-DUK-shun) is the opposite of oxidation; it is the addition of electrons to a molecule. Reduction results in an increase in the potential energy of the molecule. An example of a reduction reaction is the conversion of pyruvic acid into lactic acid: dicacitcaLdicacivuryP H COOH C OH CH3 COOH C O CH3 Reduction Add 2 H (H H ) | || | ‖ | | + + − When a substance is oxidized, the liberated hydrogen atoms do not remain free in the cell but are transferred immediately by coenzymes to another compound. Two coenzymes are commonly used by animal cells to carry hydrogen atoms: nicotinamide adenine dinucleotide (NAD), a derivative of the B vitamin niacin, and flavin adenine dinu- cleotide (FAD), a derivative of vitamin B2 (riboflavin). The oxidation and reduction states of NAD+ and FAD can be represented as follows: ReduceddezidixO decudeRdezidixO FADH2FAD H )2 H (H NAD NADH H H )2 H (H ++ ++ + − + + + − − H )2 H (H H )2 H (H ++ − + + + − − When NAD+ is reduced to NADH + H+, the NAD+ gains a hydride ion (H−), neutralizing its charge, and the H+ is released into the sur- rounding solution. When NADH is oxidized to NAD+, the loss of the hydride ion results in one less hydrogen atom and an additional positive charge. FAD is reduced to FADH2 when it gains a hydrogen ion and a hydride ion, and FADH2 is oxidized to FAD when it loses the same two ions. Oxidation and reduction reactions are always coupled; each time one substance is oxidized, another is simultaneously reduced. Such paired reactions are called oxidation–reduction or redox reactions. For example, when lactic acid is oxidized to form pyruvic acid, the two hydrogen atoms removed in the reaction are used to reduce NAD+. This coupled redox reaction may be written as follows: Lactic acid Reduced Pyruvic acid Oxidized NAD+ Oxidized NADH + H+ Reduced An important point to remember about oxidation–reduction reactions is that oxidation is usually an exergonic (energy-releasing) reaction. Cells use multistep biochemical reactions to release energy from energy-rich, highly reduced compounds (with many hydrogen atoms) to lower-energy, highly oxidized compounds (with many oxygen atoms or multiple bonds). For example, when a cell oxidizes a molecule of glucose (C6H12O6), the energy in the glucose molecule is removed in a stepwise manner. Ultimately, some of the energy is captured by trans- ferring it to ATP, which then serves as an energy source for energy- requiring reactions within the cell. Compounds with many hydrogen atoms such as glucose contain more chemical potential energy than oxidized compounds. For this reason, glucose is a valuable nutrient. Mechanisms of ATP Generation Some of the energy released during oxidation reactions is captured within a cell when ATP is formed. Briefly, a phosphate group P is added to ADP, with an input of energy, to form ATP. The two high- energy phosphate bonds that can be used to transfer energy are indicated by “squiggles” (∼): Adenosine — P ∼ P + P + energy ADP Adenosine — P ∼ P ∼ P ATP The high-energy phosphate bond that attaches the third phos- phate group contains the energy stored in this reaction. The addition of a phosphate group to a molecule, called phosphorylation (fos′- for-i-LĀ-shun), increases its potential energy. Organisms use three mechanisms of phosphorylation to generate ATP: 1. Substrate-level phosphorylation generates ATP by transferring a high-energy phosphate group from an intermediate phosphory- lated metabolic compound—a substrate—directly to ADP. In hu- man cells, this process occurs in the cytosol. c25MetabolismAndNutrition.indd Page 955 10/14/16 11:55 AM f-512 /208/WB01989/9781119287759/ch25/text_s 884 johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Underline johnup Underline johnup Cross-Out johnup Cross-Out johnup Cross-Out johnup Cross-Out johnup Cross-Out johnup Cross-Out Copyright W iley, 2020 956 CHAPTER 25 Metabolism and Nutrition acids that can be used for lipogenesis (lip-ō-JEN-e-sis), the syn- thesis of triglycerides. Triglycerides then are deposited in adipose tissue, which has virtually unlimited storage capacity. Glucose Movement into Cells Before glucose can be used by body cells, it must first pass through the plasma membrane and enter the cytosol. Glucose absorption in the gas- trointestinal tract (and kidney tubules) is accomplished via secondary active transport (Na+–glucose symporters). Glucose entry into most other body cells occurs via GluT molecules, a family of transporters that bring glucose into cells via facilitated diff usion (see