Chapter 27 Nutrition and Metabolism

1. Chapter 27Nutrition and Metabolism

2. Overview of Nutrition and Metabolism • Nutrition refers to the food (nutrients) humans eat • Malnutrition—a deficiency in the consumption of food, vitamins, and minerals • Categories of nutrients • Macronutrients—nutrients needed in large amounts (bulk nutrients) • Macromolecules such as carbohydrates, fats (lipids), and proteins • Water • Macrominerals—minerals needed in large quantity; for example, sodium, chloride, and calcium • Micronutrients—nutrients needed in very small amounts • Vitamins • Microminerals (trace elements)—minerals such as iron, iodine, zinc, that are needed only in very small quantities • Balance of nutrients is required for good health (Figure 27-1 and 27-2)

3. Overview of Nutrition and Metabolism • Metabolism—the use of nutrients—a process made up of many chemical processes (Figure 27-23) • Catabolism breaks food down into smaller molecular compounds and releases two forms of energy—heat and chemical energy • Anabolism—a synthesis process • Both processes take place inside of cells continuously and concurrently • Chemical energy released by catabolism must be transferred to ATP, which supplies energy directly to the energy-using reactions of all cells (Figure 27-3)

4. Carbohydrates • Dietary sources of carbohydrates • Complex carbohydrates • Polysaccharides—starches; found in vegetables and grains; glycogen is found in meat • Cellulose—a component of most plant tissue; passes through the digestive system without being broken down • Disaccharides—found in refined sugar; must be broken down before they can be absorbed • Monosaccharides—found in fruits; move directly into the internal environment without being processed directly • Glucose—carbohydrate most useful to the human cell; can be converted from other monosaccharides (Figure 27-4)

5. Carbohydrates • Carbohydrate metabolism—human cells catabolize most of the carbohydrate absorbed and anabolize a small portion of it • Glucose transport and phosphorylation—glucose reacts with ATP to form glucose-6-phosphate; this step prepares glucose for further metabolic reactions • This step is irreversible except in the intestinal mucosa, the liver, and the kidney tubules • Glycolysis—the first process of carbohydrate catabolism; consists of a series of chemical reactions (Figure 27-5) • Glycolysis occurs in the cytoplasm of all human cells • An anaerobic process—the only process that provides cells with energy under conditions of inadequate oxygen • It breaks down chemical bonds in glucose molecules and releases about 5% of the energy stored in them • It prepares glucose for the second step in catabolism—the citric acid cycle

6. Carbohydrates • Carbohydrate metabolism (cont.) • Citric acid cycle • Two pyruvic acid molecules from glycolysis are converted to two acetyl molecules in a transition reaction, losing one carbon dioxide molecule per pyruvic acid molecule converted • By end of transition reaction and citric acid cycle, two pyruvic acids have been broken down to six carbon dioxide and six water molecules (Figures 27-6 and 27-7) • Citric acid cycle also called tricarboxylic acid (TCA) cycle because citric acid is also called tricarboxylic acid • Citric acid cycle formerly called Krebs cycle after Sir Hans Krebs, who discovered this process

7. Carbohydrates • Carbohydrate metabolism (cont.) • Electron transport system (Figure 27-8) • High-energy electrons (along with their protons) removed during citric acid cycle enter a chain of molecules that are embedded in the inner membrane of the mitochondria • As electrons move down the chain, they release small bursts of energy to pump protons between the inner and the outer membrane of the mitochondrion • Protons move down their concentration gradient and across the inner membrane, driving ATP-synthase (Figure 27-9) • Oxidative phosphorylation—the joining of a phosphate group to ADP to form ATP by the action of ATP synthase (Figure 27-10 and 27-11)

8. Carbohydrates • Carbohydrate metabolism (cont.) • The anaerobic pathway—a pathway for the catabolism of glucose; transfers energy to ATP using only glycolysis; ultimately ends with the oxidative phosphorylation of ATP (paying the “oxygen debt”) (Figure 27-12)) • Glycogenesis—a series of chemical reactions in which glucose molecules are joined to form a strand of glucose beads; a process that operates when the blood glucose level increases above the midpoint of its normal range (Figures 26-13 and 26-14) • Glycogenolysis (Figure 27-15)—the reversal of glycogenesis; means different things in different cells • Gluconeogenesis (Figure 27-16)—the formation of new glucose; occurs chiefly in liver

9. Carbohydrates • Carbohydrate metabolism (cont.) • Control of glucose metabolism—hormonal and neural devices maintain homeostasis of blood glucose concentration (Figures 27-17 and 27-18) • Insulin—secreted by beta cells to decrease blood glucose level (Figure 27-14) • Glucagon increases blood glucose level by increasing activity of enzyme phosphorylase • Epinephrine—hormone secreted in times of stress; increases phosphorylase activity • Adrenocorticotropic hormone stimulates adrenal cortex to increase its secretion of glucocorticoids • Glucocorticoids accelerate gluconeogenesis • Growth hormone increases blood glucose level by shifting from carbohydrate to fat catabolism • Thyroid-stimulating hormone has complex effects on metabolism

10. Carbohydrates • Carbohydrate metabolism (cont.) • Hormones that cause the blood glucose level to rise are called hyperglycemic • Insulin is hypoglycemic because it causes the blood glucose level to decrease

11. Lipids • Dietary sources of lipids • Triglycerides—the most common lipids—composed of a glycerol subunit that is attached to three fatty acids • Phospholipids—an important lipid found in all foods • Cholesterol—an important lipid found only in animal foods • Dietary fats • Saturated fats contain fatty acid chains in which there are no double bonds • Unsaturated fats contain fatty acid chains in which there are some double bonds

12. Lipids • Transport of lipids—they are transported in blood as chylomicrons, lipoproteins, and fatty acids • In the absorptive state, many chylomicrons are present in the blood • Postabsorptive state—95% of lipids are in the form of lipoproteins • Lipoproteins consist of lipids and protein and are formed in the liver • Blood contains three types of lipoproteins: very low density, low density, and high density • Cholesterol lipoproteins associated with heart disease (Figure 27-19) • Fatty acids are transported from the cells of one tissue to the cells of another in the form of free fatty acids

13. Lipids • Lipid metabolism • Lipid catabolism—triglycerides are hydrolyzed to yield fatty acids and glycerol; glycerol is converted to glyceraldehyde-3-phosphate, which enters the glycolysis pathway; fatty acids are broken down by beta- oxidation and are then catabolized through the citric acid cycle (Figure 27-20) • Lipid anabolism consists of the synthesis of triglycerides, cholesterol, phospholipids, and prostaglandins • Control of lipid metabolism is through the following hormones: • Insulin • Growth hormone • ACTH • Glucocorticoids

14. Proteins • Sources of proteins • Proteins are assembled from a pool of 20 different amino acids • The body synthesizes amino acids from other compounds in the body • Only about half of the necessary types of amino acids can be produced by the body; the rest are supplied through diet—found in both meat and vegetables

15. Proteins • Protein metabolism—anabolism is primary, and catabolism is secondary • Protein anabolism—process by which proteins are synthesized by ribosomes of cells (Figure 27-21) • Protein catabolism—deamination takes place in liver cells and forms an ammonia molecule, which is converted to urea and excreted in urine, and a keto acid molecule, which is oxidized or converted to glucose or fat (Figure 27-22) • Protein balance—rate of protein anabolism balances rate of protein catabolism • Nitrogen balance—amount of nitrogen taken in equals nitrogen in protein catabolic waste • Two kinds of protein or nitrogen imbalance: • Negative nitrogen balance—protein catabolism exceeds protein anabolism; more tissue proteins are catabolized than are replaced by protein synthesis • Positive nitrogen balance—protein anabolism exceeds protein catabolism • Control of protein metabolism—achieved by hormones

16. Vitamins and Minerals • Vitamins (Table 27-3)—organic molecules necessary for normal metabolism; many attach to enzymes and help them work or have other important biochemical roles (Figures 27-24 and 27-25) • The body does not make most of the necessary vitamins; they must be obtained through diet • The body stores fat-soluble vitamins and does not store water-soluble vitamins

17. Vitamins and Minerals • Minerals (Table 27-4)—inorganic elements or salts found in the earth • Attach to enzymes and help them work and function in chemical reactions • Essential to the fluid/ion balance of internal fluid environment • Are involved in many processes in the body such as muscle contraction, nerve function, hardening of bone, etc. • Too large or too small an amount of some minerals may be harmful • Recommended mineral intakes may vary over the lifespan (Figures 27-26 and 27-27)

18. Metabolic Rates • Metabolic rate means the amount of energy released by catabolism • Metabolic rates are expressed in two ways: • Number of kilocalories of heat energy expended per hour or per day • As normal or as a percentage above or below normal • Basal metabolic rate—rate of energy expended under basal conditions • Factors: size, body composition, sex, age, thyroid hormone, body temperature, drugs, other factors (Figures 27-28 through 27-31)

19. Metabolic Rates • Total metabolic rate (Figure 27-28)—the amount of energy used in a given time • Main determinates: • Basal metabolic rate • Energy used to do skeletal muscle work • Thermic effect of foods • Energy balance and weight—the body maintains a state of energy balance • Body maintains weight when the total calories in the food ingested equals the total metabolic rate • Body weight increases when energy input exceeds energy output • Body weight decreases when energy output exceeds energy input • In starvation, carbohydrates are used up first, then fats, then proteins (Figure 27-32)

20. Mechanisms for Regulating Food Intake (Table 27-6) • Hypothalamus plays a part in food intake • Feeding centers in hypothalamus exert primary control over appetite • Appetite center • Cluster of neurons in lateral hypothalamus that, if stimulated, brings about increased appetite • Orexigenic effects—factors that trigger appetite • Satiety center • Group of neurons in ventral medial nucleus of hypothalamus that, if stimulated, brings about decreased appetite • Anorexigenic effects—factors that suppress appetite (anorexia is loss of appetite)

21. The Big Picture: Nutrition, Metabolism, and the Whole Body • Every cell in the body needs the maintenance of the metabolic pathways to stay alive • Anabolic pathways build the various structural and functional components of the cells • Catabolic pathways convert energy to a usable form and degrade large molecules into subunits used in anabolic pathways

22. The Big Picture: Nutrition, Metabolism, and the Whole Body • Cells require appropriate amounts of vitamins and minerals to produce structural and functional components necessary for cellular metabolism • Other body mechanisms operate to ensure that nutrients reach the cells