Concept 39.1 Food Provides Energy And Nutrients

• Animals are heterotrophs that derive their energy and molecular building blocks, directly or indirectly, from autotrophs. Review Figure 39.1
   
• Carbohydrates, fats, and proteins in food supply animals with metabolic energy. A measure of the energy content of food is the Calorie (Cal). Excess energy is stored as glycogen and fat.
   
• Food also provides macronutrients, which are needed in large quantities, and micronutrients, which are needed in small amounts.
   
• Food provides essential carbon skeletons that animals cannot synthesize themselves.
   
• Adult humans require eight essential amino acids and at least one essential fatty acid. Review Figure 39.2
   
• Essential minerals are chemical elements that are required in the diet. Review Table 39.1 and WEB ACTIVITY 39.1
   
• Vitamins are small organic molecules needed as micronutrients, usually functioning as enzymes or coenzymes. Review Table 39.2 and WEB ACTIVITY 39.2
   
• Malnutrition results when any essential nutrient is lacking from the diet. Chronic malnutrition causes deficiency diseases.
   

Concept 39.2 Digestive Systems Break Down Macromolecules

• Digestion involves the breakdown of complex food molecules into monomers that can be absorbed and used by cells. Hydrolytic enzymes break down proteins, carbohydrates, fats, and nucleic acids into their monomeric units.
   
• In most animals, digestion takes place in a tubular gut that has two openings. The tubular gut can be divided into a foregut that physically breaks up food, a midgut that digests and absorbs the food, and a hindgut that packages wastes. Review Figure 39.3
   
• In most vertebrates, absorptive areas of the gut are character- ized by a large surface area produced by extensive folding and numerous villi and microvilli. Review Figure 39.4
   
• Animals can be characterized by how they acquire their food: saprobes and detritivores eat dead organic matter, filter feeders strain the aquatic environment for small food items, herbivores eat plants, and carnivores eat animals.
   

Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut with Accessory Glands

• The vertebrate gut can be divided into several compartments with different functions. Review Figure 39.6 and WEB ACTIVITY 39.3
   
• The gut consists of an inner mucosa that secretes mucus, enzymes, and hormones and absorbs nutrients; a submucosa containing blood and lymph vessels; two layers of smooth muscle; and a semi-independent nerve network called the enteric nervous system. Review Figure 39.7
   
• Peristalsis moves food throughout the length of the gut. Sphincters block the gut at certain locations, but they relax as a wave of peristalsis approaches.
   
• Digestion begins in the mouth. Mammals chew their food, and their saliva contains the starch-digesting enzyme amylase.
   
• The stomach breaks up food, begins the process of protein digestion, controls the flow of chyme to the small intestine, and kills many microorganisms. Stomach cells secrete HCl, pepsinogen (the inactive form of pepsin), and mucus that protects the stomach wall. Review Figure 39.8
   
• In the duodenum, pancreatic enzymes carry out most of the digestion of food. Bile from the liver and gallbladder emulsifies fats for digestion. Bicarbonate ions from the pancreas neutralize the pH of the chyme. Review Figure 39.10
   
• The jejunum and ileum absorb most of the products of digestion, including amino acids, monosaccharides, and inorganic ions. Specific transporter proteins are sometimes involved.
   
• Fats broken down by lipases are resynthesized into triglycerides within intestinal cells, combined with cholesterol and phospholipids, and coated with protein to form chylomicrons, which move into lymph vessels. Review Figure 39.11 and ANIMATED TUTORIAL 39.1
   
• Water and ions are absorbed in the large intestine as waste matter and consolidated into feces, which are periodically eliminated.
   
• Microorganisms in some compartments of the gut digest materials that their host cannot. Review Figure 39.9
   

Concept 39.4 Food Intake and Metabolism Are Regulated

• Animals cycle between an absorptive state (food in the gut) and a postabsorptive state (no food in the gut).
   
• Neuronal reflexes coordinate much of the activity of the digestive tract.
   
• The actions of the stomach and small intestine are largely controlled by the hormones gastrin, secretin, and cholecystokinin (CCK). Review Figure 39.12
   
• Insulin from the pancreas largely controls glucose metabolism. In the postabsorptive state, lack of insulin blocks the uptake and use of glucose by most cells of the body except neurons. If blood glucose levels fall, glucagon secretion increases, stimulating the liver to break down glycogen and release glucose to the blood. Review Figure 39.13 and ANIMATED TUTORIAL 39.2
   
• The liver plays a central role in directing the traffic of fuel molecules. In the absorptive state, the liver takes up and stores fats, carbohydrates, and proteins. In the postabsorptive state, the liver can produce glucose, either via breakdown of glycogen or via gluconeogenesis from proteins or other molecules.
   
• Fat and cholesterol are shipped out of the liver as low-density lipoproteins (LDLs). High-density lipoproteins (HDLs) act as acceptors of cholesterol and are believed to bring fat and cholesterol back to the liver.
   
• Food intake is governed by sensations of hunger and satiety, which are determined by brain mechanisms responding to feedback signals such as insulin, leptin, and ghrelin. Review INTERACTIVE TUTORIAL 39.1