Chapter Summary

• Animals are heterotrophs that derive their energy and molecular building blocks, directly or indirectly, from autotrophs.

• Carbohydrates, fats, and proteins in food supply animals with energy. A measure of the energy content of food is the kilocalorie (kcal). Excess caloric intake is stored as glycogen and fat. Review Figure 50.2

• For many animals, food provides essential carbon skeletons that they cannot synthesize themselves. Review Figure 50.4

• Most researchers consider 8 amino acids to be essential for adult humans; some believe that infants require as many as 12 essential amino acids in their diet. Macronutrients are mineral elements needed in large quantities; micronutrients are needed in small amounts. Review Figure 50.5, Table 50.1, Activity 50.1

• Vitamins are organic molecules that must be obtained in food. Review Table 50.2, Activity 50.2

• Malnutrition results when any essential nutrient is lacking from the diet. Chronic malnutrition causes deficiency disease.

• Animals can be characterized by how they acquire nutrients: saprobes and detritivores, or decomposers, depend on dead organic matter, filter feeders strain the aquatic environment for small food items, herbivores eat plants, and carnivores eat other animals. Behavioral and anatomical adaptations reflect these feeding strategies. See Activity 50.3

• Digestion involves the breakdown of complex food molecules into monomers that can be absorbed and used by cells. In most animals, digestion takes place in a tubular gut. Review Figure 50.7

• Absorptive areas of the vertebrate gut are characterized by a large surface area produced by extensive folding and numerous villi and microvilli. Review Figure 50.8

• Hydrolytic enzymes break down proteins, carbohydrates, and fats into their monomeric units.

• The microbiome of the gut participates in digestive processes, makes certain nutrients available for absorption, and is sensitive to composition of diet, ingestion of drugs, and other factors. Review Investigating Life: How Does the Gut Microbiome Contribute to Obesity and Metabolic Disease?

• The vertebrate gut can be divided into several compartments with different functions. Review Figure 50.9, Activity 50.4

• The cells and tissues of the vertebrate gut are organized in the same way throughout its length. The innermost tissue layer, the mucosa, is the secretory and absorptive surface. The submucosa contains blood and lymph vessels and a nerve network that is sensory and also controls gut secretions. External to the submucosa are two smooth muscle layers. Between the two muscle layers is another nerve network that controls the movements of the gut. Review Figure 50.10

• Swallowing is a reflex that pushes a bolus of food into the esophagus. Peristalsis and segmentation movements of the gut move the bolus down the esophagus and through the entire length of the gut. Sphincters block the gut at certain locations, but they relax as a wave of peristalsis approaches. Review Figure 50.11

• Digestion begins in the mouth, where amylase is secreted with the saliva. Digestion of protein begins in the stomach, where parietal cells secrete HCl and chief cells secrete pepsinogen, which becomes pepsin when activated by low pH and autocatalysis. The mucosa also secretes mucus, which protects the tissues of the gut. Review Figure 50.12

• In the duodenum, pancreatic enzymes carry out most of the digestion of food. Bile from the liver and gallbladder emulsifies fats into micelles. Bicarbonate ions from the pancreas neutralize the pH of the chyme entering from the stomach, thereby producing an environment conducive to the actions of pancreatic enzymes such as trypsin. Review Figure 50.13, Table 50.3

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• Final enzymatic cleavage of polypeptides and disaccharides occurs among the microvilli of the intestinal mucosa. Amino acids, monosaccharides, and inorganic ions are absorbed by the microvilli. Specific transport proteins are sometimes involved. Symporters often power the absorption of nutrients.

• Fats broken down by lipases are absorbed mostly as monoglycerides and fatty acids and are resynthesized into triglycerides within the gut epithelium. The triglycerides are combined with cholesterol and phospholipids and coated with protein to form chylomicrons, which pass out of the mucosal cells and into lymphatic vessels in the submucosa. Review Figure 50.14, Animation 50.1

• 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 50.15

• Autonomic reflexes coordinate activity of the digestive tract, which has an intrinsic nervous system that can act independently of the CNS.

• The actions of the stomach and small intestine are largely controlled by the hormones gastrin, secretin, and cholecystokinin (CCK). Review Figure 50.16

• The liver plays a central role in directing the storage and release of fuel molecules. In the absorptive state, the liver takes up and stores fats and carbohydrates, converting monosaccharides to glycogen or fats. The liver also takes up amino acids and uses them to produce blood plasma proteins, and can engage in gluconeogenesis.

• Fat and cholesterol are shipped out of the liver as low-density lipoproteins (LDLs). High-density lipoproteins (HDLs) act as acceptors of cholesterol and bring fat and cholesterol back to the liver.

• Insulin largely controls fuel metabolism during the absorptive state and promotes glucose uptake as well as glycogen and fat synthesis. 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 Focus: Key Figure 50.17, Figure 50.18, Animation 50.2

• 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 Figure 50.19, Activity 50.5