Summary

Concept 30.1: Food Provides Energy and Chemical Building Blocks

• Animals are heterotrophs that derive their energy and chemical building blocks from eating other organisms.
• A measure of the energy content of food is the calorie (cal), the amount of heat required to raise the temperature of 1 gram of water by 1°C. What we typically refer to as “calories” in our food are actually kilocalories (kcal). The metric unit for measuring energy is the joule. Review Figures 30.1 and 30.2
• There are three major types of food molecules: lipids (fats and oils), carbohydrates, and proteins. Lipids contain about twice as many calories per unit of weight as carbohydrates and proteins. Animals store extra energy as lipids and glycogen.
• Food provides carbon skeletons, termed essential, that animals cannot synthesize themselves.
• Adult humans require eight essential amino acids and at least two essential fatty acids. Review Figure 30.3
• Vitamins are essential carbon molecules needed in tiny amounts. They are either water-soluble or lipid-soluble. Review Table 30.1 and ACTIVITY 30.1
• Essential minerals are chemical elements that are required in the diet in addition to carbon, hydrogen, oxygen, and nitrogen. Review Table 30.2 and ACTIVITY 30.2
• Malnutrition results when any essential nutrient is lacking from the diet. Chronic malnutrition causes deficiency diseases.

Concept 30.2: Animals Get Food in Three Major Ways

• Animals can be characterized by how they acquire their food. Some animals feed on easily visible food items that they eat by targeting them individually. Suspension feeders (or filter feeders) are aquatic animals that feed indiscriminately on large numbers of tiny food particles that they collect from the surrounding water. Review Figures 30.4 and 30.5
• Some animals rely on symbiosis with microbes to obtain at least some of their nutrients. Reef-building corals, for example, contain photosynthetic algae that supply carbon compounds to the coral. Microbes in the rumen of ruminants break down cellulose that the animals are unable to digest. Review Figures 30.6 and 30.7

Concept 30.3: The Digestive System Plays a Key Role in Determining the Nutritional Value of Foods

• In most animals, digestion takes place in a tubular gut that has two openings: the mouth and the anus. Food is processed within the gut lumen. Review Figure 30.8
• Digestion is the enzymatic breakdown of food molecules into smaller molecules that can cross the gut epithelium. The process of transporting molecules from the gut lumen into the blood is called absorption.
• Each digestive enzyme can break only specific types of chemical bonds in food molecules. For example, the sugars trehalose and lactose are broken down by trehalase and lactase, respectively. Review Figure 30.9
• The nutritional value of a food depends on the ability of an animal’s digestive tract to process the food in such a way that nutrients in the food can be absorbed.
• Animal species vary in the digestive enzymes they produce and thus in their ability to gain nutritional value from specific food molecules.
• An individual animal can adjust its digestive and absorptive capabilities in response to changes in its diet.

Concept 30.4: The Vertebrate Digestive System Is a Tubular Gut with Accessory Glands

• The vertebrate digestive system consists of a tubular gut and several secretory organs—notably the liver and pancreas—that aid in digestion. Review Figure 30.11 and ACTIVITY 30.3
• The gut consists of an inner gut epithelium that secretes mucus, enzymes, and hormones and absorbs nutrients; a submucosa containing blood and lymph vessels; two layers of smooth muscle; and a nerve network called the enteric nervous system, which is part of the autonomic nervous system. Review Figure 30.12
• Peristalsis moves food along the length of the gut. Sphincters block food passage at certain locations but relax at controlled moments to let food through.
• Digestive enzymes are categorized on the basis of the types of food molecules they hydrolyze and by where they function (intraluminal, membrane-associated, or intracellular).
• The vertebrate gut can be divided into several compartments with different functions. The foregut includes the mouth, esophagus, and stomach. The midgut is the principal site of digestion and absorption. The hindgut stores waste between defecations and reabsorbs water and salts from the feces.
• Digestion begins in the mouth in mammals. Mammals chew their food, and their saliva contains the starch-digesting enzyme amylase.
• The stomach breaks up food, begins the process of protein digestion, and controls the flow of food materials to the small intestine. Stomach cells secrete HCl, pepsinogen (the inactive form of pepsin), and mucus that protects the stomach wall.
• In most vertebrates, absorptive areas of the gut are characterized by a large surface area produced by extensive folding, including villi and microvilli. Review Figure 30.13
• Digestive juices secreted by the pancreas and emulsifying bile secreted by the liver flow into the lumen of the early part of the midgut. Bicarbonate ions from the pancreas neutralize the pH of the food entering from the stomach.
• Some animals have fermentation chambers where symbiotic microbes help break down foods, synthesize B vitamins, and recycle nitrogen. The host animal is classified as a foregut fermenter, midgut fermenter, or hindgut fermenter, based on the location of the fermentation compartment within the digestive tract.

Concept 30.5: The Processing of Meals Is Regulated

• Animals alternate between an absorptive state (food in the gut) and a postabsorptive state (no food in the gut).
• The actions of the stomach and small intestine are controlled by hormones such as gastrin, secretin, and cholecystokinin (CCK). Review Figure 30.14
• Food intake is governed by sensations of hunger and satiety, which are determined by brain mechanisms responding to feedback signals provided in part by the hormones ghrelin and leptin. Review Figure 30.15 and ANIMATED TUTORIAL 30.1
• Insulin and glucagon from the pancreas control the glucose concentration of the blood. Insulin stimulates the uptake and use of glucose by many cells of the body. If blood glucose concentration falls, glucagon secretion increases, stimulating the liver to break down glycogen and release glucose to the blood. Review Figure 30.16 and ANIMATED TUTORIAL 30.2