43.1 Animals employ a digestive system to prepare food for assimilation by cells.
Types of Digestive Systems
• Heterotrophs are divided into three groups based on their primary food source: Herbivores eat plants, carnivores eat meat, and omnivores eat both plants and animals. (p. 888)
• Most organisms digest their food extracellularly within a digestive cavity. (p. 888)
• In digestion, fragmentation of food particles is followed by chemical digestion of food molecules, which involves hydrolysis to separate subunit molecules so they can be absorbed into the bloodstream. (p. 888)
Vertebrate Digestive Systems
• Vertebrate digestive systems consist of a tubular gastrointestinal tract and accessory digestive organs composed of different tissue layers. (p. 889)
43.2 Food is ingested, swallowed, and transported to the stomach.
Mouth and Teeth
• Carnivorous mammals have pointed teeth lacking a flat grinding surface that are adapted for cutting. Herbivores have large, flat teeth with complex ridges for pulverizing plant cells composed of cellulose. (p. 890)
• Birds, which lack teeth, break up food using ingested pebbles inside of a gizzard. (p. 890)
• The tongue mixes food with a mucous solution, and saliva moistens and lubricates food, making it easier to swallow. (pp. 890-891)
Esophagus and Stomach
• The esophagus connects the pharynx to the stomach. (p. 892)
• Successive waves of contraction move food along the esophagus to the stomach, and contraction of a sphincter restricts food from moving backward into the esophagus from the stomach. (p. 892)
• The stomach churns food and mixes it with gastric juices to assist in chemical digestion. Hydrochloric acid creates an acidic environment, and the enzyme pepsin breaks up proteins into polypeptides. (pp. 892-893)
• Dissolved food (chyme) is transferred through the pyloric sphincter into the small intestine. (p. 893)
43.3 The small and large intestines have very different functions.
Small Intestine
• The capacity of the small intestine is limited; thus, for maximal efficiency, only small amounts of chyme must enter at each time interval. (p. 894)
• The small intestine is the site of nutrient absorption and is divided into the duodenum, the jejunum, and the ileum. (p. 894)
• The epithelial wall is covered with villi and microvilli to increase surface area, and consequently increase absorptive capacity. (p. 894)
• The pancreas, liver, and gallbladder are all accessory organs to the small intestine and provide secretions to aid in digestion. (p. 895)
• Amino acids and monosaccharides enter the blood via the hepatic portal vein. The products of fat digestion are first absorbed into the lymphatic capillaries. (p. 896)
Large Intestine
• The large intestine (colon) is much shorter than the small intestine and engages in no digestion and very little absorption. (p. 897)
• The large intestine's primary function is to remove water and concentrate waste for excretion. (p. 897)
Variations in Vertebrate Digestive Systems
• While most animals lack the enzymes necessary to digest cellulose, the digestive tracts of some animals contain bacteria and protists that convert cellulose into digestible substances. (p. 898)
• Ruminants possess multichambered stomachs and regurgitate and rechew rumen contents. (p. 898)
• Horses, deer, and lagomorphs possess a large cecum, which contains microorganisms that digest cellulose. (p. 898)
• Rodents and lagomorphs also engage in coprophagy. (p. 898)
43.4 Neural, hormonal, and accessory organ regulation controls digestion.
Neural and Hormonal Regulation of the Digestive Tract
• The activities of the gastrointestinal tract are coordinated by the nervous system and the endocrine system (p. 900)
• Protein in food stimulates secretion of gastrin, which in turn stimulates secretion of pepsinogen and HCl. (p. 900)
• Enterogastrones are secreted by the duodenum and promote digestion in the duodenum while inhibiting gastric functions. (p. 900)
Accessory Organ Function in Nutrient Processing and Distribution
• The liver chemically modifies absorbed substances in the gastrointestinal tract and converts toxins and poisons into less toxic forms. (p. 902)
• Insulin stimulates the liver to remove excess glucose from the blood and store it as the polysaccharide glycogen. Glucagon stimulates the breakdown of glycogen, which releases glucose into the blood. (p. 902)
43.5 All animals require food energy and essential nutrients.
Food Energy and Energy Expenditure
• The ingestion of food provides a source of energy and the raw materials the animal is unable to manufacture for itself. (p. 903)
• Basal metabolic rate (BMR) is the minimum rate of energy consumption under defined resting conditions. (p. 903)
• The amount of energy consumed by a body per day is determined by the BMR and the level of physical activity. (p. 903)
• Obesity may occur if the amount of food ingested per day exceeds the daily energy expenditure for a long period of time. (p. 903)
Essential Nutrients
• Essential nutrients are substances that an animal cannot manufacture for itself, but that are necessary for the maintenance of the organism's health. (p. 904)
• Many vertebrates cannot synthesize one or more of the 20 essential amino acids, and thus must obtain them from the food they eat. In addition, all vertebrates have lost the ability to synthesize certain unsaturated fatty acids. (p. 904)