Fueling Body Activities: Digestion

Concept Outline 51.1 Animals employ a digestive system to prepare food for assimilation by cells. Types of Digestive Systems. Some invertebrates have a gastrovascular cavity, but vertebrates have a digestive tract that chemically digests and absorbs the food. Vertebrate Digestive Systems. The different regions of the gastrointestinal tract are adapted for different functions. The Mouth and Teeth. Carnivores, herbivores, and omnivores display differences in the structure of their teeth. Esophagus and Stomach. The esophagus delivers food to the stomach, which secretes hydrochloric acid and pepsin. 51.3 The small and large intestines have very different functions. The Small Intestine. The small intestine has mucosal folds called villi and smaller folds called microvilli that absorb glucose, amino acids, and fatty acids into the blood. The Large Intestine. The large intestine absorbs water, ions, and vitamin K, and excretes what remains as feces. Variations in Vertebrate Digestive Systems. Digestive systems are adapted to particular diets. Accessory Organs. The pancreas secretes digestive enzymes and the hormones insulin and glucagon. The liver produces bile, which emulsifies fat; the gallbladder stores the bile. Neural and Hormonal Regulation of Digestion. Nerves and hormones help regulate digestive functions. Food Energy and Energy Expenditure. The intake of food energy must balance the energy expended by the body in order to maintain a stable weight. Essential Nutrients. Food must contain vitamins, minerals, and specific amino acids and fatty acids for health. P lants and other photosynthetic organisms can produce the organic molecules they need from inorganic components. Therefore, they are autotrophs, or self-sustaining. Animals are heterotrophs: they must consume organic molecules present in other organisms (figure 51.1). The molecules heterotrophs eat must be digested into smaller molecules in order to be absorbed into the animal's body. Once these products of digestion enter the body, the animal can use them for energy in cell respiration or for the construction of the larger molecules that make up its tissues. The process of animal digestion is the focus of this chapter. FIGURE 51.1 Animals are heterotrophs. All animals must consume plant material or other animals in order to live. The nuts in this chipmunk's cheeks will be consumed and converted to body tissue, energy, and refuse.