ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Forty-Five Review Answers 1. The vertebrate gastrointestinal tract, from the inside out, is composed of an epithelial mucosal layer responsible for secretion and absorption, an intermediate muscular muscularis layer responsible for propelling materials through the digestive tract, and an outer serosa layer composed of connective tissue, that anchors the gut in place and through which the gut communicates via blood vessels and nerves. 2. Carnivores have pointed teeth and lack flat grinding surfaces. Herbivores have large, flat teeth to pulverize the cellulose plant wall. Omnivores have mixed tooth surfaces to accommodate both kinds of food-carnivore-like teeth in the front and herbivore-like teeth in the back. The four types of teeth in adult humans are incisors for biting, canines for tearing food, and premolars and molars, both for grinding and crushing. Birds lack teeth. 3. Salivary amylase initiates digestion of starch. It is secreted by the salivary glands and starts working on starch immediately, even while still in the buccal cavity. 4. The palate elevates to steer food to the rear of the pharynx and keep it out of the nasal cavity; this triggers a swallowing reflex that elevates the pharynx slightly, at the same time forcing the epiglottis down over the opening into the trachea, preventing food from entering there as well. The two types of muscles that propel food down the esophagus are skeletal in the upper third of the esophagus and smooth in the lower two-thirds. The rhythmic contractions of these muscles that propel food down the esophagus are termed peristalsis. 5. The parietal cells secrete hydrochloric acid. Pepsinogen is secreted by the chief cells. Hydrochloric acid activates pepsinogen and turns it into the active protein-digesting enzyme pepsin. 6. Villi are projections of the mucosal wall into the lumen of the digestive tract that increase its overall surface area. Microvilli are smaller fingerlike projections that occur across the surface of the epithelial cells of the mucosa, forming a brush border. This greatly extends the surface area of the intestine to facilitate absorption. 7. The three products of digestion are absorbed as follows: sugars and amino acids are absorbed via carrier systems into the blood; fatty acids diffuse into the lymphatic circulation via lacteals in intestinal villi. 8. Secretory cells in the exocrine portion of the pancreas secrete enzymes which digest carbohydrates, proteins, and fats; and bicarbonate that neutralizes stomach acid, which enables enzymes to operate at a neutral pH. The endocrine secretion from the pancreas is insulin, a hormone important for regulating blood glucose levels. 9. The liver produces bile salts, cholesterol, and lecithin, all of which solubilize fats. The gallbladder stores bile. 10. The liver regulates the composition of the blood by receiving the products of digestion directly through the hepatic portal system, permitting it to modify or detoxify materials before they are returned to the heart for pumping throughout the body. In this way, the liver can break down drugs, alcohol, or toxins to which the body has been exposed. Through the secretion of glycogen, the liver can also help control blood glucose levels. The main function of the colon is to concentrate solids and reabsorb water. 11. All of these hormones are stimulated by the presence of food or chyme in the appropriate region of the digestive tract. Gastrin stimulates secretion of HCl and pepsinogen by the stomach. GIP retains material in the stomach and stimulates release of insulin. CCK stimulates the gallbladder to contract to release bile for the emulsification of fats. Secretin stimulates the secretion of bicarbonate by the pancreas in response to lowered pH levels in the duodenum. 12. Herbivorous vertebrates need bacteria to digest cellulose. Symbionts have protists instead of bacteria to do the same thing. Cows are physiologically better able to use plants as food because they digest cellulose in the rumen and at the front end of the digestive tract, and can digest the materials a second time; rodents and rabbits, lacking the digestive specializations of the ruminants, consume their own feces to have access to the fecal microorganisms that digest cellulose. 13. A vitamin is formally defined as an essential organic substance used in small amounts that must be consumed in the diet because the organism is incapable of synthesizing it. The essential amino acids are those which cannot be synthesized by an organism and therefore must be ingested.