Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 51 (p. 1036)

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.

3. Regurgitation in humans is normally prevented by a sphincter that can open when pressure is exerted on it. Horses have a true sphincter that does not open with such pressure.

4. The parietal cells secrete hydrochloric acid.

5. The two products of digestion are absorbed as follows: sugars and amino acids are absorbed via carrier systems into the blood.

6. 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, at the front end of the digestive tract, and can digest the materials a second time.

7. Secretory cells in the exocrine portion of the pancreas secrete (1) enzymes, which digest carbohydrates, proteins, and fats; and (2) bicarbonate, which neutralizes stomach acid, enabling enzymes to operate at a neutral pH. The endocrine secretion from the pancreas is insulin, a hormone important for regulating blood glucose levels.

8. Bile salts act as a detergent, dispersing large fat drops into smaller droplets, forming an emulsion. When emulsified the fat has a greater surface area to be acted upon by lipase so digestion proceeds more rapidly.

9. Gastrin stimulates secretion of HC1 and pepsinogen by the stomach. Secretin stimulates the secretion of bicarbonate by the pancreas in response to lowered pH levels in the duodenum.

10. 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 that cannot be synthesized by an organism and therefore must be ingested.