Key Concepts

1. Key Concepts • Animals require an array of nutrients to stay healthy, including specific amino acids, vitamins, and elements, as well as organic compounds that act as building blocks in chemical synthesis or have high potential energy. • There is usually a close correspondence between the structure of an animal’s mouthparts and the function of those mouthparts in capturing and processing food.

2. Key Concepts • Digestion occurs in the digestive tract, which is compartmentalized into organs that have specialized functions in the ingestion and digestion of food, absorption of nutrients and water, or excretion of wastes. • Lack of homeostasis with respect to nutrients such as glucose and cholesterol can cause disease.

3. Overview: The Need to Feed • Food is taken in, taken apart, and taken up in the process of animal nutrition. • In general, animals fall into three categories: • Herbivoreseat mainly autotrophs (plants and algae). • Carnivoreseat other animals. • Omnivoresregularly consume animals as well as plants or algal matter.

4. An animal’s diet must supply chemical energy, organic molecules, and essential nutrients • An animal’s diet provides chemical energy, which is converted into ATP and powers processes in the body. • Animals need a source of organic carbon and organic nitrogen in order to constructorganic molecules. • Essential nutrients are required by cells andmust be obtained from dietary sources.

5. Essential Nutrients • There are four classes of essential nutrients: • Essential amino acids • Essential fatty acids • Vitamins • Minerals

6. Essential Amino Acids • Animals require 20 amino acids and can synthesize about half from molecules in their diet. • The remaining amino acids, the essential amino acids must be obtained from food in preassembled form. • A diet that provides insufficient essential amino acids causes malnutrition called protein deficiency.

7. Meat, eggs, and cheese provide all the essential amino acids and are thus “complete” proteins. • Most plant proteins are incomplete in amino acid makeup. • Individuals who eat only plant proteins need to eat specific plant combinations to get all essential amino acids. • Some animals have adaptations that help them through periods when their bodies demand extraordinary amounts of protein.

8. Essential amino acids from a vegetarian diet 8 Essential amino acids for adults Beans and otherlegumes Methionine Valine Threonine Phenylalanine Leucine Corn (maize)and other grains Isoleucine Tryptophan Lysine

9. Essential Fatty Acids • Animals can synthesize most of the fatty acids they need. • The essential fatty acids are certain unsaturated fatty acids that must be obtained from the diet. • Deficiencies in fatty acids are rare.

10. Vitamins • Vitaminsare organic molecules required in the diet in small amounts. Many vitamins function as coenzymes. • 13 vitamins essential to humans have been identified. • Vitamins are grouped into two categories: fat-soluble and water-soluble.

12. Minerals • Minerals are simple inorganic nutrients, usually required in small amounts. Minerals serve a variety of important functions including enzymes cofactors.

14. Dietary Deficiencies • Undernourishment is the result of a diet that consistently supplies less chemical energy than the body requires. • Malnourishment is the long-term absence from the diet of one or more essential nutrients.

15. Undernourishment • An undernourished individual will • Use up stored fat and carbohydrates • Break down its own proteins • Lose muscle mass • Suffer protein deficiency of the brain • Die or suffer irreversible damage.

16. Malnourishment • Malnourishment can cause deformities, disease, and death. Malnourishment can be corrected by changes to a diet. • Insights into human nutrition have come from epidemiology, the study of human health and disease in populations. • Neural tube defects were found to be the result of a deficiency in folic acid in pregnant mothers.

17. The main stages of food processing are ingestion, digestion, absorption, and elimination • Ingestionis the act of eating. There are a variety of types of eating: • Suspension feeders • Substrate feeders • Fluid feeders • Bulk feeders

18. Suspension Feeders • Many aquatic animals are suspension feeders, which sift small food particles from the water. Baleen Humpback whale, a suspension feeder

19. Substrate feeders are animals that live in or on their food source. Leaf miner caterpillar, a substrate feeder Caterpillar Feces

20. Fluid feeders suck nutrient-rich fluid from a living host. Mosquito, a fluid feeder

21. Bulk feeders eat relatively large pieces of food. Rock python, a bulk feeder

22. Digestionis the process of breaking food down into soluble molecules - small enough to absorb. • In chemical digestion, the process of enzymatic hydrolysissplits bonds in molecules with the addition of water. • Absorptionis uptake of nutrients by body cells. • Eliminationis the passage of undigested material out of the digestive compartment.

23. The four stages of food processing Smallmolecules Piecesof food Chemical digestion(enzymatic hydrolysis) Nutrientmoleculesenter bodycells Mechanicaldigestion Undigestedmaterial Food Ingestion Digestion Mechanical & Chemical Digestion Elimination Absorption 2 4 1 3

24. How Are Nutrients Digested and Absorbed? • Four processes are necessary for an animal to obtain energy from its food: ingestion, digestion, absorption, and elimination. • Digestion is the breakdown of food into small enough pieces to allow for absorption—the uptake of nutrients.

25. An Introduction to the Digestive Tract • Digestion takes place in the digestive tract—also called the alimentary canal or gastrointestinal (GI) tract—beginning at the mouth and ending at the anus. • Digestive tracts come in two general designs: • Incomplete digestive tracts. • Complete digestive tracts.

26. An Introduction to the Digestive Tract • Incomplete digestive tracts have a single opening through which food is ingested and wastes are eliminated. The mouth opens into a chamber, called a gastrovascular cavity, where digestion takes place.

28. An Introduction to the Digestive Tract • Complete digestive tracts have two openings—they start at the mouth and end at the anus. The interior of this tube communicates directly with the external environment via these openings. • Complete digestive tracts have three advantages: • Animals can feed on large pieces of food. • Chemical and physical processes can be separated within the canal, so that they occur independently of each other and in a prescribed sequence. • Material can be ingested and digested continuously.

30. An Overview of Digestive Processes • Digestion begins in the mouth, starting with the tearing and crushing activity of teeth during chewing. • Distinct chemical changes occur as food moves through each compartment in the digestive tract.

31. An Overview of Digestive Processes • Each of the three major types of macromolecules—carbohydrates, lipids, and proteins—must be broken down during digestion. This chemical processing, started in the mouth, continues in the stomach and finishes in the small intestine. • The small molecules that result from this digestion are absorbed in the small intestine, along with water, vitamins, and ions. • More water is absorbed in the large intestine, producing feces that eventually exit the body at the anus.

33. The Mouth and Esophagus • During chewing, enzymes in saliva start to break down some components of the food. • Salivary amylase is the most important catalyst in the breakdown of carbohydrates. • Cells in the tongue also secrete lipase, which begins the breakdown of lipids. • Salivary glands (3 pairs of salivary glands called parotid, submandibular, and sublingual gland) in the mouth release water and glycoproteins called mucins. When mucins contact water, they form the slimy substance called mucus.

35. The Mouth and Esophagus • The combination of water and mucus makes food soft and slippery enough to be swallowed. • Food then enters the esophagus—a muscular tube connecting the mouth and stomach—and is propelled to the stomach by a wave of muscle contractions called peristalsis. • The system is a reflex—an automatic reaction to a stimulus—that is stimulated by the act of swallowing.

37. A Modified Esophagus: the Bird Crop • In an array of bird species, the esophagus has a prominent, widened segment called the crop where food can bestored and, in some cases, processed. • In many groups, the crop is a simple storage sac and is thought to be an adaptation that allows individuals to eat a large amount in a short time, then retreat to a safe location while digestion occurs. • However, two groups of leaf-eating birds have an enlarged crop that contain bacteria capable of breaking down cellulose.

38. The Stomach • The stomach is a tough, muscular pouch bracketed on both ends by valves called sphincters. • After eating, muscular contractions in the stomach result in churning that mixes and breaks down the food mechanically. • The lumen of the stomach is highly acidic; the predominant acid in the stomach is hydrochloric acid (HCl).

40. The Stomach as a Site of Protein Digestion • Churning mixes the food with gastric juice, which contains hydrochloric acid (HCl) and the enzyme pepsin to begin the digestion of proteins. • Specialized stomach cells called chief cells contain a pepsin precursor called pepsinogen, which is converted to active pepsin by contact with the acidic environment of the stomach. • Secretion of a protein-digesting enzyme in inactive form is important: It prevents destruction of proteins in the cells where the enzyme is synthesized.

42. Which Cells Produce Stomach Acid? • The stomach epithelium contains several types of secretory cells, each of which is specialized for a particular function. • Parietal cells are the source of HCl in gastric juice. • Gastric juice can have a pH as low as 1.5. • Mucous cells secrete mucus, which lines the gastric epithelium and protects the stomach from damage by HCl.

43. How Do Parietal Cells Secrete HCl? • The enzyme carbonic anhydrase is found in high concentration in parietal cells. • This enzyme catalyzes the formation of carbonic acid (H2CO3) from carbon dioxide and water. In solution, the carbonic acid formed immediately dissociates into the bicarbonate ion (HCO3–) and a proton: CO2 + H2O  H2CO3 H+ + HCO3– • The protons formed by the dissociation of carbonic acid are actively pumped into the lumen of the stomach.

45. Ulcers as an Infectious Disease • An ulcer is a hole in an epithelium that damages the underlying basement membrane and tissues. • Ulcers in the stomach or the first part of the small intestine can result in intense abdominal pain. • Ulcers were initially thought to result from excess acid in the stomach, but Robin Warren and Barry Marshall determined that ulcers were caused by a bacterium called Helicobacter pylori.

46. The Small Intestine • Partially digested food passes from the stomach into the small intestine, a six-meter-long tube. The food mixes with secretions from the pancreas and liver and begins to move through the tube. • At the end of the small intestine, digestion is complete and most nutrients—along with much water—has been absorbed. • The small intestine has an enormous surface area for absorption of nutrients due to projections called villi, which in turn have projections called microvilli.

47. The Small Intestine • The enormous surface area of the small intestine increases the efficiency of nutrient absorption. • Because each villus contains blood vessels and a lymphatic vessel called a lacteal, nutrients pass quickly from epithelial cells into the body’s transport systems.

49. Protein Processing by Pancreatic Enzymes • Enzymes in the small intestine called proteases digest polypeptides to monomers. There are many types of proteases, each specific for a different kind of polypeptide. • Proteases are synthesized in inactive form by the pancreas, transferred through the pancreatic duct to the small intestine, and activated there. • Pancreatic enzymes are activated by another enzyme known as enterokinase.

50. Protein Processing by Pancreatic Enzymes • Enterokinase activates a pancreatic enzyme called trypsinogen by phosphorylating it, resulting in the active enzyme trypsin. • Trypsin, in turn, activates other enzymes that are synthesized by the pancreas and secreted in an inactive form. After they are activated, each enzyme begins cleaving specific peptide bonds. • Eventually polypeptides are broken up into amino acid monomers.