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UNIT B: Human Body Systems Chapter 8: Human Organization Chapter 9: Digestive System: Section 9.3 Chapter 10: Circulatory System and Lymphatic System Chapter 11: Respiratory System Chapter 12: Nervous System Chapter 13: Urinary System Chapter 14: Reproductive System

Chapter 9: Digestive System UNIT B Chapter 9: Digestive System Chapter 9: Digestive System In this chapter, you will learn about the structure and function of the digestive system. What is our current understanding of the structure and function of the digestive system, including the stomach? Which substances produced by the stomach and other organs help with the chemical breakdown of foods? Chapter opener figure background: Canadian Alexis St. Martin became the unexpected live test subject for research that shed new light on how the digestive system works. St. Martin was working in the fur trade in 1822 on Mackinac Island in Lake Huron when an accident left him with gunshot wounds to his thorax and abdomen. Under the care of United States army surgeon Dr. William Beaumont, St. Martin survived his severe injuries. However, St. Martin’s stomach wound did not close over; instead, it healed permanently attached to surrounding muscle and skin, leaving a direct opening from his stomach to the outside world. This unusual situation led Beaumont to perform groundbreaking research on the stomach’s role in digestion. For almost 11 years, Beaumont made direct observations into the workings of St. Martin’s stomach. He placed a variety of foods attached to a string into his patient’s stomach and later removed them. He also drew samples of stomach fluids for investigation. His research led to a new understanding of the mechanical and chemical nature of the digestive action of the stomach. Alexis St. Martin was an unexpected live test subject for Dr. William Beaumont’s research on how the digestive system works. TO PREVIOUS SLIDE

9.3 Digestive Enzymes UNIT B Chapter 9: Digestive System Section 9.3 9.3 Digestive Enzymes Digestive enzymes help break down the major components of food: carbohydrates, proteins, nucleic acids, and fats. TO PREVIOUS SLIDE

Carbohydrate Digestion by Enzymes UNIT B Chapter 9: Digestive System Section 9.3 Carbohydrate Digestion by Enzymes The digestion of starch (a carbohydrate) begins in the mouth. Salivary amylase (produced by the salivary glands) digests starch into maltose (a disaccharide) Pancreatic amylase (produced by the pancreas) and maltase (produced by the small intestine) then convert maltose in the small intestine to glucose (a monosaccharide). Glucose can be absorbed by the small intestine. salivary amylase: an enzyme in saliva that digests starch to maltose pancreatic amylase: a pancreatic enzyme that digests starch maltase: an enzyme secreted by the surface cells of the small intestine villi; completes the digestion of starch to glucose TO PREVIOUS SLIDE

UNIT B Chapter 9: Digestive System Section 9.3 Figure 9.11 Digestion and absorption of nutrients. a. The breakdown of carbohydrates, such as starch, involves amylase enzymes. TO PREVIOUS SLIDE

Carbohydrate Digestion by Enzymes UNIT B Chapter 9: Digestive System Section 9.3 Carbohydrate Digestion by Enzymes Other disaccharides, such as lactose, have their own enzyme that digests them in the small intestine. Lactase is an enzyme that digests lactose, a sugar found in milk. Individuals who have a lactase deficiency often have symptoms of lactose intolerance (diarrhea, gas, cramps) caused by the fermentation of non-digested lactose by intestinal bacteria TO PREVIOUS SLIDE

Protein Digestion by Enzymes UNIT B Chapter 9: Digestive System Section 9.3 Protein Digestion by Enzymes The digestion of proteins begins in the stomach. Pepsin is an enzyme produced by gastric glands that acts on proteins to produce peptides. Trypsin (produced by the pancreas) and peptidases (produced in the small intestine) break down peptides into amino acids pepsin: an enzyme that acts on proteins to produce peptides trypsin: a pancreatic enzyme that digests protein peptidases: enzymes secreted by the surface cells of the small intestine villi; complete the digestion of protein to amino acids TO PREVIOUS SLIDE

UNIT B Chapter 9: Digestive System Section 9.3 Figure 9.11 Digestion and absorption of nutrients. b. Protein digestion involves the action of protease enzymes. TO PREVIOUS SLIDE

Fat Digestion by Enzymes UNIT B Chapter 9: Digestive System Section 9.3 Fat Digestion by Enzymes Lipase (produced by the pancreas) acts in the small intestine and digests fat molecules in the fat droplets after they have been emulsified by bile salts Glycerol and fatty acids enter the cells of the villi, where they are rejoined and repackaged as lipoprotein droplets (chylomicrons) before entering the lacteals lipase: a pancreatic enzyme that digests fat molecules after they have been emulsified by bile salts TO PREVIOUS SLIDE

UNIT B Chapter 9: Digestive System Section 9.3 Figure 9.11 Digestion and absorption of nutrients. c. For fat digestion, bile salts emulsify the fats so that lipase enzymes can digest the particles. TO PREVIOUS SLIDE

Regulation of Digestive Enzymes UNIT B Chapter 9: Digestive System Section 9.3 Regulation of Digestive Enzymes Enzymes function best at an optimum temperature and pH that helps maintain the proper shape to fit their substrate. Since the digestive system is maintained at a constant 37ºC, enzymatic activity is largely controlled by pH The pH of the stomach is between 1 and 2 but can increase to around 7.4 to 7.8 when sodium bicarbonate in pancreatic juice is released from the pancreas This increase in pH occurs after chyme enters the duodenum, and allows different digestive enzymes to be active depending on the pH TO PREVIOUS SLIDE

Check Your Progress UNIT B Chapter 9: Digestive System Section 9.3 Check Your Progress Describe where in the digestive tract the chemical digestion of each of the following types of nutrients occurs: carbohydrates, proteins, and fats. Identify the final molecule (monomer) resulting from the digestion of carbohydrates, proteins, and fats. Explain how the structure of the digestive system assists in regulating the digestive enzymes. ANSWERS 1. Carbohydrate digestion begins in the mouth with salivary amylase breaking starch into maltose. It then continues in the small intestine with maltase and pancreatic amylase. Protein digestion begins in the stomach where pepsin breaks proteins into peptides. It continues in the small intestine where trypsin continues the breakdown of proteins into peptides and peptidases break protein into amino acids. Fat breakdown mainly occurs with the secretion of lipase from the pancreas into the small intestine where fats are broken down into glycerol and fatty acids.   2. Carbohydrates are broken down into glucose monomers, proteins are broken down into amino acids, and fats are broken down into fatty acids and glycerol. 3. The digestive system is compartmentalized, which provides the optimum environment and pH for the enzymes to operate. For example, pepsin is active in the stomach at a pH 2-3 but inactivated when it reaches the duodenum, where the pH is more basic. TO PREVIOUS SLIDE

UNIT B Chapter 9: Digestive System Section 9.3 TO PREVIOUS SLIDE