Chapter 8: Human Organization
Chapter 10: Circulatory System and
Lymphatic System
Chapter 11: Respiratory System
Chapter 12: Nervous System
Chapter 13: Urinary System
Chapter 14: Reproductive System

UNIT B Chapter 9: Digestive System
In this chapter, you will learn about the structure and function of the digestive system.
Alexis St. Martin was an unexpected live test subject for Dr. William Beaumont’s research on how the digestive system works.
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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?

UNIT B Chapter 9: Digestive System
Digestive enzymes help break down the major components of food: carbohydrates, proteins, nucleic acids, and fats.
Section 9.3
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UNIT B Chapter 9: Digestive System
Carbohydrate Digestion by Enzymes
Section 9.3
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.
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UNIT B Chapter 9: Digestive System
Figure 9.11 Digestion and absorption of nutrients.
a. The breakdown of carbohydrates, such as starch, involves amylase enzymes.
Section 9.3
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UNIT B Chapter 9: Digestive System
Carbohydrate Digestion by Enzymes
Section 9.3
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.
o 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
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UNIT B Chapter 9: Digestive System
Protein Digestion by Enzymes
Section 9.3
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
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UNIT B Chapter 9: Digestive System
Figure 9.11 Digestion and absorption of nutrients.
b. Protein digestion involves the action of protease enzymes.
Section 9.3
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UNIT B Chapter 9: Digestive System
Fat Digestion by Enzymes
Section 9.3
• 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
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UNIT B Chapter 9: Digestive System
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.
Section 9.3
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UNIT B Chapter 9: Digestive System
Regulation of Digestive Enzymes
Enzymes function best at an optimum temperature and pH that helps maintain the proper shape to fit their substrate.
Section 9.3
• Since the digestive system is maintained at a constant
37ºC, enzymatic activity is largely controlled by pH o 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 o This increase in pH occurs after chyme enters the duodenum, and allows different digestive enzymes to be active depending on the pH
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UNIT B Chapter 9: Digestive System
1. Describe where in the digestive tract the chemical digestion of each of the following types of nutrients occurs: carbohydrates, proteins, and fats.
2. Identify the final molecule (monomer) resulting from the digestion of carbohydrates, proteins, and fats.
3. Explain how the structure of the digestive system assists in regulating the digestive enzymes.
Section 9.3
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UNIT B Chapter 9: Digestive System Section 9.3
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