Human Biology: Digestive System

1. Human Biology: Digestive System Lesson 2: Chemical Digestion and Absorption

2. Today’s Objectives • Analyse the functional inter-relationships of the structures of the digestive system, including: • Identify the pancreas as the source gland for insulin, and describe the function of insulin in maintaining blood sugar levels • Explain the role of bile in the emulsification of fats • Describe the functions of anaerobic bacteria in the colon • Describe how the small intestine is specialized for chemical and physical digestion and absorption • Describe the structure of the villus, including microvilli, and explain the functions of the capillaries and lacteals within it

3. Today’s Objectives • Describe the components, pH, and digestive actions of salivary, gastric, pancreatic, and intestinal juices, including: • Relate the following digestive enzymes to their glandular sources and describe the digestive reactions they promote: salivary amylase, pancreatic amylase, proteases (pepsin, pepsinogen, trypsin), lipase, peptidase, maltase, nuclease • Describe the role of water as a component of digestive juices • Describe the role of sodium bicarbonate in pancreatic juice • Describe the role of hydrochloric acid in gastric juice • Describe the role of mucus in gastric juice • Describe the importance of the pH level of various regions of the digestive tract

4. Review • Last day we learned the major structures of the digestive tract, and the difference between chemical and physical digestion • Chemical digestion occurs when special digestive enzymes are used to break down the molecules in food • Chemical digestion of carbohydrates begins in the mouth • Chemical digestion of proteins begins in the stomach • Chemical digestion of fats begins in the duodenum (small intestine)

5. Chemical Digestion: Carbs • Digestion occurs as a result of hydrolysis • Salivary Amylase: Enzymes in saliva • Acts on starch to break it into many molecules of maltose • Maltose is later broken down in the system to glucose Amylase STARCH + WATER -----------> MALTOSE

6. Chemical Digestion: Carbs • Pancreatic Amylase: Also acts on starch to convert it to maltose • Occurs in the duodenum but produced by the pancreas • Maltase: Converts maltose to glucose • Produced in the small intestine • maltase • MALTOSE + WATER ----------------> 2 GLUCOSE

7. Chemical Digestion: Protein • Proteases: Break down proteins to peptides • Two types of protease: • Pepsin: Produced by the gastric glands of the stomach • Trypsin: Produced by the pancreas • Peptidases: Break down peptides into amino acids • Produced by the small intestine Pepsin/Trypsin Protein + Water ----------------> Peptides Peptidases Peptides + Water ----------------> amino acids

8. Chemical Digestion: Fats • Bile: Breaks down fat into fat droplets in the duodenum • Produced by the liver • Stored in the gall bladder • Bile is not an enzyme • Lipase: Breaks down fat droplets into glycerol and 3 fatty acids • Produced by the pancreas Bile Fat -----------> Fat Droplets Lipase Fat droplets + Water --------------> Glycerol + 3 Fatty Acids

9. Emulsification • Emulsification is the process that breaks down fats into fat droplets • A person who has had his gall bladder removed will have trouble digesting fatty foods • The gall bladder stores bile for use at the proper time during the digestive process

10. Emulsification • Emulsifiers (such as bile) can cause fats to mix with water • They contain molecules with a nonpolar and a polar end • The molecules position themselves in the fat droplet so that their nonpolar ends point inward into the droplet, and the polar ends point outward • Now the droplets can disperse in water

11. Digestive actions of Gastric, Pancreatic, and Intestinal Juices • Pancreatic Juice: pancreatic amylase, trypsin, lipase, and sodium bicarbonate • Formed in the pancreas • Secreted into the duodenum via the pancreatic duct • Gastric juice: hydrochloric acid • Formed in the stomach • HCl changes pepsinogen into pepsin for digestion of protein HCl (hydrochloric acid) + pepsinogen ----> Pepsin

12. Digestive actions of Gastric, Pancreatic, and Intestinal Juices • Intestinal Juice: maltase, peptidase • Formed in the small intestine • Maltase breaks down maltose, peptidase breaks down peptides • Nuclease: • Formed in the small intestine, Pancreas • Nuclease breaks down RNA and DNA into nucleotides

13. Control of Gastric (stomach) Secretions • The following occurs especially after eating a protein-rich meal • Gastrin: a hormone produced in the lower part of the stomach • Gastrin enters the bloodstream and later stimulates gastric glands in the upper part of the stomach to produce pepsinogen and HCl • Pepsinogen and HCl react with each other to produce pepsin

14. Control of Gastric (stomach) Secretions • HCl can burn the lining of the stomach, so mucous is produced to protect the stomach lining • If a portion of the stomach does get burned, it is called an ulcer

15. Control of Intestinal Secretions • The duodenal wall produces hormones, the most important of which are secretin and cholecystokinin (CCK), in response to the presence of acid chyme • Secretin stimulates the release of pancreatic juice from the pancreas • CCK stimulates the release of bile from the gall bladder

16. Control of Intestinal Secretions • Acid, especially HCl, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCK • These hormones then enter the bloodstream

17. The Role of Insulin INSULIN • Insulin: A hormone produced by the pancreas • Secreted when blood sugar concentration is high • Causes liver and muscles to take up and store excess glucose as glycogen • Also promotes synthesis of protein and fats • As a result, insulin lowers blood sugar level High Blood Sugar Low Blood Sugar GLUCAGON

18. The Role of Glucagon • Glucagon: Another pancreatic hormone • Secreted when blood sugar concentration is low • Causes liver and muscles to break down glycogen into glucose • Stops protein and fat synthesis • As a result, glucagon raises blood sugar level

19. Pancreas • The Pancreas is called both an Exocrine and an Endocrine organ • Exocrine: produces some enzymatic substances • Endocrine: produces hormones