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, enzymes
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: 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
Low Blood Sugar
High Blood Sugar
INSULIN
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