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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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