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

AmylaseSTARCH + 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/TrypsinProtein + Water ---------------->

Peptides

PeptidasesPeptides + 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

BileFat -----------> Fat Droplets

LipaseFat 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 proteinHCl (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