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
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