Human Biology: Digestive System
Lesson 2: Chemical Digestion and Absorption
Today’s Objectives
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Analyse the functional inter-relationships of the
structures of the digestive system, including:
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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
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Describe the components, pH, and digestive actions of
salivary, gastric, pancreatic, and intestinal juices, including:
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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
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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
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Digestion occurs as a result of hydrolysis
Salivary Amylase: Enzymes in saliva
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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
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Pancreatic Amylase: Also acts on starch to convert it to
maltose
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Maltase: Converts maltose to glucose
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Occurs in the duodenum but produced by the pancreas
Produced in the small intestine
maltase
MALTOSE + WATER ----------------> 2 GLUCOSE
Chemical Digestion: Protein
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Proteases: Break down proteins to peptides
Two types of protease:
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Pepsin: Produced by the gastric glands of the stomach
Trypsin: Produced by the pancreas
Pepsin/Trypsin
Protein + Water ----------------> Peptides
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Peptidases: Break down peptides into amino acids
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Produced by the small intestine
Peptidases
Peptides + Water ----------------> amino acids
Chemical Digestion: Fats
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Bile: Breaks down fat into fat droplets in the duodenum
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Produced by the liver
Stored in the gall bladder
Bile is not an enzyme
Bile
Fat -----------> Fat Droplets
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Lipase: Breaks down fat droplets into glycerol and 3
fatty acids
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Produced by the pancreas
Lipase
Fat droplets + Water --------------> Glycerol + 3 Fatty Acids
Emulsification
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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
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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
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Pancreatic Juice: pancreatic amylase, trypsin, lipase, and
sodium bicarbonate
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Formed in the pancreas
Secreted into the duodenum via the pancreatic duct
Gastric juice: hydrochloric acid
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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
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Intestinal Juice: maltase, peptidase
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Formed in the small intestine
Maltase breaks down maltose, peptidase breaks down peptides
Nuclease:
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Formed in the small intestine, Pancreas
Nuclease breaks down RNA and DNA into nucleotides
Control of Gastric (stomach) Secretions
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The following occurs especially after eating a proteinrich 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
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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
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The duodenal wall produces hormones, the most
important of which are secretin and cholecystokinin
(CCK), in response to the presence of acid chyme
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Secretin stimulates the release of pancreatic juice from the
pancreas
CCK stimulates the release of bile from the gall bladder
Control of Intestinal Secretions
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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
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Insulin: A hormone produced by the pancreas
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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
INSULIN
High Blood Sugar
Low Blood Sugar
GLUCAGON
The Role of Glucagon
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Glucagon: Another pancreatic hormone
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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
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The Pancreas is called both an Exocrine and an
Endocrine organ
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Exocrine: produces some enzymatic substances
Endocrine: produces hormones