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BIOL 100C:
Introductory Biology III
The Digestive System
Dr. P. Narguizian
Fall 2012
Principles of Biology
OBTAINING
AND PROCESSING
FOOD
Copyright © 2009 Pearson Education, Inc.
Animals ingest their food in a variety of ways
 Most animals have one of three kinds of diets
– Herbivores, plant-eaters—cattle, snails, sea urchins
– Carnivores, meat-eaters—lions, hawks, spiders
– Omnivores, eating both plants and other animals—
humans, roaches, raccoons, crows
Copyright © 2009 Pearson Education, Inc.
Animals ingest their food in a variety of ways
 Animals obtain and ingest their food in different
ways
– Suspension feeding
– Substrate feeding
– Fluid feeding
– Bulk feeding
Copyright © 2009 Pearson Education, Inc.
Caterpillar
Feces
Overview: Food processing occurs in four stages
 Food is processed in four stages
– Ingestion
– Digestion
– Absorption
– Elimination
Copyright © 2009 Pearson Education, Inc.
Small
molecules
Pieces
of food
Chemical digestion Nutrient
Mechanical (hydrolysis)
molecules
digestion
enter body
cells
Undigested
material
Food
1 Ingestion
2 Digestion
3 Absorption
4 Elimination
Overview: Food processing occurs in four stages
 Mechanical digestion breaks food down into smaller
pieces
– Smaller pieces are easier to swallow
– Smaller pieces have more surface area exposed to
digestive fluids
Copyright © 2009 Pearson Education, Inc.
Overview: Food processing occurs in four stages
 Chemical digestion breaks down large organic
molecules into their components
– Proteins split into amino acids
– Polysaccharides and disaccharides into
monosaccharides
– Nucleic acids into nucleotides
Copyright © 2009 Pearson Education, Inc.
Macromolecule
Components
Proteindigesting
enzymes
Protein
Polysaccharide
Amino acids
Carbohydratedigesting
enzymes
Disaccharide
Monosaccharides
Nucleic aciddigesting
enzymes
Nucleic acid
Nucleotides
Fat-digesting
enzymes
Fat
Glycerol Fatty acids
Digestion occurs in specialized compartments
 Sponges digest food in vacuoles
 Most animals digest food in compartments
– Enzymes break down the food
– Food particles move into cells lining the compartment
– Undigested materials are expelled
Copyright © 2009 Pearson Education, Inc.
Digestion occurs in specialized compartments
 Cnidarians and flatworms have a gastrovascular
cavity with a single opening, the mouth
– Food enters the mouth
– Undigested food is expelled back out the mouth
Copyright © 2009 Pearson Education, Inc.
Mouth
Tentacle
1 Digestive
enzymes
released from
a gland cell
2 Soft tissues
digested
3 Food particle
Food
(Daphnia,
a water
flea)
engulfed
Gastrovascular
cavity
4 Food particle
digested in
food vacuole
Digestion occurs in specialized compartments
 Most animals have an alimentary canal with
– Mouth
– Anus
– Specialized regions
Copyright © 2009 Pearson Education, Inc.
Mouth
Pharynx
Esophagus
Crop
Anus
Gizzard
Intestine
Dorsal fold
Interior of intestine
Wall of intestine
Earthworm
Esophagus
Midgut
Anus
Mouth
Crop
Grasshopper
Gastric pouches
Hindgut
Stomach
Gizzard
Intestine
Mouth
Esophagus
Crop
Anus
Bird
HUMAN DIGESTIVE SYSTEM
Copyright © 2009 Pearson Education, Inc.
Digestive System I: The Upper Gastrointestinal Tract
 http://educationportal.com/academy/lesson/digestive-system-i-theupper-gastrointestinal-tract.html
The human digestive system consists of an
alimentary canal and accessory glands
 Alternating waves of contraction and relaxation by
smooth muscle in the walls of the canal move food
along in a process called peristalsis
 Sphincters control the movement of food into and
out of digestive chambers
Copyright © 2009 Pearson Education, Inc.
The human digestive system consists of an
alimentary canal and accessory glands
 The pyloric sphincter
– Regulates the passage of food from the stomach to the
small intestine
– Limits the upward movement of acids into the
esophagus
Copyright © 2009 Pearson Education, Inc.
Mouth
Salivary
glands
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Oral
cavity
Tongue
Mouth
Pharynx
Salivary
glands
Esophagus
Liver
Esophagus
Sphincter
Stomach
Sphincter
Gallbladder
Pancreas
Small
intestine
Large
intestine
Rectum
Anus
Small
intestine
Digestion begins in the oral cavity
 Teeth break up food, saliva moistens it
– Salivary enzymes begin the hydrolysis of starch
– Buffers neutralize acids
– Antibacterial agents kills some bacteria ingested with
food
 The tongue tastes, shapes the bolus of food, and
moves it toward the pharynx
Copyright © 2009 Pearson Education, Inc.
Teeth
Incisors
Canine
Premolars
Molars
Tongue
Salivary
glands
Opening of a
salivary gland
duct
“Wisdom”
tooth
After swallowing, peristalsis moves food through
the esophagus to the stomach
 The trachea conducts air to the lungs
 The esophagus conducts food from the pharynx to
the stomach
Copyright © 2009 Pearson Education, Inc.
Bolus of
food
Tongue
Pharynx
Epiglottis
up
Larynx
up
Larynx
Trachea
Epiglottis
down
Epiglottis
up
Larynx
down
Esophageal
sphincter
Esophagus
Sphincter contracted
Esophagus
Sphincter relaxed
Sphincter contracted
After swallowing, peristalsis moves food through
the esophagus to the stomach
 The swallowing reflex
– Food moves from the pharynx into the esophagus
– The swallowing reflex prevents food from entering the
trachea
– A coughing reflex helps expel materials that
accidentally enter the trachea
Copyright © 2009 Pearson Education, Inc.
Esophageal sphincter
(contracted)
Bolus of
food
Bolus of
food
Muscles contract,
constricting passageway
and pushing bolus down
Muscles relax,
allowing passageway
to open
Stomach
The stomach stores food and breaks it down with
acid and enzymes
 Acid
– pH 2
– Parietal cells secrete hydrogen and chloride ions, which
combine to make HCl
– Acid kills bacteria and breaks apart cells in food
 Pepsinogen and HCl produce pepsin
– Pepsin production activates more pepsinogen
production—positive feedback
– Pepsin begins the chemical digestion of proteins
– Acidic gastric juices mix with food to produce acid
chyme
Copyright © 2009 Pearson Education, Inc.
The stomach stores food and breaks it down with
acid and enzymes
 What prevents the gastric juices from digesting the
walls of the stomach?
– Mucus helps protect against HCl and pepsin
– New cells lining the stomach are produced about every
3 days
Copyright © 2009 Pearson Education, Inc.
Esophagus
Sphincter
Lumen (cavity)
of stomach
Sphincter
Small
intestine
Interior surface
of stomach
Stomach
Gastrin
Interior surface
of stomach
Pits
Release of gastric juice
(mucus, HCl, and pepsinogen)
Pepsinogen
Epithelium
Mucous
cells
Gastric
gland
3
Pepsin
2
(active
HCl enzyme)
1
H+
Cl–
Chief cells
Parietal cells
The small intestine is the major organ of chemical
digestion and nutrient absorption
 Small intestine is named for its smaller diameter—it
is about 6 meters long
 Alkaline pancreatic juice neutralizes acid chyme and
its enzymes digest food
 Bile, made in the liver and stored in the gall bladder,
emulsifies fat for attack by pancreatic enzymes
Copyright © 2009 Pearson Education, Inc.
Liver
Bile
Gallbladder
Stomach
Acid chyme
Intestinal
enzymes
Duodenum of
small intestine
Pancreatic juice
Pancreas
Digestive System II: The Lower Gastrointestinal
Tract
 http://educationportal.com/academy/lesson/digestive-system-ii-thelower-gastrointestinal-tract.html
The small intestine is the major organ of chemical
digestion and nutrient absorption
 Enzymes from cells of the intestine continue
digestion
Copyright © 2009 Pearson Education, Inc.
The small intestine is the major organ of chemical
digestion and nutrient absorption
 Surface area for absorption is increased by
– Folds of the intestinal lining
– Fingerlike villi
Copyright © 2009 Pearson Education, Inc.
Vein
with blood
en route to
the liver
Muscle
layers
Lumen
Large
circular folds
Villi
Nutrient
absorption
Intestinal wall
The small intestine is the major organ of chemical
digestion and nutrient absorption
 Nutrients pass across the epithelium and into blood
 Blood flows to the liver where nutrients are
processed and stored
Copyright © 2009 Pearson Education, Inc.
Lumen of intestine
Lumen of intestine
Nutrient absorption
into epithelial cells
Nutrient
absorption
Microvilli
Epithelial
cells
Amino
acids
and
sugars
Fatty
acids
and
glycerol
Fats
Blood
capillaries
Lymph
vessel
Blood
Lymph
Villi
Epithelial cells
lining villus
One of the liver’s many functions is processing
nutrient-laden blood from the intestines
 Blood from the digestive tract drains to the liver
 The liver performs many functions
– Glucose in blood is converted to glycogen and stored
in the liver
– Liver synthesizes many proteins including blood
clotting proteins and lipoproteins that transport fats
and cholesterol
– Liver changes toxins to less toxic forms
– Liver produces bile
Copyright © 2009 Pearson Education, Inc.
Heart
Kidneys
Liver
Intestines
Hepatic
portal
vein
The large intestine reclaims water and compacts
the feces
 Diarrhea occurs when too little water is reclaimed
 Constipation occurs when too much water is
reclaimed
 Feces are stored in the rectum
 Colon bacteria produce vitamins—biotin, vitamin K,
B vitamins
Copyright © 2009 Pearson Education, Inc.
The large intestine reclaims water and compacts
the feces
 Appendix
– Located near the junction of the small intestine and
colon
– Makes a minor contribution to immunity
Copyright © 2009 Pearson Education, Inc.
Large
intestine
(colon)
Small
intestine
Sphincter
End
of small
intestine
Appendix
Cecum
Rectum
Anus
Unabsorbed
food material
EVOLUTION CONNECTION: Evolutionary
adaptations of vertebrate digestive systems
often relate to diet
 The length of the digestive tract often correlates
with diet
– Herbivores and omnivores have relatively longer
digestive tracts than carnivores
Copyright © 2009 Pearson Education, Inc.
Stomach
Small intestine
Cecum
Colon
(large
intestine)
Carnivore
Herbivore
EVOLUTION CONNECTION: Evolutionary
adaptations of vertebrate digestive systems
often relate to diet
 Many herbivores have specializations of the gut
that promote the growth of cellulose-digesting
microbes
– Rumen
– Reticulum
– Omasum
– Abomasum
Copyright © 2009 Pearson Education, Inc.
Intestine
Omasum
Rumen
Esophagus
Rumen
Abomasum
Reticulum
a.
b.
g.
h.
i.
c.
d.
e.
j.
k.
l.
f.
m.
You should now be able to
1. Describe the four stages of food processing
2. Describe the main components of the human
digestive tract and their functions
3. Explain how teeth and saliva help us swallow
4. Explain why the stomach does not digest itself
5. Compare the structures and functions of the
small and large intestines
6. Compare the digestive tracts of carnivores and
herbivores
Copyright © 2009 Pearson Education, Inc.