DIGESTION & ABSORPTION
Chapter 3
Outline


Digestive Tracts
The 4 Stages
 Ingestion
 Digestion
 Absorption
 Egestion

Nutrition
Overview: Digestion and Absorption


Food is taken in, taken apart, and taken up in the
process of animal digestion and absorption.
In general, animals are heterotrophs which fall into
three categories:
 Holozoic
needs to ingest organic or non-organic food and
digestion is via alimentary canal
 Saprophytes digest food extracellularly and absorption is
via cell walls
 Parasites live and obtain food from another living
organism
The 4 Main Stages : Ingestion

Ingestion is the act of eating

Four types of feeders:
 Suspension
 Substrate
 Fluid
 Bulk
Feeders
Feeders
Feeders
feeders
Fig. 41-6a
Baleen
Humpback whale, a suspension feeder
Fig. 41-6b
Leaf miner caterpillar,
a substrate feeder
Caterpillar
Feces
Fig. 41-6c
Mosquito, a fluid feeder
Fig. 41-6d
Rock python, a bulk feeder

Digestion is the process of breaking food down into
molecules small enough to absorb
 In
chemical digestion, the process of enzymatic hydrolysis
splits bonds in molecules with the addition of water


Absorption is uptake of nutrients by body cells
Elimination is the passage of undigested material
out of the digestive compartment
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
(enzymatic hydrolysis)
Nutrient
molecules
enter body
cells
Undigested
material
Food
1 Ingestion
2 Digestion
3 Absorption
4 Elimination
Digestive Compartments





Most animals process food in specialized compartments
These compartments reduce the risk of an animal
digesting its own cells and tissues
In intracellular digestion, food particles are engulfed
by endocytosis and digested within food vacuoles
Extracellular digestion is the breakdown of food
particles outside of cells
It occurs in compartments that are continuous with the
outside of the animal’s body
Fig. 41-8
Tentacles
Food
Mouth
Epidermis
Gastrodermis
Gastrovascular
cavity




Animals with simple body plans have a gastrovascular
cavity that functions in both digestion and distribution of
nutrients
More complex animals have a digestive tube with two
openings, a mouth and an anus
This digestive tube is called a complete digestive tract
or an alimentary canal
It can have specialized regions that carry out digestion
and absorption in a stepwise fashion
Digestive Tracts

Incomplete versus Complete Tracts


Incomplete tract has a single opening
Ex: Planarian
Food enters through mouth and muscular pharynx
 Wastes exit through mouth and muscular pharynx




Lacks specialized parts
Complete Tract has two openings
Ex: Earthworm

Food enters through mouth

Wastes exit through anus
Incomplete Digestive Tract of a
Planarian
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mouth
pharynx
digestive tract
Golgi
apparatus
lysosome
a.
gastrovascular cavity
b.
Complete Digestive Tract of an
Earthworm
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
typhlosole
anus
mouth
pharynx
intestine
esophagus
crop
gizzard
Fig. 41-9a
Crop
Esophagus
Gizzard
Intestine
Pharynx
Anus
Mouth
Typhlosole
Lumen of intestine
(a) Earthworm
Fig. 41-9b
Foregut
Midgut
Esophagus
Rectum
Anus
Crop
Mouth
(b) Grasshopper
Hindgut
Gastric cecae
Fig. 41-9c
Stomach
Gizzard
Intestine
Mouth
Esophagus
Crop
Anus
(c) Bird
The Mammalian Digestive System:
Organs Specialization





The mammalian digestive system consists of an
alimentary canal and accessory glands that secrete
digestive juices through ducts
Mammalian accessory glands are the salivary
glands, the pancreas, the liver, and the gallbladder
Food is pushed along by peristalsis, rhythmic
contractions of muscles in the wall of the canal
Valves called sphincters regulate the movement of
material between compartments
Humans digestive tract is complete and
extracellular
Fig. 41-10a
Tongue
Sphincter
Oral cavity
Salivary glands
Pharynx
Esophagus
Sphincter
Liver
Stomach
Ascending
portion of
large intestine
Gallbladder
Duodenum of
small intestine
Pancreas
Small
intestine
Small
intestine
Large
intestine
Rectum
Anus
Appendix
Cecum
Fig. 41-10b
Salivary
glands
Mouth
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Fig. 41-10
Tongue
Sphincter
Salivary
glands
Oral cavity
Salivary glands
Mouth
Pharynx
Esophagus
Esophagus
Sphincter
Liver
Stomach
Ascending
portion of
large intestine
Gallbladder
Duodenum of
small intestine
Pancreas
Small
intestine
Small
intestine
Large
intestine
Rectum
Anus
Appendix
Cecum
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
The Oral Cavity, Pharynx, and Esophagus


The first stage of digestion (ingestion) is mechanical
and takes place in the oral cavity
Mouth
 Three
major pairs of salivary glands
 Saliva
contains salivary amylase (Digestion)
 Salivary amylase initiates starch digestion
 Tongue
 Mixes
is composed of striated muscle
chewed food with saliva
 Forms mixture into bolus
Pharynx and Esophagus

Pharynx



Where digestive and respiratory passages come together
Soft palate closes off nasopharynx
Epiglottis
Covers opening into trachea
 Keeps food from air passages (most of the time)


Esophagus


Takes food to stomach by peristalsis
Peristalsis - Rhythmical contraction to move contents in
tubular organs
25
Fig. 41-11-1
Food
Epiglottis
up
Tongue
Pharynx
Esophageal
sphincter
contracted
Glottis
Larynx
Trachea
Esophagus
To To
lungs stomach
Fig. 41-11-2
Food
Epiglottis
up
Tongue
Pharynx
Esophageal
sphincter
contracted
Glottis
Larynx
Trachea
Epiglottis
down
Esophagus
To To
lungs stomach
Glottis up
and closed
Esophageal
sphincter
relaxed
Fig. 41-11-3
Food
Epiglottis
up
Tongue
Epiglottis
up
Pharynx
Esophageal
sphincter
contracted
Glottis
Larynx
Trachea
Epiglottis
down
Esophagus
To To
lungs stomach
Glottis up
and closed
Esophageal
sphincter
relaxed
Glottis
down
and open
Esophageal
sphincter
contracted
Relaxed
muscles
Relaxed
muscles
Stomach
Contracted
muscles
Sphincter
relaxed
Peristalsis in the Digestive Tract
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esophagus
bolus
Stage 2: Digestion (Stomach)


The stomach stores food and secretes gastric juice
Stomach
 Stomach
wall has deep folds
 Folds disappear as the stomach fills to an approximate
volume of one liter
 Epithelial lining of the stomach has millions of gastric pits,
which drain gastric glands
 Pepsin
is a hydrolytic enzyme that acts on protein to produce
peptides

Food mixing with gastric juices becomes acid chyme
Fig. 41-12a
Esophagus
Sphincter
Stomach
5 µm
Sphincter
Interior surface
of stomach
Small
intestine
Folds of
epithelial
tissue
Fig. 41-12b
Interior surface
of stomach
Epithelium
3
Pepsinogen
2
1 Pepsinogen and HCl
are secreted.
HCl
Gastric gland
2 HCl converts
1
Mucus cells
Pepsin
Cl–
pepsinogen to pepsin.
H+
3 Pepsin activates
more pepsinogen.
Chief cells
Chief cell
Parietal cells
Parietal cell
Fig. 41-12
Esophagus
Sphincter
Stomach
5 µm
Sphincter
Interior surface
of stomach
Small
intestine
Folds of
epithelial
tissue
Epithelium
3
Pepsinogen
2
HCl
Gastric gland
1 Pepsinogen and HCl
are secreted.
Pepsin
2 HCl converts
pepsinogen to pepsin.
1
Mucus cells
Cl–
3 Pepsin activates
more pepsinogen.
H+
Chief cells
Chief cell
Parietal cells
Parietal cell
STAGE 2: Digestion (Small Intestine)



The small intestine is the longest section of the
alimentary canal
It is the major organ of digestion and absorption
The first portion of the small intestine is the
duodenum, where acid chyme from the stomach
mixes with digestive juices from the pancreas, liver,
gallbladder, and the small intestine itself
Gallbladder
Bile
Liver
Stomach
Pancreas
Duodenum of
small intestine
Duodenum: The Pancreas




The pancreas produces proteases trypsin and
chymotrypsin, protein-digesting enzymes that are
activated after entering the duodenum
Its solution is alkaline and neutralizes the acidic chyme
The pancreas produces proteases trypsin and
chymotrypsin, protein-digesting enzymes that are
activated after entering the duodenum
Its solution is alkaline and neutralizes the acidic chyme
Duodenum: The Liver


In the small intestine, bile aids in digestion and
absorption of fats
Bile is made in the liver and stored in the gallbladder
 Bile
contains bile salts which break up fat into fat droplets
via emulsification
 Helps maintain glucose concentration in blood by converting
excess into glycogen
Small Intestine



The epithelial lining of the duodenum, called the brush
border, produces several digestive enzymes
Enzymatic digestion is completed as peristalsis moves
the chyme and digestive juices along the small
intestine
Most digestion occurs in the duodenum; the jejunum
and ileum function mainly in absorption of nutrients
and water
Fig. 41-13a
Carbohydrate digestion
Polysaccharides
Oral cavity,
pharynx,
esophagus
(starch, glycogen)
Disaccharides
(sucrose, lactose)
Salivary amylase
Smaller polysaccharides,
maltose
Stomach
Polysaccharides
Lumen of
small
intestine
Epithelium
of small
intestine
(brush
border)
Pancreatic amylases
Maltose and other
disaccharides
Disaccharidases
Monosaccharides
Fig. 41-13b
Protein digestion
Stomach
Proteins
Pepsin
Small polypeptides
Polypeptides
Pancreatic trypsin and
chymotrypsin
Lumen of
small
intestine
Smaller
polypeptides
Pancreatic carboxypeptidase
Amino acids
Small peptides
Epithelium
of small
intestine
(brush
border)
Dipeptidases, carboxypeptidase,
and aminopeptidase
Monosaccharides
Amino acids
Fig. 41-13c
Nucleic acid digestion
DNA, RNA
Lumen of
small
intestine
Pancreatic
nucleases
Nucleotides
Nucleotidases
Epithelium
of small
intestine
(brush
border)
Nucleosides
Nucleosidases
and
phosphatases
Nitrogenous bases,
sugars, phosphates
Fig. 41-13d
Fat digestion
Fat globules
Bile salts
Lumen of
small
intestine
Fat droplets
Pancreatic lipase
Glycerol, fatty
acids, monoglycerides
Fig. 41-13
Carbohydrate digestion
Oral cavity,
pharynx,
esophagus
Polysaccharides
Disaccharides
(starch, glycogen)
(sucrose, lactose)
Protein digestion
Nucleic acid digestion
Fat digestion
Salivary amylase
Smaller polysaccharides,
maltose
Stomach
Proteins
Pepsin
Small polypeptides
Lumen of
small intestine
Polysaccharides
Pancreatic amylases
Polypeptides
Pancreatic trypsin and
chymotrypsin
DNA, RNA
Fat globules
Pancreatic
nucleases
Bile salts
Maltose and other
disaccharides
Nucleotides
Fat droplets
Smaller
polypeptides
Pancreatic lipase
Pancreatic carboxypeptidase
Glycerol, fatty
acids, monoglycerides
Amino acids
Epithelium
of small
intestine
(brush
border)
Small peptides
Disaccharidases
Monosaccharides
Nucleotidases
Nucleosides
Dipeptidases, carboxypeptidase,
and aminopeptidase
Amino acids
Nucleosidases
and
phosphatases
Nitrogenous bases,
sugars, phosphates
STAGE 3: Absorption (Small Intestines)


The small intestine has a huge surface area, due to
villi and microvilli that are exposed to the intestinal
lumen
The enormous microvillar surface greatly increases
the rate of nutrient absorption
Fig. 41-15
Microvilli (brush
border) at apical
(lumenal) surface
Vein carrying blood
to hepatic portal vein
Lumen
Blood
capillaries
Muscle layers
Epithelial
cells
Basal
surface
Large
circular
folds
Villi
Epithelial cells
Lacteal
Key
Nutrient
absorption
Intestinal wall
Villi
Lymph
vessel
Fig. 41-15a
Vein carrying blood
to hepatic portal vein
Muscle layers
Large
circular
folds
Villi
Key
Nutrient
absorption
Intestinal wall
Fig. 41-15b
Microvilli (brush
border) at apical
(lumenal) surface
Lumen
Blood
capillaries
Epithelial
cells
Basal
surface
Epithelial cells
Lacteal
Villi
Key
Nutrient
absorption
Lymph
vessel
Fig. 41-16
Lumen
of small
intestine
Triglycerides
Fatty acids
Monoglycerides
Epithelial
cell
Triglycerides
Phospholipids,
cholesterol,
and proteins
Chylomicron
Lacteal
STAGE 3: Absorption (Large Intestines)



The colon of the large intestine is connected to the
small intestine
The cecum aids in the fermentation of plant material
and connects where the small and large intestines
meet
The human cecum has an extension called the
appendix, which plays a very minor role in immunity
Junction of the Small Intestine and the Large
Intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
large intestine
small intestine
cecum
vermiform appendix
Fig. 41-17
STAGE 4: Egestion (Large Intestines)






A major function of the colon is to recover water that has
entered the alimentary canal
Wastes of the digestive tract, the feces, become more
solid as they move through the colon
Feces are stored in the rectum until they can be
eliminated
Two sphincters between the rectum and anus control
bowel movements
Feces pass through the rectum and exit via the anal
canal where the opening is call the anus.
The colon houses strains of the enterobacteria Escherichia
coli, some of which produce vitamins