Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 36
Digestion and Human Nutrition
(Sections 36.1 - 36.6)
Albia Dugger • Miami Dade College
36.1 The Battle Against Bulge
• When you eat more food than you need, you store the excess
energy – mostly as fat in adipose tissue
• In the US about two-thirds of adults are overweight or obese,
which increases risk of type 2 diabetes, cardiovascular
disease, some cancers, and other disorders
• Increasing portion sizes and fast-food meals contribute to the
obesity trend in adults and children
Battle Against Bulge (cont.)
• Genetics contribute to weight control
• The ob gene encodes a hormone (leptin) made by adipose
cells that suppresses appetite – people who can’t make leptin
are severely obese
• Another obesity-related gene, fto, affects food intake – people
with the high-risk allele eat more food before they feel full
Effects of a Mutant ob Gene
ABC Video: Diet Soda and Weight Gain
36.2 Animal Digestive Systems
• Animals are heterotrophs that typically digest food inside their
body, but outside of their cells
• The digestive system mechanically and chemically breaks
food down to molecules that are absorbed into the internal
environment, and expels unabsorbed residues
• The digestive system interacts with other organ systems to
maintain homeostasis for the body as a whole
Incomplete and Complete Systems
• Some invertebrates have an incomplete digestive system,
but most invertebrates and all vertebrates have a complete
digestive system
• incomplete digestive system
• Saclike digestive system; food enters and leaves through
the same opening
• complete digestive system
• Tubelike digestive system; food enters through one
opening and wastes leave through another
Incomplete Digestive System
Incomplete Digestive System
branching saclike gut
A Incomplete digestive
system of a flatworm.
pharynx
single opening takes in
food, expels waste
Fig. 36.2a, p. 596
Complete Digestive System
Complete Digestive System
pharynx
mouth
with
tongue
liver
gallbladder
pancreas
B Complete digestive
system of a frog.
esophagus
stomach
small intestine
large intestine
cloaca (opening
through which
digestive wastes,
urinary wastes,
and gametes exit
the body)
Fig. 36.2b, p. 596
Five Tasks of the Digestive System
1. Mechanical processing and motility:
• Movements that break up, mix, and directionally propel
food material
2. Secretion:
• Release of substances, especially digestive enzymes, into
the lumen (the space inside the tube)
3. Digestion:
• Breakdown of food into particles, then to nutrient
molecules small enough to be absorbed
Five Tasks of the Digestive System
4. Absorption:
• Uptake of digested nutrients and water across the gut wall,
into extracellular fluid
5. Elimination:
• Expulsion of undigested or unabsorbed solid residues
Diet-Related Structural Adaptations
• Features of an animal digestive systems are shaped by
natural selection and adapt the animal to a particular diet
• A bird’s beak-size and shape determines what kinds of food it
can process
• Teeth of mammals differ, depending on their diet:
• Carnivores have long, sharp canine teeth
• Herbivores (e.g. antelopes) have prominent incisors for
grazing, and flat molars for grinding food
Herbivore and Human Molars
Herbivore and Human Molars
gumline
crown
root
antelope molar
crown
root
human molar
Fig. 36.3a, p. 597
Diet-Related Adaptations (cont.)
• Animal guts also are
specialized to their diets
• Seed-eating birds have
a large crop for storing
food; and a large, tough
gizzard for grinding food
beak
mouth
Diet-Related Adaptations
(cont.)
esophagus
crop
glandular part
of stomach
gizzard
intestines
cloaca
C Complete digestive system of a bird. The crop is an expandable
organ for storing food. The muscular gizzard grinds up food.
Fig. 36.2c, p. 596
ANIMATION: Examples of digestive
systems
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Diet-Related Adaptations (cont.)
• Ruminants have multiple stomach chambers that maximize
nutrients extracted from plant foods rich in cellulose
• Microbes ferment cellulose in the first two chambers
• Solids in the second chamber form “cud” that is
regurgitated for a second chewing
• ruminant
• Hoofed mammal with a multiple-chamber stomach that
adapts it to a cellulose-rich diet
A Ruminant Stomach
A Ruminant Stomach
ingestion, regurgitation, reswallowing
of food through esophagus
stomach
chamber 1
stomach
chamber 2
stomach
chamber 3
stomach
chamber 4
B
to small
intestine
Fig. 36.3b, p. 597
ANIMATION: Antelope stomach function
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Diet-Related Adaptations (cont.)
• Some meat-eaters have stomachs that can stretch
enormously – a python may eat only once or twice a year
Key Concepts
• Animal Digestive Systems
• Some animal digestive systems are saclike, but most are
a tube with two openings
• The structure of an animal’s digestive system often
reveals adaptations that allow the animal to eat a specific
diet
• For example, antelopes have adaptations that allow them
to eat grasses
36.3 The Human Digestive System
• Humans have a complete digestive system with specialized
regions for digesting food, absorbing nutrients, and
concentrating and storing unabsorbed waste
• Salivary glands, the pancreas, and the liver are accessory
organs that secrete enzymes and other substances into the
tubular portion of this system
Overview of the Digestive System
• Food taken into the human mouth is swallowed and moves
into the pharynx, which opens onto the esophagus
• Peristalsis of the esophagus conveys food to the stomach
• From the stomach, food enters the small intestine, then the
large intestine (colon)
• Wastes are stored in the rectum until they are eliminated
through the anus
Key Terms
• esophagus
• Muscular tube between the throat and stomach
• peristalsis
• Wavelike smooth muscle contractions that propel food
through the digestive tract
• stomach
• Muscular organ that mixes food with acid and digestive
enzymes (gastric fluid) that it secretes
Key Terms
• small intestine
• Longest portion of the digestive tract
• Receives secretions from the liver and pancreas
• Site of most digestion and absorption
• colon (large intestine)
• Receives digestive waste from the small intestine and
concentrates it as feces
• Absorbs water and ions
Key Terms
• rectum
• Region where feces are stored prior to excretion
• anus
• Opening through which digestive waste is expelled from a
complete digestive system
Major Organs and Accessory Organs
A Major Organs
Mouth
Oral cavity. Teeth break food into
bits. Tongue mixes food with saliva.
Pharynx (throat)
Entrance to the gut and respiratory
system. Action of the epiglottis keeps
food from entering the trachea.
B Accessory Organs
Salivary Glands
Produce and secrete
saliva, which moistens
food and begins the
process of carbohydrate
digestion.
Esophagus
Muscular tube through which food
moves to the stomach.
Stomach
J-shaped muscular sac that receives
food and mixes it with gastric fluid
secreted by cells in its lining.
Liver
Produces bile, which
aids digestion and
absorption of fats.
Small Intestine
Longest tube of the gut. Its first part receives
secretions from the liver, gallbladder, and
pancreas. These secretions help complete the
process of digestion. Most water and products
of digestion are absorbed across the highly
folded wall of this organ.
Large Intestine (colon)
Wider than the small intestine, but shorter.
It absorbs most remaining water, thus
concentrating any undigested waste and
forming the feces.
Rectum Expandable sac that stores feces.
Anus
Opening through which feces
are expelled from the body.
Gallbladder
Stores and concentrates
bile, then secretes it into
the small intestine.
Pancreas
Secretes enzymes and
bicarbonate (a buffer)
into the small intestine.
Major Organs and
Accessory Organs
Fig. 36.5, p. 598
ANIMATION: Human digestive system
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Key Concepts
• Human Digestive System
• In humans, food taken into the mouth is mechanically
broken down by teeth
• Saliva starts the process of chemical digestion
• The stomach stores and digests food
• Digestion is completed and absorption begins in the small
intestine
• The large intestine concentrates wastes
ANIMATION: Digestion
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36.4 Digestion in the Mouth
• Mechanical digestion begins in the mouth when teeth break
food into smaller pieces
• Human adults have four types of teeth: incisors, canines,
premolars, and molars
• Each tooth consists of bonelike dentin, with a crown covered
by enamel, the hardest material in the body
Human Teeth
Human Teeth
Fig. 36.6a, p. 599
Human Teeth
A Adult teeth.
Incisors shear
off bits of food.
Canines tear
meats.
Premolars and
molars have
broad, bumpy
crowns that are
platforms for
grinding and
crushing food.
molars
(12)
premolars
(8)
canines (4)
incisors
(8)
lower jaw
upper jaw
Fig. 36.6a, p. 599
Human Teeth
Fig. 36.6b, p. 599
Human Teeth
B Cross-section of one
human tooth. The crown
is the portion extending
above the gum; the root is
embedded in the jaw.
enamel
dentin
crown
pulp cavity
(contains
nerves and
blood vessels)
gingiva
(gum)
ligaments
root
root canal
periodontal
membrane
bone
Fig. 36.6b, p. 599
Digestion in the Mouth (cont.)
• Chemical digestion (enzymatic breakdown of food into
molecular subunits) also begins in the mouth
• Saliva, secreted by salivary glands, contains enzymes,
bicarbonate, and mucins
• Salivary amylase begins breakdown of starch; salivary lipase
begins breakdown of fats
• salivary gland
• Exocrine gland that secretes saliva into the mouth
3D Animation: Digestive Motility
36.5 Food Storage and
Digestion in the Stomach
• The stomach is a muscular, highly-folded, stretchable sac
with a sphincter at either end
• It stores food and continues the process of digestion that
began in the mouth
• sphincter
• Ring of muscle that controls passage through a tubular
organ or body opening
Structure of the Stomach
Structure of the Stomach
gastroesophageal
sphincter
esophagus
serosa
longitudinal
muscle
pyloric sphincter
circular
muscle
oblique
muscle
submucosa
small intestine
mucosa
Fig. 36.7, p. 600
Functions of the Stomach
• The stomach has three functions:
1. It stores food and controls the rate of passage to the
small intestine
2. It mechanically breaks down food
3. It secretes substances that aid in chemical digestion
Digestion in the Stomach
• Cells of the glandular epithelium (mucosa) lining the stomach
secrete gastric fluid, which includes mucus, hydrochloric
acid, and pepsinogen, an inactive form of the proteindigesting enzyme pepsin
• gastric fluid
• Fluid secreted by the stomach lining
• Contains digestive enzymes, acid, and mucus
Digestion in the Stomach (cont.)
• Action potentials generated in the upper portion of the
stomach cause rhythmic stomach contractions that mix
gastric fluid with food to form chyme
• chyme
• A semiliquid mixture of food and gastric fluid
Chemical Digestion of Protein
• Arrival of food in the stomach triggers endocrine cells to
secrete gastrin into the blood, which stimulates acid secretion
by cells of the stomach lining
• Acid in denatures proteins and exposes their peptide bonds
• Acids also converts pepsinogen into pepsin, which breaks
peptide bonds, cutting proteins into smaller polypeptides
Stomach Disorders
• Gastroesophageal reflux:
• When the gastroesophageal sphincter does not close
properly, acidic chyme splashes back into the esophagus,
causing heartburn or acid indigestion
• Stomach ulcers:
• When the stomach’s protective mucus layer is disrupted,
gastric fluid and enzymes can erode the stomach lining
• Most ulcers result from a bacterial infection (H. pylori)
ANIMATION: Peristalsis
36.6 Structure of the Small Intestine
• Chyme leaves the stomach through the pyloric sphincter and
enters the duodenum, the first part of the small intestine
• The small intestine is the longest segment of the gut, and has
a highly folded lining with many projections that increase
surface area for digestion and absorption
Lining of the Small Intestine
• Longitudinal cross
section through
the small intestine
showing its highly
folded lining
Increasing Surface Area
• Millions of hairlike villi containing blood and lymph vessels
project from the intestinal lining
• Epithelial cells called brush border cells at the surface of a
villus have even smaller projections called microvilli
• Collectively, folds and projections of the small intestinal lining
increase its surface area to about the size of a tennis court
Key Terms
• villi
• Multicelled projections at the surface of each fold in the
small intestine
• brush border cell
• In the lining of the small intestine, an epithelial cell with
microvilli at its surface
• microvilli
• Thin projections that increase the surface area of some
epithelial cells
Wall of the Small Intestine
Wall of the Small Intestine
autonomic
nerves
longitudinal circular
muscle
muscle
B Structure of
the wall of the
small intestine.
villi
blood
vessels
lymph
vessel
C One intestinal
fold with hair-like
villi at its surface.
D One villus
with brush
border cells at
its surface.
E A brush
border cell
with microvilli
at its free
surface.
F Micrograph
of microvilli at
the surface of
a brush border
cell.
Fig. 36.8b-f, p. 601