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39
Nutrition, Digestion, and
Absorption
Chapter 39 Nutrition, Digestion, and Absorption
Key Concepts
• 39.1 Food Provides Energy and Nutrients
• 39.2 Digestive Systems Break Down
Macromolecules
• 39.3 The Vertebrate Digestive System Is a
Tubular Gut with Accessory Glands
• 39.4 Food Intake and Metabolism Are
Regulated
Chapter 39 Opening Question
Why are some ethnic groups of humans
much more prone to obesity and
diabetes than other groups?
Concept 39.1 Food Provides Energy and Nutrients
Animals are heterotrophs and derive their
nutrition by eating other organisms.
Autotrophs use solar energy or inorganic
chemical energy to synthesize their
necessary nutrients.
Heterotrophs depend on this synthesis and
have adapted to take advantage of it.
Figure 39.1 Heterotrophs Get Energy from Autotrophs (Part 1)
Figure 39.1 Heterotrophs Get Energy from Autotrophs (Part 2)
Concept 39.1 Food Provides Energy and Nutrients
Energy needs can be measured.
Measures of heat energy:
A calorie is the amount of heat needed to
raise 1 gram of water 1°C
A kilocalorie (kcal) = 1,000 calories
The Calorie (Cal) is the same as a
kilocalorie and = 1,000 calories
A joule = 0.239 calories
Concept 39.1 Food Provides Energy and Nutrients
The metabolic rate measures energy needs
of an animal that are met by food intake
and digestion.
Foods that provide energy are fats,
carbohydrates, and proteins.
Basal metabolic rate (BMR) is the minimum
energy needed to sustain life while an
animal is at rest.
Concept 39.1 Food Provides Energy and Nutrients
Animals must store food between meals.
Carbohydrates are stored in liver and
muscle cells as glycogen—enough for
about a day’s energy needs.
Most fuel is stored as fat—stores more
energy per gram and with little water,
which makes it more compact.
Concept 39.1 Food Provides Energy and Nutrients
Essential nutrients are required but cannot
be synthesized.
Macronutrients—nutrients required in large
amounts, like calcium
Micronutrients—nutrients required in tiny
amounts, like iron
Amino acids are the building blocks of
proteins.
Each species has essential amino acids
that they cannot synthesize.
Concept 39.1 Food Provides Energy and Nutrients
Complementary diets can supply all eight
essential amino acids for adult humans.
Humans must also obtain essential fatty
acids.
Linoleic acid is one that helps synthesize
other unsaturated fatty acids.
Figure 39.2 A Strategy for Vegetarians
Concept 39.1 Food Provides Energy and Nutrients
An essential mineral is a chemical element
required in the diet.
Calcium is a macronutrient, important in
structure of bones and teeth and nerve
and muscle function.
Osteoporosis is due to insufficient calcium.
Iron is a micronutrient, important in blood
and enzyme structure.
Iron-deficiency anemia—too few red blood
cells—is common.
Table 39.1 Mineral Elements Required by Animals
Concept 39.1 Food Provides Energy and Nutrients
Vitamins—carbon compounds that are
micronutrients
Species-specific, some can be synthesized
Scurvy—lack of vitamin C—cured by
ascorbic acid in citrus fruits
Vitamins are water- or fat-soluble. Excess
water-soluble ones are excreted in the
urine—fat-soluble ones like Vitamin D may
build up in the liver.
Table 39.2 Vitamins in the Human Diet
Concept 39.1 Food Provides Energy and Nutrients
Nutrient deficiency leads to malnutrition—
chronic malnutrition leads to a deficiency
disease:
• Scurvy and anemia
• Beriberi—due to deficiency of B1
• Goiter and hypothyroidism—due to iodine
deficiency
Concept 39.2 Digestive Systems Break Down Macromolecules
The function of the digestive system, or gut,
is to break down food into molecules for
absorption.
Food is broken down through hydrolysis by
enzymes produced by the digestive
system.
Enzymes are classified by the food they
break down: proteases, carbohydrases,
peptidases, lipases, and nucleases.
Concept 39.2 Digestive Systems Break Down Macromolecules
Digestion usually begins in a body cavity.
Gastrovascular cavities connect to the
outside through a single opening.
Example: A jellyfish stings prey and brings it
into its gastrovascular cavity—enzymes
break down food, and undigested particles
are released through the same opening.
Concept 39.2 Digestive Systems Break Down Macromolecules
Tubular guts have an opening at each end:
A mouth takes in food, molecules are
digested, and wastes are eliminated
through the anus.
In most animals the gut can be divided into
three sections: foregut, midgut, and
hindgut.
Figure 39.3 Compartments for Digestion and Absorption (Part 1)
Figure 39.3 Compartments for Digestion and Absorption (Part 2)
Figure 39.3 Compartments for Digestion and Absorption (Part 3)
Concept 39.2 Digestive Systems Break Down Macromolecules
Food is broken up in the mouth cavity by
teeth, radula (snails), or mandibles
(arthropods).
From the mouth the food moves through an
esophagus to a stomach, or storage sac.
Some animals have two organs in a row—a
crop that holds food and a muscular
gizzard that grinds up food.
Concept 39.2 Digestive Systems Break Down Macromolecules
Small food particles are delivered into the
midgut—a long thin midgut is the small
intestine.
Nutrients are absorbed in the midgut
through villi—fingerlike projections that
increase surface area for absorption.
Villi have microvilli to provide even more
surface area.
Figure 39.4 Intestinal Surface Area and Nutrient Absorption (Part 1)
Figure 39.4 Intestinal Surface Area and Nutrient Absorption (Part 2)
Figure 39.4 Intestinal Surface Area and Nutrient Absorption (Part 3)
Concept 39.2 Digestive Systems Break Down Macromolecules
The hindgut, or large intestine, recovers
ions and water and stores undigested
waste as feces.
The end of the digestive tract is the anus.
In birds, amphibians and reptiles, the
cloaca expels both urinary and digestive
wastes.
Most digestive tracts contain symbiotic
bacteria.
Concept 39.2 Digestive Systems Break Down Macromolecules
Heterotrophs can be classified by how they
acquire food:
Saprobes (decomposers) absorb nutrients
from dead organic matter.
Detritivores actively feed on dead organic
matter.
Predators feed on living organisms.
Concept 39.2 Digestive Systems Break Down Macromolecules
Predators feed on other animals:
• Herbivores—consume plants
• Carnivores—prey on animals
• Omnivores—prey on both
• Filter feeders filter small organisms from
an aquatic environment
• Fluid feeders include mosquitoes
Concept 39.2 Digestive Systems Break Down Macromolecules
Mammalian teeth have shapes and
organization adapted to specific diets:
• Incisors—used for cutting, chopping, or
gnawing
• Canines—for stabbing, gripping, or ripping
• Molars or premolars—shearing, crushing,
or grinding
Figure 39.5 Mammalian Teeth
Concept 39.2 Digestive Systems Break Down Macromolecules
Diet affects the size of the digestive system.
Carnivores have short digestive tracts,
because meat is easy to digest.
Herbivores have longer digestive tracts,
often with compartments for cellulosedigesting bacteria to aid in plant digestion.
Traits in humans indicate we are omnivores.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The vertebrate digestive system is a tubular
gut, running from mouth to anus.
It has several accessory glands, including
liver and pancreas.
Processes occur in sequence in different
sections.
Figure 39.6 The Vertebrate Digestive System
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The vertebrate gut has a layered plan:
• Lumen—the gut cavity
• Mucosa—layer of epithelial cells, mucosal
epithelium, that secrete mucus, digestive
enzymes, or hormones; some absorb
nutrients through microvilli
• The submucosa has blood and lymph
vessels, and nerves
Figure 39.7 Tissue Layers of the Vertebrate Gut
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Two layers of smooth muscle are outside
the submucosa:
• The circular muscle layer—innermost cells
oriented around the gut that constrict the
gut
• The longitudinal muscle layer—outermost
cells oriented along the gut that shorten
the gut
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Between layers of smooth muscle are
nerves that coordinate movement of the
gut.
Nerves in the enteric nervous system only
form synapses with other nerves in the
network.
The peritoneum is a membrane that
surrounds the gut and lines the wall of the
cavity.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
In mammals, digestion begins in the
mouth—chewing mixes food with saliva,
containing amylase to digest starch.
A mouthful of food is a bolus—when
swallowed it passes into the esophagus.
Food is kept out of the trachea by the
epiglottis, which closes off the larynx.
The bolus moves toward the stomach
through peristalsis, coordinated by an
anticipatory wave of relaxation.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The stomach holds large amounts of food,
breaks it up, and begins protein digestion.
Gastric pits in the stomach are lined with
three types of secretory cells:
• Chief cells
• Parietal cells
• Mucus-secreting cells
Figure 39.8 Action in the Stomach (Part 1)
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Chief cells secrete pepsinogen, the inactive
form of a proteolytic enzyme, pepsin.
Parietal cells secrete hydrochloric acid
(HCl), which kills ingested organisms,
breaks down food, and activates pepsin.
Newly active pepsin activates other
pepsinogen molecules—a process called
autocatalysis.
Mucus-secreting cells protect the stomach
from the formation of ulcers.
Figure 39.8 Action in the Stomach (Part 2)
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Parietal cells produce HCl and keep the
stomach pH below 1.
They contain carbonic anhydrase, which
catalyzes the formation of H2CO3 from
CO2; H2CO3 dissociates into HCO3–
and H+.
One antiporter exchanges HCO3– for Cl– and
another exchanges H+ ions for K+.
K+ leaks back into the cells and H+ is
continually returned to the stomach.
Figure 39.8 Action in the Stomach (Part 3)
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Smooth muscle contractions in the stomach
create chyme—a mixture of gastric juice
and partly digested food.
The stomach walls contract and move
chyme to the bottom of the stomach.
The pyloric sphincter allows small amounts
to enter the small intestine.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Ruminants have four-chambered
stomachs:
The rumen and reticulum contain
microorganisms that metabolize the
cellulose into nutrients for the host.
Food then travels to the omasum where it is
concentrated by water absorption.
The abomasum is the true stomach.
Microorganisms are also digested by the
host and provide protein.
Figure 39.9 A Ruminant’s Stomach
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Most chemical digestion occurs in the small
intestine.
The small intestine has three sections:
• Duodenum—the initial section and site of
most digestion
• Jejunum and ileum—carry out most
absorption
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The duodenum contains epithelial cells that
produce enzymes:
• Peptidases cleave peptides into
absorbable amino acids
• Other enzymes cleave disaccharides into
monosaccharides
• Lipases that digest fats
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The liver produces and secretes bile that is
stored in the gallbladder.
Bile flows to the duodenum via the common
bile duct.
Bile’s function is to emulsify fat—bile
contains bile salts that prevent fat
molecules from sticking together.
Greater surface area exposes more of the
fat molecule to the lipases for digestion.
Figure 39.10 The Liver, Gallbladder, and Pancreas
Figure 39.11 Digestion and Absorption of Fats
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
The pancreas is an endocrine (hormonereleasing) and an exocrine (digestive juicesecreting) gland.
It produces insulin, glucagon, and digestive
enzymes.
The pancreas secretes enzymes as
zymogens.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
One zymogen, trypsinogen, is activated by
enterokinase in the duodenum—produces
active trypsin that can activate other
zymogens.
The pancreas also secretes HCO3– to
neutralize chyme in intestine.
Table 39.3 Major Digestive Enzymes of Humans
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Breakdown of complex molecules continues
in the jejunum.
Absorption of the breakdown products takes
place in the jejunum and ileum.
The microvilli absorb nutrients and inorganic
ions through several methods.
• Na+ and other ions are actively transported
—sometimes with “hitchhikers.”
• Water moving through spaces between
cells can carry nutrients.
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
• Lipid-soluble fats pass through the villi
membrane.
Fats are then re-formed into chylomicrons
with a protein coat that makes them
soluble in water.
Chylomicrons flow through the lymphatic
system and then enter the blood stream
through the thoracic ducts.
Figure 39.11 Digestion and Absorption of Fats
Concept 39.3 The Vertebrate Digestive System Is a Tubular Gut
with Accessory Glands
Blood leaving the digestive tract goes to the
liver via the hepatic portal vein.
Liver cells absorb nutrients and store them
or convert them for use.
Contents of the small intestine pass into the
large intestine, or colon, that absorbs
water and ions, and produces feces, which
are stored in the rectum.
Too much water absorption in the colon
leads to constipation; too little leads to
diarrhea.
Concept 39.4 Food Intake and Metabolism Are Regulated
Animals do not eat continuously and are in
one of two states:
Absorptive state—the period after a meal
when food is in the gut and nutrients are
absorbed
Postabsorptive state—stomach and small
intestine are empty and metabolism runs
on stored nutrients
Concept 39.4 Food Intake and Metabolism Are Regulated
Digestion is governed by neuronal and
hormonal controls.
The enteric nervous system is able to
coordinate digestion.
Many hormones regulate the digestive tract
and its accessory organs.
Concept 39.4 Food Intake and Metabolism Are Regulated
Digestive hormones and sites of production:
• Gastrin—released by the stomach into the
blood when food is present
• Secretin—produced when chyme arrives
in the duodenum, causes pancreas to
secrete bicarbonate ions
• Cholecystokinin (CCK)—released by
duodenal cells, causes gallbladder to
release bile, stimulates pancreas, slows
stomach
Figure 39.12 Hormones Control Digestion
Concept 39.4 Food Intake and Metabolism Are Regulated
Insulin is the major hormone in blood
glucose regulation.
Insulin is released by β cells of the islets of
Langerhans in the pancreas when blood
glucose rises.
Glucose transporters are inserted into cell
membranes in response to insulin—more
glucose can move in to cells.
Concept 39.4 Food Intake and Metabolism Are Regulated
Blood glucose falls in the postabsorptive
period.
Insulin release is lower and glucose uptake
is slowed.
If glucose level is very low, glucagon is
released and causes the liver to break
down glycogen and begin
gluconeogenesis.
Figure 39.13 Regulating Glucose Levels in the Blood
Concept 39.4 Food Intake and Metabolism Are Regulated
Diabetes mellitus is a lack of insulin or the
inability to respond to insulin.
Type I is the lack of cells that produce
insulin, also called juvenile diabetes—
treated by supplemental insulin.
Type II, adult diabetes, is a poor response
to insulin and cells are unable to absorb
enough to meet metabolic needs—treated
by changes in diet and lifestyle.
Concept 39.4 Food Intake and Metabolism Are Regulated
The liver coordinates changes between
states.
During absorptive state the liver stores fuel
as glycogen and fats, and synthesizes
blood plasma proteins.
During postabsorptive state the liver can
deliver nutrients into the blood from its
reserves.
Concept 39.4 Food Intake and Metabolism Are Regulated
Lipoproteins aid in fat transport and are
produced in the liver:
• High-density lipoproteins (HDLs)
remove cholesterol from tissue and carry it
to liver
• Low-density lipoproteins (LDLs)
transport cholesterol in body
• Very-low-density lipoproteins (VLDLs)
transport triglycerides to fat cells
Concept 39.4 Food Intake and Metabolism Are Regulated
The hypothalamus provides signals of
hunger or satiety and governs how much
food is eaten.
Leptin is a hormone produced by adipose
cells with receptors in the hypothalamus.
Leptin provides feedback about body fat to
the brain—the more fat a cell contains, the
more leptin it produces.
Concept 39.4 Food Intake and Metabolism Are Regulated
Leptin was discovered in a strain of mice
that overeat and become obese.
These mice have a mutation in the gene for
leptin—injections of leptin caused them to
lose weight and eat less.
This effect was not the same in humans—
most obese people have higher levels of
leptin, but do not respond to it normally.
Concept 39.4 Food Intake and Metabolism Are Regulated
Other feedback signals regulate food intake.
Ghrelin is a hormone secreted by the
stomach—its levels rise before a meal and
promote hunger.
The hormone peptide YY reduces appetite
in rodents and humans.
Figure 39.14 A Single-Gene Mutation Leads to Obesity in Mice (Part 1)
Figure 39.14 A Single-Gene Mutation Leads to Obesity in Mice (Part 2)
Answer to Opening Question
Genetics and lifestyle both contribute to the
Pimas’ diabetes and obesity.
The Arizona Pimas have a much greater
insulin response to glucose than people of
other heritage do—one effect of that is
increased fat synthesis.
Their lifestyle as compared to the Mexican
Pimas is much more sedentary, with a
poorer diet.