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CHAPTER 21
DIGESTION &
NUTRITION
OBTAINING
AND PROCESSING
FOOD
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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
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Overview: Food processing occurs in four stages
• Food is processed in four stages
– Ingestion
– Digestion
– Absorption
– Elimination
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Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
(hydrolysis)
Nutrient
molecules
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
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Overview: Food processing occurs in four stages
– Chemical digestion (HYDROLYSIS) breaks down large
organic molecules into their components
– Proteins split into amino acids
– Carbohydrates into monosaccharides
(including GLUCOSE)
– Nucleic acids into nucleotides
– Lipids into glycerol & fatty acids
Why does your body need these
monomers???
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WHAT DOES YOUR BODY NEED FOR
DIGESTION???
• Lots of WATER
• And ???
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
What conditions are necessary for
Digestion???
HUMAN DIGESTIVE SYSTEM
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Oral
cavity
Tongue
Mouth
Salivary
glands
Pharynx
Salivary
glands
Mouth
Esophagus
Esophagus
Liver
Gallbladder
Liver
Pancreas
Esophagus
Stomach
Sphincter
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
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 (CARBOHYDRASES) begin the
hydrolysis of CARBOHYDRATES
– Buffers neutralize acids (Need pH???)
– Antibacterial agents kills some bacteria ingested
with food
• The tongue tastes, shapes the bolus of food, and
moves it toward the pharynx
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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
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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
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 1-2
– 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 begins the chemical DIGESTION of PROTEIN
What happens to the carbohydrate digestion
that started in the oral cavity???
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Esophagus
Sphincter
Lumen (cavity)
of stomach
Sphincter
Small
intestine
Interior surface
of stomach
Stomach
Gastrin
CONNECTION: Digestive ailments include acid reflux
and gastric ulcers
• Acid reflux into esophagus—heartburn
• Bacterial infections (Helicobacter pylori)
in the stomach and duodenum can
produce ulcers
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Bacteria
Mucous
layer of
stomach
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 the stomach
acid (pH back to 7)
• Bile, made in the liver and stored in the gall
bladder, emulsifies fat so it can be digested by
pancreatic enzymes
• Digestion of ALL molecules occurs
(carbohydrates, proteins, nucleic acids, fats)
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Liver
Bile
Gallbladder
Stomach
Acid chyme
Intestinal
enzymes
Pancreatic juice
Duodenum of
small intestine
Pancreas
The small intestine is the major organ of chemical
digestion and nutrient absorption
– Enzymes from cells of the intestine continue digestion
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The small intestine is the major organ of chemical
digestion and nutrient absorption
– Surface area forABSORPTION is
increased by
– Folds of the intestinal lining
– Fingerlike villi
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Vein
with blood
en route to
the liver
Lumen of intestine
Nutrient
absorption
Epithelial
cells
Muscle
layers
Amino
Fatty
acids
acids
and
and
sugars glycerol
Lumen
Fats
Blood
capillaries
Large
circular folds
Lymph
vessel
Villi
Blood
Lymph
Nutrient
absorption
Villi
Intestinal wall
Lumen of intestine
Nutrient absorption
into epithelial cells
Microvilli
Epithelial cells
lining villus
The LARGE INTESTINE (COLON) 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
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Large
intestine
(colon)
Small
intestine
Sphincter
End
of small
intestine
Appendix
Cecum
Rectum
Anus
Unabsorbed
food material
NUTRITION
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Overview: A healthy diet satisfies THREE NEEDS
• FUEL (ENERGY) to power the body
• Organic Molecules to build molecules
• Essential Nutrients—raw materials that
animals cannot make for themselves
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Chemical energy (ATP) powers the body
• Nutrients are oxidized inside cells to make ATP
• ATP is the main energy “currency” in a cell
• Proteins, carbohydrates, and fats are the main
sources of dietary calories
WHAT CHEMICAL REACTION USES GLUCOSE FROM
CARBOHYDRATES TO PRODUCE ATP
MOLECULES?
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Chemical energy powers the body
– Basal metabolic rate (BMR) is the energy a resting
animal requires each day
– Metabolic rate is the BMR plus the energy needed for
physical activity
– Excess energy is stored as GLYCOGEN or FAT
– Metabolic rates usually decrease through adulthood
• So what should you do to maintain a
healthy weight???
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An animal’s diet must supply Essential Nutrients
– Animals cannot produce eight of the 20 amino
acids
– These eight amino acids are essential amino acids
– These eight amino acids must come from the diet
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CONNECTION: Vegetarians must be sure to obtain
all eight essential amino acids
• The eight essential amino acids can be obtained
from
– Animal protein
– The proper combination of plant foods
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Essential
amino acids
Methionine
Valine
(Histidine)
Threonine
Phenylalanine
Leucine
Corn
Isoleucine
Tryptophan
Lysine
Beans and
other
legumes
A healthy diet includes 13 vitamins and many
essential minerals
• Essential vitamins and minerals
– Required in very small amounts
– Extreme excesses can be dangerous
– Excess water-soluble vitamins can be eliminated in
urine
– Excess fat-soluble vitamins can accumulate to
dangerous levels in body fat
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A healthy diet includes 13 vitamins and many
essential minerals
• Minerals are simple inorganic nutrients usually
required in small amounts
– Calcium and phosphorus are required in larger
amounts
– Most people ingest more salt than they need, but
some needed
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CONNECTION: Do you need to take vitamin and
mineral supplements?
• Recommended Dietary Allowances (RDAs) are
debated
• Excessive doses of vitamin A and iron can be
dangerous
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CONNECTION: Do you need to take vitamin and
mineral supplements?
• Extra doses of some vitamins are recommended
by the USDA
– Extra B12 for people over 50
– Extra vitamin D for people with dark skin or who
get little sun
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CONNECTION: What do food labels tell us?
• Food labels indicate
– Serving size
– Calories per serving
– Amounts of selected nutrients per serving and as a
percentage of daily value
– Recommendations for daily limits of selected
nutrients
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Ingredients: whole
wheat flour, water,
high fructose corn
syrup, wheat gluten,
soybean or canola
oil, molasses, yeast,
salt, cultured whey,
vinegar, soy flour,
calcium sulfate
(source of calcium).
EVOLUTION CONNECTION: The human health
problem of obesity may reflect our
evolutionary past
• Overnourishment: consuming more food energy
than is needed
• World Health Organization recognizes obesity as
a major global health problem
• Of people in the United States
– 30% are obese
– 35% are overweight
– 15% of children and adolescents are overweight
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EVOLUTION CONNECTION: The human health
problem of obesity may reflect our
evolutionary past
– Obesity leads to
–
–
–
–
Diabetes
Cancer
Cardiovascular disease
300,000 deaths per year in the United States
How might this affect the debate on
HEALTH CARE REFORM in the U.S.???
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CONNECTION: Diet can influence cardiovascular
disease and cancer
• A healthy diet may reduce the risk of
– Cardiovascular disease
– Cancer
•
Two main types of cholesterol
– LDL—contributes to blocked blood vessels and
higher blood pressure
– HDL—tends to reduce blocked blood vessels
– Exercise increases HDL levels
– Smoking decreases HDL levels
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CONNECTION: Diet can influence cardiovascular
disease and cancer
• The relationship between food and health is
complex
• The American Cancer Society recommends
– Regular exercise
– A diverse diet of healthy foods with an emphasis on
plant sources
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