Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 36
Digestion and Human Nutrition
(Sections 36.7 - 36.12)
Albia Dugger • Miami Dade College
36.7 Digestion and Absorption
in the Small Intestine
• Chemical and mechanical digestion are completed in the
small intestine, and most nutrients are absorbed
• The small intestine receives chyme from the stomach,
enzymes and bicarbonate from the pancreas, and bile from
the gallbladder
• Bicarbonate raises the pH of chyme enough for digestive
enzymes to function
8 Steps in Digestion and Absorption
• Carbohydrate digestion:
1. Pancreatic amylase breaks polysaccharides into
disaccharides; specific enzymes of the brush border cells
(e.g. sucrase) split disaccharides into simple sugars
(monosaccharides)
2. Monosaccharides are actively transported into brush border
cells, then out into interstitial fluid in a villus – where they
enter the blood
8 Steps in Digestion and Absorption
• Protein digestion:
3. Pancreatic proteases (trypsin, chymotrypsin) break proteins
into polypeptides; enzymes at the surface of brush border
cells break polypeptides into amino acids
4. Amino acids are actively transported into brush border cells,
then out into interstitial fluid – where they enter the blood
8 Steps in Digestion and Absorption
• Fat digestion:
5. Movements of the intestinal wall break water-insoluble fat
(triglyceride) globules into small droplets
• Bile salts made in the liver and concenrated in the
gallbladder coat (emulsify) the droplets, so that globules
can’t reform
Key Terms
• bile
• Mix of salts, pigments, and cholesterol produced in the
liver, then stored and concentrated in the gallbladder;
emulsifies fats when secreted into the small intestine
• gallbladder
• Organ that stores and concentrates bile
• emulsification
• Suspension of fat droplets in a fluid
8 Steps in Digestion and Absorption
• Fat digestion (cont.):
6. Pancreatic lipases break triglycerides droplets into lipidsoluble fatty acids and monoglycerides
7. Monoglycerides and fatty acids enter a villus by diffusing
across the membrane’s lipid bilayer, into brush border cells
8 Steps in Digestion and Absorption
• Fat digestion (cont.):
8. In brush border cells, triglycerides form and become coated
with proteins
• The resulting lipoproteins move by exocytosis into the
interstitial fluid inside the villus
• From interstitial fluid, triglycerides enter lymph vessels that
drain into the general blood circulation
Summary of Digestion
Summary of Digestion
1
carbohydrates
3 proteins
monosaccharides
fat globules
(triglycerides)
+
amino acids
bile salts
5 emulsification
droplets
6
free fatty acids,
monoglycerides + bile salts
7
2
Lumen of Small Intestine4
triglycerides + proteins
Brush
Border
Cell
lipoproteins
8
Internal Environment
(interstitial fluid inside a villus)
1 Enzymes break
polysaccharides
down to simple
sugars, or
monosaccharides.
3 Proteins are
broken into
polypeptides,
then amino
acids.
5 Movements of
theintestinal wall
break up fat
globules into small
droplets. Bile salts
coat the droplets,
2 Monosaccharides 4 Amino acids are so that globules
are actively
actively transported cannot form again.
transported into
into brush border
brush border cells, cells, then out into
then out into
interstitial fluid.
interstitial fluid.
6 Pancreatic
8 In a brush border
enzymes digest
the droplets to
fatty acids and
monoglycerides.
cell, the products of
fat digestion form
triglycerides, which
associate with
proteins. The
7 Monoglycerides resulting
and fatty acids
lipoproteins are then
diffuse across the expelled by
plasma
exocytosis into the
membrane’s lipid interstitial
bilayer, into brush fluid inside the
border cells.
villus.
Fig. 36.9, p. 602
Summary of Digestion
1
carbohydrates
3 proteins
monosaccharides
fat globules
(triglycerides)
+
bile salts
amino acids
2
5 emulsification
droplets
6
free fatty acids,
monoglycerides
+ bile salts
7
4
Lumen of Small Intestine
Brush
Border
Cell
triglycerides + proteins
lipoproteins
8
Internal Environment
(interstitial fluid inside a villus)
Stepped Art
Fig. 36.9, p. 602
ANIMATION: Digestion and absorption in
the small intestine
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Fluid Absorption
• Each day, 7 to 9 liters of fluid from food, drink, and secretions
from your stomach, accessory glands, and intestinal lining
enter the small intestine
• About 80 percent of the water moves across the intestinal
lining and into the internal environment, following an osmotic
gradient created by transport of salts, sugars, and amino
acids across brush border cells
Disorders: Lactose Intolerance
• People who do not produce sufficient lactase to digest lactose
at the surface of brush border cells are lactose intolerant
• Lactose enters the large intestine, where resident bacteria
break it down in reactions that produce hydrogen gas as a byproduct, causing flatulence, bloating, and cramps
Disorders: Gallstones
• Sometimes cholesterol
or bilirubin in bile
accumulates as hard
pellets (gallstones)
• The gallbladder or
gallstones may have to
be removed surgically if
they block or become
lodged in a duct
Disorders: Pancreatitis
• Gallstones can irritate the pancreas, leading to inflammation
(pancreatitis) and upper abdominal pain
• As a result of inflammation, pancreatic enzymes begin to
digest the pancreas itself, causing further inflammation and
narrowing the duct leading to the small intestine
• Chronic pancreatitis may require pills that provide enzymes to
replace those normally provided by a healthy pancreas
Key Concepts
• A Closer Look at Digestive Organs
• The stomach is an expandable sac that secretes acidic
gastric fluid
• The small intestine has a highly folded interior and a huge
surface area
• It receives enzymes from the pancreas and bile from the
gallbladder
• The bile is made in the liver
Animation: Absorption
36.8 The Large Intestine
• Indigestible material, dead bacteria and mucosal cells,
inorganic substances, and some water from the small
intestine enters the colon (large intestine)
• The large intestine completes the process of absorption, then
concentrates, stores, and eliminates wastes as feces
• feces
• Unabsorbed food material and cellular waste that is
expelled from the digestive tract
Structure of the Colon
• The first part of the colon is a pouch called the cecum, from
which the appendix projects
•
Beyond the cecum, the colon ascends the wall of the
abdominal cavity, extends across the cavity, descends, and
connects to the rectum
• appendix
• Wormlike projection from the first part of the large intestine
Cecum and Appendix
Cecum and Appendix
ascending
portion of
large intestine
cecum
appendix
last
portion
of small
intestine
Fig. 36.11a, p. 604
Colon Function
• Contraction of smooth muscle in the colon wall mixes the
colon’s contents and slowly propels material along
• Conditions favor growth of normal flora such as E. coli, which
makes vitamin B12 that we absorb across the colon lining
• Signals from autonomic nerves cause the colon to contract
forcefully and propel feces to the rectum, which activates a
defecation reflex to expel feces
Health and the Colon
• Emotional stress, a diet low in fiber, minimal exercise,
dehydration, and some medications can lead to constipation
• Infections can cause an episode of diarrhea; autoimmune
disorders (Crohn’s disease) can cause chronic diarrhea
• Appendicitis often occurs after a bit of feces lodges in the
appendix and infection sets in
• Some people are genetically predisposed to develop colon
polyps, which can become cancerous
Polyps in the Transverse Colon
Polyps in the Transverse Colon
transverse colon
colon polyp
descending colon
Fig. 36.11b, p. 604
36.9 Fate of Absorbed Compounds
• Cells continually synthesize and break down carbohydrates,
fats, and proteins into small organic compounds (sugars,
amino acids, and triglycerides)
• These compounds are burned as fuel in aerobic respiration,
stored, or used in synthesis of larger organic compounds
• The nervous and endocrine systems regulate this molecular
turnover
Major Pathways of Organic Metabolism
Major Pathways of Organic Metabolism
Food Intake
dietary carbohydrates, lipids
dietary proteins, amino acids
Cytoplasmic Pool
of Carbohydrates, Fats
(interconvertible forms)
Cytoplasmic Pool
of Amino Acids
ammonia
storage
forms
(e.g.,
glycogen)
building
blocks for
cell
structures
specialized
derivatives
(e.g., steroids,
acetylcholine)
instant
energy
sources
for cells
urea
excreted
in urine
nitrogencontaining
derivatives
(e.g., hormones,
nucleotides)
building
blocks for
structural
proteins,
enzymes
Stepped Art
Fig. 36.12a, p. 605
Liver Functions
• The liver has a central role in metabolism and homeostasis:
• Forms bile (assists fat digestion), rids body of excess
cholesterol and blood’s respiratory pigments
• Controls amino acid levels in the blood; converts
potentially toxic ammonia to urea
• Controls glucose level in blood; stores glycogen
• Removes hormones that served their functions from blood
• Removes ingested toxins, such as alcohol, from blood
• Breaks down worn-out red blood cells; stores iron
• Stores vitamins A and B12
36.10 Human Nutritional Requirements
• Diet profoundly affects your body’s structure and function
• The US Department of Agriculture (USDA) and other
government agencies research diets that may help prevent
diabetes, cancers, and other health problems
• You can generate your own healthy eating plan by visiting the
USDA web site: www.mypyramid.gov
USDA Dietary Recommendations
• USDA nutritional guidelines:
• Lower intake of refined grains, saturated fats, trans fatty
acids, added sugar or caloric sweeteners, and salt
• Eat more vegetables and fruits with a high potassium and
fiber content, and whole grains
• Eat fat-free or low-fat milk products
• About 55% of daily calories should come from
carbohydrates (preferably complex carbohydrates)
USDA Nutritional Guidelines
Energy-Rich Carbohydrates
• Fresh fruits, whole grains, and vegetables, especially
legumes (peas and beans), provide complex carbohydrates
• Provide energy (glucose), essential vitamins, and fiber
• Soluble fiber helps lower cholesterol levels
• Insoluble fiber helps prevent constipation
• Processed carbohydrates (white flour, refined sugar, corn
syrup) provide “empty calories”
Essential Fatty Acids
• Lipids are used to build cell membranes, as energy stores,
and as a reservoir for fat-soluble vitamins
• Linoleic acid and alpha-linolenic acid are essential fatty
acids, required in the diet
• essential fatty acid
• Fatty acid that the body cannot make and must obtain
from the diet
Good Fat
• Monounsaturated oleic acid (in olive oil) may help prevent
heart disease when substituted for saturated fats
• Polyunsaturated omega-3 fatty acids (in oily fish such as
sardines) have special health benefits:
• Reduce risk of cardiovascular disease
• Lessen inflammation associated with rheumatoid arthritis,
• Help diabetics control their blood glucose
Bad Fat
• Diets high in saturated fats and cholesterol (in dairy products
and meats) increase risk for heart disease, stroke, and some
cancers
• Artificial trans fats have a molecular structure that is even
worse for the heart than saturated fats
• All food labels are now required to show amounts of trans
fats, saturated fats, and cholesterol per serving
Types of Dietary Lipids
Reading a Food Label
Essential Amino Acids
• Cells can make some amino acids, but eight essential amino
acids must come from the diet
• essential amino acids
• Amino acids the body can’t make and must obtain from
food
• Methionine (or cysteine), isoleucine, leucine, lysine,
phenylalanine, threonine, tryptophan, and valine
Body-Building Proteins
• Most proteins in meat are “complete” – their amino acid ratios
match a human’s nutritional needs
• Most plant proteins are “incomplete” – they lack one or more
amino acids essential for the human diet
• Vegetarians can obtain all required amino acids by combining
complementary foods such as beans and rice
Animation: Nutrition Calculator
36.11 Vitamins and Minerals
• To function properly, the body requires small amounts of
vitamins and minerals in addition to major nutrients
• vitamin
• Organic substance required in small amounts for normal
metabolism
• mineral
• Inorganic substance that is required in small amounts for
normal metabolism
Major Vitamins
Major Vitamins
Major Minerals
Major Minerals
Major Minerals
A Healthy Diet
• Most people can get all the vitamins and minerals they need
from a well-balanced diet
• In addition to vitamins and minerals, a healthy diet should
include a variety of phytochemicals (phytonutrients), which
may reduce the risk of certain disorders
• Example: Lutein and zeaxanthin in leafy green vegetables
reduce the risk of macular degeneration-related blindness
Key Concepts
• Human Nutrition
• Nutrients absorbed from the gut are raw materials for
synthesis of complex carbohydrates, lipids, proteins, and
nucleic acids
• A healthy diet is low in salt, simple sugars, and saturated
fats; and provides all nutrients, vitamins, and minerals
necessary to support metabolism
36.12 Maintaining a Healthy Weight
• Maintaining a healthy weight requires balancing energy inputs
with energy expenditures
• For most people, this means eating recommended portions of
low-calorie, nutritious foods and exercising regularly
What Is a Healthy Weight?
• Calculation of body mass index (BMI) help assess health risks
associated with weight gains:
• BMI = [weight (pounds) X 703] ÷ [height (inches)2]
• Overweight = BMI 25 to 25.9
• Obese = BMI > 30
What Is a Healthy Weight? (cont.)
• Body weight distribution also predicts risks:
• Fat deposits above the waist (“beer belly”) are associated
with heart problems
• A Harvard study linking weight and risk of cardiovascular
disorders gives estimates of “ideal” weights for men and
women
“Ideal” Weights for Men and Women
Calculating Calorie Requirements
• To estimate your kilocalorie (C) requirement, multiply your
weight (in pounds) by a physical activity factor (10 for not
active, 15 for moderately active, 20 for highly active)
• Subtract one of the following amounts from the result:
Age: 25–34
Subtract: 0
35–44
100
45–54
200
55–64
300
Over 65
400
Why Is Obesity Unhealthy?
• Obesity increases the risk of type 2 diabetes, high blood
pressure, heart disease, breast and colon cancer, arthritis,
and gallstones
• Excess triglycerides distends fat cells and impairs function:
• Triggers inflammatory response (chronic inflammation)
• Interferes with action of insulin
Eating Disorders
• Anorexia nervosa
• A person who has access to food routinely eats too little to
maintain a healthy weight
• Damages organ systems throughout the body
• Bulimia nervosa (“binge and purge” eating)
• Induced vomiting erodes teeth, increases risk of cancer of
the esophagus, disrupts iron and pH balance
Key Concepts
• Maintaining a Healthy Body Weight
• Maintaining body weight requires balancing calories taken
in with calories burned in metabolism and physical activity
• A body weight far above or below normal increases the
risk of health problems
The Battle Against Bulge (revisited)
• Leptin decreases appetite; ghrelin increases it
• A low-fat, low-calorie diet increases the concentration of
ghrelin in the bloodstream – increasing hunger
• Gastric bypass surgery may be more effective than diets
because it lowers ghrelin levels
• Gastric bypass surgery also has risks of anesthesia, surgical
complications, and vitamin and mineral deficiencies
ANIMATION: Chronology of Leptin
Research
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Animation: Body Mass Index