Digestion and
Human Nutrition
Digestive System Tasks
• Break up, mix, and move food material
• Secrete enzymes into tube where digestion
occurs
• Digest (break down) food particles into
smaller molecules
– Proteins – amino acids
– Carbs – monosaccharids (simple sugars)
– Lipids – fatty acids
– Nucleic acids - nucleotides
• Absorb nutrients and fluids
• Eliminate wastes and residues
Two Types of Systems
• Incomplete digestive
system
– One opening, saclike
digestive cavity
• Complete digestive
system
– A tube with an
opening at each end
– Digestive system is
often subdivided into
functional regions
Figure 41.3a,b
Page 726
Human Digestive System
• A complete system with many
specialized organs
• About 6.5 to 9 meters long if extended
• Lined with mucus-secreting epithelium
• Movement is one way, from mouth to
anus
Human Digestive System
Major Components
•
•
•
•
Mouth (oral cavity)
Pharynx (throat)
Esophagus
Gut
– Stomach
– Small intestine
– Large intestine
– Rectum
– Anus
Figure 41.6
Page 728
Accessory Organs
• Salivary glands
– Secrete saliva
• Liver
– Secretes bile
• Gallbladder
– Stores and concentrates bile
• Pancreas
– Secretes digestive enzymes
Digestion Starts in the Mouth
Teeth
enamel
Lower jaw
molars
dentin
premolars
canines
incisors
• Normal adult
number is 32
Figure 41.7
Page 729
Saliva
• Produced by salivary glands at back of
mouth and under tongue
• Saliva includes
– Salivary amylase (enzyme that breaks
down starch)
– Bicarbonate (buffer – maintains mouth
pH)
– Mucins (bind food into bolus - soft mass
of chewed food )
– Water
Swallowing
• Complex reflex
• Tongue forces food into pharynx
• Epiglottis and vocal cords close off
trachea; breathing temporarily ceases
• Bolus moves into esophagus, then through
esophageal (cardiac) sphincter (circular
band of muscles) into stomach
• peristalsis – waves of smooth muscle
contraction: happens throughout digestive
system
Structure of the Stomach
• J-shaped organ lies
below the diaphragm
• Sphincters at both ends
– ring of smooth
muscles that close off a
passageway
• Outer serosa covers
smooth muscle layers
• Inner layer of glandular
epithelium faces lumen
sphincters
serosa
muscle
mucosa
Figure 41.9a
Page 731
Stomach Secretions
• Secreted into lumen (gastric fluid)
– Hydrochloric acid (HCl)
– Mucus (protective)
– Pepsinogen (inactive form of a proteindigesting enzyme)
• Stomach cells also secrete the hormone
gastrin into the bloodstream which stimulates
secretion of gastric acid
Mixing Chyme
• Chyme = A thick mixture
of food and gastric fluid
• High acidity kills many
pathogens
• Mixed and moved by
waves of stomach
contractions
Figure 41.9b
Page 731
Protein Digestion in Stomach
• High acidity of gastric fluid denatures
proteins and exposes peptide bonds
• Pepsinogen secreted by stomach lining
is activated to pepsin by HCl
• Pepsin breaks proteins into fragments
Ulcer
• An erosion of the wall of the stomach or
small intestine
• Can result from undersecretion of
mucus and buffers, or oversecretion of
pepsin
• Most ulcers involve Helicobacter pylori
bacteria and can be treated with
antibiotics
Into the Small Intestine
• three regions
-duodenum - receives chyme from
stomach and fluid from 3 organs
(pancreas, liver and gallbladder)
-jejunum – most nutrients are digested and
absorbed
-ileum – absorbs some nutrients, delivers
unabsorbed material to large intestine
Intestinal Secretions
• Wall of the duodenum secretes
– Disaccharidases - digest
disaccharides to monosaccharides
– Peptidases - break protein fragments
down to amino acids
– Nucleases - digest nucleotides down
to nucleic acids and monosaccharides
Pancreatic Enzymes
• Secreted into duodenum
• Pancreatic amylase –break down polysaccharaides
• Trypsin and chymotrypsin – break down proteins
into fragments
• Carboxypeptidase - break protein fragments into
amino acids
• Lipase - break down triglycerides
• Pancreatic nucleases – break down nucleic acids
• also secretes bicarbonate to help neutralize pH from
stomach
Fat Digestion
• Liver produces bile
• Bile is stored in gallbladder, then
secreted into duodenum
• Bile emulsifies fats; breaks them into
small droplets
• This gives enzymes a greater surface
area to work on
Hormones and Digestion
• Gastrin - a hormone that stimulates secretion of
gastric acid
• Secretin - Causes the stomach to make pepsin, the
liver to make bile, and the pancreas to make a
digestive juice.
• Cholecystokinin (CCK) - stimulates contraction of
the gallbladder and increases secretion of
pancreatic juice
• GIP (glucose insulinotropic peptide) - stimulates
insulin secretion from pancreas
Walls of Small Intestine
• Projections (microvilli) into
the intestinal lumen
increase the surface area
available for absorption
One villus
Figure 41.10
Page 733
submucosa
gut lumen
circular
muscle
serosa
longitudinal
muscle
blood vessels
mesh of nerves
(plexus)
Fig. 41-8a2, p.725
Small
Intestine
highly folded mucosa
Fig. 41-8a1, p.725
Absorption of Nutrients
-the passage of nutrients, water, salts, and vitamins
into the internal environment
- most nutrient absorption occurs in the small
intestine (jejunum & ileum)
-water – by osmosis
-mineral ions – selectively absorbed
-monosacc’s and amino acids – transport
proteins, then enter blood
-fatty acids and monoglycerides – diffuse (lipid
soluble), enter lymph
INTESTINAL LUMEN
Absorption
Mechanisms
carbohydrates
monosaccharides
Monosaccharides &
amino acids are
proteins
actively transported
across plasma
EPITHELIAL
CELL
membrane of
epithelial cells, then
from cell into internal
environment
Figure 41.11
Page 734
INTERNAL
ENVIRONMENT
amino acids
Fat Absorption
bile
salts
bile salts
+
micelles
fat globules
(triglycerides)
emulsification
droplets
fatty acids,
monoglycerides
triglycerides + proteins
EPITHELIAL
CELL
chylomicrons
Figure 41.11
Page 734
INTERNAL ENVIRONMENT
Into the Blood
• Glucose and amino acids enter blood
vessels directly
• Triglycerides enter lymph vessels,
which eventually drain into blood
vessels
Large Intestine (Colon)
• Concentrates and stores
feces
ascending
portion of
large intestine
• Sodium ions are actively
transported out of lumen
and water follows
• Lining secretes mucus
and bicarbonate
cecum
appendix
•starts at the cecum (a cup shaped pouch - the
appendix projects here), ascending on right
side, transverse, then descending down left side
Bacteria in Colon
• Slow movement of material through
colon allows growth of bacteria
• Harmless--unless they escape into
abdominal cavity
• Some produce vitamin K, which is
absorbed through intestinal wall
Movement through Colon
• During a meal, gastrin and autonomic
signals trigger contraction of ascending
and transverse colon
• Material moves along to make room for
incoming food
• Feces is stored in last part of colon
“Enzyme”
Produced in
Breaks Down
Used in
Pancreatic Amylase
Pancreas
Starch
Small intestine
Maltase
Small intestine
maltose
Small intestine
Sucrase
Small intestine
Sucrose
Small intestine
Lactase
Small intestine
Lactose
Small intestine
Salivary Amylase
Salivary glands
Starch
Mouth
Pepsin
Stomach
Proteins
Stomach
Trypsin/chymotrypsin Pancreas
Proteins
Small intestine
Peptidase
(carboxypeptidase)
Nuclease
Small intestine or
pancreas
Pancreas
Peptides
Small intestine
Nucleic acids
Small intestine
Nucleosidases
Small intestine
Nucleotides
Small intestine
Lipase
Pancreas
Fat droplets
Small intestine
**Bile salts
Liver
Emulsify fats
Small intestines
**Bicarbonate
Pancreas
Neutralize pH
Small intestine
nutrients
Carbohydrates
• Body’s main energy source
• Foods high in complex carbohydrates
are usually high in fiber; promote colon
health
• Simple sugars lack fiber as well as
minerals and vitamins of whole foods;
intake should be minimized
Lipids
• Most fats can be synthesized
• Essential fatty acids must be obtained
from food
• Fats should be about 30 percent of diet
• Excess saturated fats can raise
cholesterol level and contribute to heart
disease
Proteins
• Body cannot build eight of the twenty
amino acids
• These essential amino acids must be
obtained from diet
• Animal proteins are complete; supply all
essential amino acids
• Plant proteins are incomplete; must be
combined
Pathways of Organic Metabolism
Food intake
dietary carbohydrates, lipids
POOL OF CARBOHYDRATES AND FATS
structural
components
of cells
some surface
secretion, cell
sloughing
storage
forms
specialized
derivatives
(e.g., steroids,
acetylcholine)
used as
cellular
energy
source
cell
activities
excreted as
CO2 via lungs
dietary proteins, amino acids
NH3
POOL OF AMINO ACIDS
urea
nitrogencontaining
derivatives
(e.g., hormones,
nucleotides)
components
of structural
proteins,
enzymes
excreted
in urine
cell
activities
some cell
sloughing
Dietary Essentials
• Vitamins
– Essential organic substances
• Minerals
– Essential inorganic substances
Vitamins
Fat soluble
• Excess
accumulates in
tissue
• Vitamins A, D,
E, K
•
•
•
•
•
Fat insoluble (Water soluble)
B vitamins
Pantothenic acid
Folate
Biotin
Vitamin C
Major Minerals
Calcium
Chloride
Copper
Fluorine
Iodine
Iron
Magnesium
Phosphorus
Potassium
Sodium
Sulfur
Zinc