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Unit H: Digestion
I. Digestion begins in the
mouth
• Two types of digestion occur here:
mechanical and chemical.
• Mechanical digestion of all nutrient
types occurs with chewing and mixing
with saliva.
• Chemical digestion of carbohydrates
(starch) occurs with the enzyme:
salivary amylase.
Inside of Mouth:
• Saliva is produced at three glands. It
dissolves materials, lubricates for
swallowing, and contains amylase.
• Teeth of three types: incisors for
biting,canines for tearing and
premolars/molars for grinding food.
(a) Omnivore
(human)
incisors
canine
premolars
molars
II. Swallowing:
• The food bolus moves to back of throat.
• Tongue pushes food into pharynx,
swallowing reflex occurs.
• Larynx is blocked by the epiglottis, food
moves into esophagus.
• Peristalsis moves food to stomach.
(a) Before swallowing
roof of mouth
food
pharynx
tongue
epiglottis
esophagus
larynx
(b) During swallowing
epiglottis
(folds over
larynx)
food
(enters
esophagus)
larynx
(moves up)
III. Stomach Digestion:
• Entrance to stomach is the cardiac sphincter. The
stomach contains gastric juice containing HCl at
a pH of 3 which dissolves most food substances.
• The gastric glands makes HCl, mucus protects
stomach from eating itself (ulcer).
• Pepsin is made in gastric glands, first as
pepsinogen which changes to pepsin when it
contacts HCl.
• Pepsin breaks down protein to polypeptides.
• Stomach churns, causing mechanical digestion
of all nutrients.
Duodenum and ducts:
please sketch…
V. Pancreas secretes 6
substances:
1) HCO3- : bicarbonate neutralizes stomach’s
acid and brings pH to aprox 8
2) H2O: acts as a solvent and lubricant
Digestive enzymes:
polypeptides
3) Trypsin: protein ->
4) Nuclease: Nucleic acids -> nucleotides
maltose
5) Pancreatic amylase: Carbohydrates6) Lipase: Triglycerides-> 3 fatty acids + glycerol
6 Functions of
the Liver:
Gall bladder
Bile Duct
• Converts glucose to
glycogen to regulate
blood sugar levels.
• Synthesizes blood
proteins like
prothrombin,
albumin and
fibrinogen.
• Makes bile from
cholesterol and
bilirubin.
• Recycles old blood
cells and their iron.
• Produces urea from
ammonia and cell
wastes.
• Detoxifies poisons
like alcohol from the
blood.
Digestion Animation
VI. Function of Bile
• Bile is produced in the liver, and stored in
the gall bladder.
• When fat is detected in the duodenum, a
hormone called CCK brings about the
release of bile from the gall bladder and it is
added to the duodenum.
• Bile emulsifies fat (physically breaks it down
into smaller droplets).
• Bile can be blocked by crystals in gall
bladder: gallstones.
VII. Intestine Digestion
• Nutrients broken down by pancreatic
enzymes finish their chemical digestion
with enzymes found in the small
intestine:
– 1) Maltase: converts maltose to 2 glucoses
– 2) Peptidase: polypeptides to Amino acids
VIII. Small Intestine
specialized for Absorption
•
•
•
•
Very long (aprox 25 feet in humans)
Inner folds increase surface area
Folds lined with bumps called villi
Cells on villi posses special cell
membranes called microvilli.
• Mitochondria produce ATP to power the
active transport of nutrients into cells.
(a) Small intestine
(c) Villus
(b) Fold of intestinal
lining
(d) Cells of villi
lacteal
microvilli
villi
capillaries
fold of
intestinal
lining
intestinal
gland
arteriole
lymph
vessel
venule
(a) Small intestine
fold of
intestinal
lining
(b) Fold of intestinal
lining
villi
(c) Villus
lacteal
capillaries
intestinal
gland
arteriole
lymph
vessel
venule
microvilli
Villus is main absorption
structure:
• Amino acids,
glucose, & nucleic
acids are
transferred to the
blood capillaries
here.
•Fats are absorbed
by the lacteal and
enter the lymphatic
system, later enter
the blood at the
subclavian vein.
IX. Large Intestine:
Food enters large
intestine through the
ileo-caecal valve.
The appendix no
longer functions
in humans, can
get infected with
bacteria.
Large Intestine Functions
There are 3 parts to
large intestine:
 Ascending colon
 Transverse colon
 Descending colon
Functions:
1)Reabsorb water.
2) Get more vitamins
out of your food.
3) Escherichia coli (E
Coli) bacteria in the
large intestine
release some
vitamins
Rectum
• Storage of feces and final water removal.
• Contains stretch receptors that tell the brain
when full. Brain interprets signal, but has
conscious control over the timing of
defecation.
• Rectum opens to anus, two sets of circular
muscles called sphincters act as valves.