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The Digestive System
Accelerated Biology
Functions of
the Digestive System
• Take in food
• Break it down into
molecules small enough
for the body to absorb
• Get rid of undigested
molecules and waste
• Brainpop animation
Stages of Digestion
• Ingestion – act of eating or drinking
• Digestion – breaking down food into molecules the body
can use
• Absorption – uptake of small nutrient molecules
• Elimination – passage of undigested material from the
digestive tract
Types of Digestion
• Mechanical – chewing in mouth and churning in stomach
break food into smaller pieces
• Chemical – enzymes break the bonds between molecules
– Macromolecules  monomer units
Pathway through
the Digestive System
• One long tube, 26 feet!
• Mouth  pharynx  esophagus  stomach  small
intestine  large intestine  rectum  anus
• The liver and pancreas are accessory organs that deliver
secretions into the digestive tract through ducts.
• Passage regulated at each step by sphincters
Mouth
• Mechanical Digestion
– Teeth – rip and chew
food into shreds
– Tongue – mixes food
with saliva
Mouth
• Chemical Digestion
– Salivary glands –
secrete saliva which
moistens and lubricates
food so it can be
swallowed more easily
– Saliva – contains the
enzyme amylase that
begins the break down
of carbohydrates into
monosaccharides
Esophagus
• A long tube that connects
the mouth to the stomach
• No digestion takes place
• Peristalsis – successive
rhythmic waves of smooth
muscle contractions move
the food to the stomach
• Takes 5–10 seconds for
food to reach the stomach
Peristalsis
Stomach
• Mechanically breaks down
food by using peristaltic
waves
• Produces gastric juices that
is a mixture of HCl acid
and the enzyme pepsin
• Pepsin – breaks down
proteins into amino acids.
Only effective in acidic
environments
Stomach
• Food spends 2–6 hours in
the stomach
• The stomach produces 2 L
of HCl a day!
Ulcers
• A coating of mucous
protects the lining of the
stomach from the gastric
acids
• Ulcers result when this
lining is damaged
Heartburn
• Has nothing to do with the
heart!
• Overtime, the sphincter
that separates the
esophagus and stomach
can become relaxed
• Acids from the stomach
burn the lining of the
esophagus
Small Intestine
• 6 meters long
• Functions in the digestion
and absorption of nutrients
into the blood
• Covered with villi – tiny
fingerlike projections that
increase the surface area
for absorption
Villi
Small Intestine
• Three sections:
• Duodenum – first section
that receives secretions
from the liver, pancreas,
and gallbladder
• Jejunum – middle section
• Ileum – last section
connects the small intestine
to the large intestine
Small Intestine
Small Intestine
• Bile – produced by the
liver and enters duodenum.
Breaks up fat globules into
tiny droplets
• Pancreatic lipase – enzyme
secreted by the pancreas
and enters the duodenum.
Breaks down lipids into
fatty acids and glycerol in
the small intestine.
Large Intestine
•
•
•
•
Also called the colon
No digestion takes place
Absorbs minerals & water
Prepares wastes for
elimination from the body.
All food components that
are not for energy
production are considered
waste
Large Intestine
• Water balance is important!
• If food is rushed through
you end up with diarrhea
(watery feces)
• If food remains in the colon
too long you become
constipated (hard feces)
Large Intestine
• Most of the contents are
dead cells, mucus,
secretions.
• A colony of bacteria live in
the colon and help the body
synthesize many
compounds you cant get
from food
– Vitamin K
– Several B vitamins
Rectum
• Rectum – solid waste is
stored here until you find a
socially acceptable time
and place! 
• Anus – solid waste
eliminated through here
Liver
• Accessory organ
• Secretes the bile that aids
in the emulsification of fat
• Converts extra sugar into
glycogen for storage
• The liver then breaks down
glycogen when it is needed
for energy.
Liver
• The liver also detoxifies
poisons
• Cirrhosis – scaring of the
liver caused by chronic
drug use, drinking, viral
infections.
Liver
• Jaundice – a condition in
which the eyes, skin, and
urine become abnormally
yellow as a result of
increased amounts of bile
pigments in the blood
Gallbladder
• Concentrated and stores
the bile produced by the
liver until it is needed in
the small intestine
Pancreas
• Accessory organ
• Secretes lipase and
protease that aid in the
chemical breakdown of
lipids and proteins in the
small intestine
Appendix
• A vestigial structure on the
the large intestine
• Appendicitis – when the
appendix becomes
inflamed. If immediate
medical attention is not
received, the appendix may
burst.
Digestive Enzymes
Substrate
Organ where
breakdown occurs
Enzyme
Product
Carbohydrates
(polysaccharides)
Mouth
Small Intestine
Salivary amylase
Pancreatic amylase
(made by pancreas)
Simple sugars
Carbohydrates
(disaccharides)
Small intestine
Sucrase, maltase,
lactase
Monosaccharides
Protein
Stomach
Small Intestine
Pepsin
Pancreatic protease
Amino acids
Lipids
Small intestine
Pancreatic lipase
Simple fats
Nucleic acids
Small intestine
Nuclease
Nucleotides