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The Digestive System
Guts, teeth and glands!
Images from: http://www.whfreeman.com/life/update/
Why Guts?
• Multicellular animals must have
specialized structures for obtaining and
breaking down their food.
• There are two processes: feeding and
digestion.
• Animals are heterotrophs, they must
absorb nutrients or ingest food sources.
How to dine...
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Ingestive eaters (us).
Absorptive feeders (tapeworm)
Filter feeders (clam)
Substrate feeders (earthworms)
Fluid feeders (mosquito)
Vertebrate Digestion
• The digestive system uses mechanical and
chemical digestion to breakdown food.
Tube in a tube
• Vertebrates have a tube-within-a-tube system.
• digestion occurs in the lumen with the nutrient
molecules being transferred to the blood.
Stages of digestion
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Movement of food
Secretion of digestive juices
Digestion of food into molecules
Absorption of molecules
Elimination of undigested food and
wastes
The Digestive System
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Mouth, pharynx,
esophagus, stomach
small intestine
large intestine
anus
salivary glands
pancreas
liver and gall bladder
The Start
• In the mouth, teeth, jaws and the tongue begin
the mechanical breakdown of food.
• Chemical breakdown of starch by amylase
• Mucus moistens food and lubricates the
esophagus.
• The chewed food and saliva is then pushed into
the pharynx and esophagus.
• The esophagus uses peristalsis to send the food
to the stomach.
Move the food
• Food is chewed
and passed to
the stomach
through the
esophagus. The
name of the
movement is
peristalsis. (see
video)
The Stomach
• Holds 1 to 2L (folds)
• The stomach secretes mucus, hydrochloric
acid and pepsin. HCl lowers pH of the
stomach to activate pepsin.
• Pepsin hydrolysis of proteins into peptides.
• The stomach also mechanically churns the
food. Chyme, leaves the stomach and enters
the small intestine.
• Ulcers occur when mucus lining is reduced.
The Small Intestine
• 3 m long tube with coils and folding plus
villi. Very large surface area!
• Final digestion of all food and absorbtion.
• Villi produce enzymes which complete the
digestion of peptides and sugars.
• The absorption process in the villi.
Villi
• Sugars and amino acids go into the
bloodstream via capillaries in each villus.
• Glycerol and fatty acids go into the
lymphatic system. Absorption is an active
transport, requiring cellular energy.
Duodenum - busy place
• Secretions from the liver and pancreas
are used for digestion in the duodenum.
• The pancreas secretes digestive enzymes
and stomach acid-neutralizing
bicarbonate.
• The liver produces bile, which is stored in
the gall bladder before entering the bile
duct into the duodenum.
Small intestine - more
• Digestion of carbohydrates, proteins, and
fats continues in the small intestine. See
table in text - page 166.
• Bile emulsifies fats so that lipases can
completely digested lipids.
• Most absorption occurs in the ileum and
jejeunum (second third of the small
intestine).
Liver and Gall Bladder
• The liver produces bile and helps to
detoxify of blood
• synthesis of blood proteins
• destruction of old erythrocytes
• storage of glucose as glycogen
• De-amination amino groups and
ammonia. (this produces urea, less toxic)
Glycogen-Glucose
• Low glucose levels in the blood cause
glucagon to stimulate breakdown of
glycogen into glucose.
• Insulin helps store glucose and glycogen
in the liver (see page 929)
• When no glucose or glycogen is available,
amino acids are converted into glucose in
the liver.
The Large Intestine what to do
with left overs!
• The large intestine produces an alkaline
mucus that neutralizes acids produced by
bacterial metabolism.
• Water, salts, and vitamins are absorbed, the
remaining contents in the lumen form feces
(mostly cellulose, bacteria, bilirubin).
• Bacteria in the large intestine, such as E.
coli, produce vitamins (including vitamin K)
that are absorbed.
Nutrition
• See the basics of nutrition in your text.
• Have a hand out!