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Key Concepts
• Why eat? Eat what?
• Design of digestive systems
• Processing steps and their hormonal
control
• Challenge of herbivory
Animal
Nutrition
Animals are heterotrophs,
obtain nutrition from other organisms
• herbivores
• carnivores
• omnivores
• detritivores
• frugivores
What do animals get from food?
1. Energy from chemical bonds
2. C skeletons (ex. acetyl, some amino acids)
3. Minerals - macro- and micronutrients
4. Vitamins
Extracting these substances is
an engineering ‘problem’
Different diets need different processing
machinery
• Carnivores large
canines, slashing
premolars
• Herbivores sharp
incisors and molar
grinding surfaces
• Omnivores relatively
unspecialized teeth, to
do a little of everything
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• vegetarian mosquito larva
has brushy mouthparts to
produce currents
• carnivorous mosquito
larva has sorry brushes
and large nasty jaws
• vegetarian mosquito larva
has brushy mouthparts to
produce currents
• carnivorous mosquito
larva has weak brushes
and large nasty jaws
gut=long tube
Specialized
sections
• continuous
space with
outside of
organism
• extracellular
digestion
• specialization
of different
regions
Tissue layers are
similar along its
length AND in
different animals
From the inside
out • mucosa
• submucosa
• muscle layers
•
•
•
•
esophagus
stomach
small intestine
large intestine
Esophagus
Storage?
Humans
In other animals, can be used for food storage.
Honeypot ants –
enormous storage
capacity
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Stomach
Gastric glands contain
3 types of secretory cells
• Extra muscle
• Preliminary
digestion of protein
• Some absorption of
small molecules
• mucus
• hydrochloric
acid
• pepsinogen
• Mucus protects cell
surfaces
• Hydrochloric acid
dissolves extracellular
matrix, kills most
bacteria, low pH (2)
• Pepsinogen?
Zymogen activation - they are
activated when their catalytic action is
appropriate
trypsinogen
zymogens
Pepsinogen is an inactive form of a
protein-digesting enzyme, pepsin
Pepsinogen is an inactive form of a
protein-digesting enzyme, pepsin
trypsin
How is pesinogen
activated?
• By low pH
• Pepsin works well
at low pH
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The sequence of digestion along the long
tube is coordinated by hormones
food
Gastric mucosal
cells secrete a
hormone, gastrin,
in response to
food in the
stomach
hormone secretion
initiates digestion
product inhibits
food
gastrin secretion
into bloodstream
stomach secretes gastric juices
including HCl
low pH (very acid)
Each hormone can stimulate more than
one response
food
gastrin secretion
stomach secretes gastric juices
increase stomach
movement
low pH (very acid)
moves mix into
small intestine
Most digestion and absorption
takes place small intestine
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The duodenum
is a receiving
center.
Structure of the
intestine
LIVER
It receives
material from
the stomach,
the liver, and
the pancreas.
• large circular
folds
• villi
• muscle layers
PANCREAS
Villi
Surface area of small intestine
•
•
•
•
Tube 3 cm x 6 meters, SA
Circular folds add
Villi add
Microvilli add
~0.6 m2
~ 3x
~ 10x
~ 20x
600 x
• microvilli
• capillaries
• lymph duct
(lacteal)
TOTAL SURFACE AREA = 360 m2
What is this S.A. for?
agents of digestion
Increased surface
area for absorption general feature of
animal digestive
systems
• Liver Æ bile which
emulsifies fats
• Pancreas Æ
enzymes to digest
carbos, proteins
and fats
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Protein
Amino
Acids
Carbohydrate
Monosaccharides
This means
The products of both protein and
carbohydrate digestion are water soluble
• they CANNOT passively cross cell
membranes (lipid bilayer),
• but they CAN dissolve in the blood
Actively transported
Fat and the products of fat digestion
are NOT water soluble
Once they reach the
blood they are
carried in solution.
This means
•they CANNOT dissolve in the intestine
or in blood, but
•they CAN passively cross cell
membranes
Transport of fat across wall
Digestion of fats
1. bile salts stabilize small fat droplets
2. lipase cuts fats into fatty acids and
monoglycerides
3. bile salts are recycled
1.
2.
3.
Fatty acids are lipid soluble. They can pass through
cell membrane
In the intestinal cell, fatty acids are repackaged into
chylomicrons
Chylomicrons are transported out of cell into lymph
system
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Control of digestion in the SMALL INTESTINE
by hormones
food delivered from stomach
hormone secretion
initiates nutrientspecific digestion
muscle
movement
muscle movement
mix enters the
small intestine
fats and
proteins
low pH
cholecystokinin (CKK)
released by intestinal cells
1. gall bladder releases
bile salts
secretin released
by intestinal cells
pancreas secretes
bicarbonate which
neutralizes acid
2. pancreas secretes
digestive enzymes
Examples from the small intestine:
1. Cholecystokinin
2. Secretin
chyme enters the
small intestine
fats and
proteins
cholecystokinin (CKK)
released by intestinal cells
1. gall bladder contracts to
release bile salts
2. pancreas secretes digestive
enzymes
low pH
secretin released
by intestinal cells
pancreas secretes
bicarbonate
NEXT ...
the large intestine, where water
and ions are reabsorbed
*
vestigial
cecum
If evolution is so clever, why do we
have appendices?
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Remember that...
Different diets require different
processing machinery
These requirements are ALSO
reflected in the ‘design’ of the
digestive system
Plants have tough cell walls.
Cellulose:
most animals
cannot digest
(without help from
other kingdoms)
It isn’t easy being an herbivore
What to do, gutwise?
1. Longer guts for longer processing
time
2. Help from bacteria and protists
that have CELLULASE or other
useful enzymes
3. Chew well
Carnivore
Vegetarian
Ruminant herbivores
•
•
•
•
•
•
Cattle
Sheep
Goats
Deer
Giraffes
Antelope
and their relatives, the camels and llamas
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Ruminant herbivores
and their relatives, the camels and llamas, have
• multiple stomach compartments
• bacterial fermentation aiding
digestion
• cud-chewing
Ruminants
have 4
stomach
compartments
1. rumen
2. reticulum
3. omasum
4. abomasum
Other cellulose eaters with symbionts
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