Transcript St. Francis Xavier University Digestion

Biology 201
Dr. Edwin DeMont
Digestion
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Feeding
Animals exhibit a wide variety of feeding mechanisms
and styles.
Continuous feeders include aquatic
suspension feeders (filter feeders); they
are typically sedentary or sessile
Discontinuous feeders have more specializations, as
they “capture” their food, and attempt to avoid
predation at the same time. They are often more
active than suspension feeders. They gather and store
the food, digesting it gradually.
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Digestion
Digestive processes involve the uptake of food
particles by a variety of processes and its
subsequent enzymatic breakdown.
Two different types of
digestion exist:
(1) Intracellular
(2) Extracellular.
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Digestion
Protists and sponges
employ intracellular
digestion—they do not
have a gut.
The possession of a gut
allows extracellular
digestion of food within the
lumen of the gut cavity.
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Invertebrates
Invertebrate digestive tracts range from
gastrovascular cavities to complete, complex
digestive systems.
An incomplete digestive tract limits
feeding, as the animal can only feed
anew after undigested wastes have
been expelled.
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Invertebrates
When a complete gut is present there is a one-way
flow of food through the animal. This makes
possible a gut with distinct processing regions.
And it works well
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Invertebrates
Insects have a complete digestive tract with both storage
and grinding regions. They use extracellular digestion.
Digestion begins in the mouth
and is usually completed in the
gastric cecae. Most nutrient
absorption occurs in the
intestine and undigested foods
pass to the rectum. Water and
ions are absorbed from the
fecal mass, and solid pellets
pass out via the anus.
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Mammals
The mammalian digestive
system may be taken as an
example of a typical
(nonspecialized) vertebrate
digestive system.
Video
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Mammals
The oral cavity consists of
lips (sensory), teeth
(grinding), an epiglottis (to
divide food for the air
headed to the lungs), and
secretions by the salivary
glands (to moisten food and
add enzymes – eg. amylase.)
Food (called a bolus) is then
passed via the esophagus to
the stomach.
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Mammals
The stomach of a
mammal stores food,
mixes the food bolus,
secretes enzymes,
mucus, and
hydrochloric acid, and
controls the passage of
the mixed, liquified
food (chime) into the
small intestine.
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Mammals
There are 3 kinds of cells in
the stomach:
1. parietal cells secrete
hydrochloric acid;
2. chief cells secrete
pepsinogen (pepsin);
3. numerous mucus cells
secrete mucus to protect the
gut from HCl.
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Mammals
The small intestine is the
primary site of digestion, as
well as absorption; it has a
lot of surface area. The
length of the small intestine
is related to the diet of the
animal; herbivores have
longer intestines.
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Mammals
Villi (macroscopic) and microvilli (microscopic) both increase
the absorptive surface area of the small intestine. Each villus
has a capillary system and a lacteal to absorb nutrients.
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Mammals
The pancreas secretes a
number of enzymes into
the small intestine
facilitating digestion of
nutrients and neutralizing
acids.
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Mammals
The liver functions in a
variety of digestive ways,
including breakdown of
proteins, production of the
plasma proteins, metabolism
of carbohydrates and lipids
and synthesis of some
vitamins and bile salts.
Further, the liver stores
glycogen, which can be
converted into glucose
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Mammals
The large intestine
primarily functions in
absorption of water and
minerals.
Nutrients diffuse into
the blood from the
large intestine.
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Mammals
The digestive tract is composed of an outer serosa,
smooth muscle layers (typically one longitudinal and
one circular), and the submucosa and the inner mucosa
(connective tissues with blood and lymph vessels).
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Mammals
Peristalsis (movement of
food through the digestive
cavity) and segmentation
(churning) result from
muscular movements within
the gastrointestinal tube.
Sphincters also regulate food
movement, and close off
certain gut regions. Control
of gastrointestinal activity is
regulated by the volume and
composition of food in the
lumen of the gut.
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