Feeding and Digestion –
Part 1
Biology 155
A. Russo-Neustadt
I. Definitions
A. Ingestion = to bring food into the digestive
system
B. Digestion = to break food down into smaller
molecules that can be absorbed by the body
1. Mechanical – physical, makes use of chewing,
grinding, etc.
2. Chemical - makes use of enzymes
C. Egestion = elimination of non-digestible
materials
II. Feeding Mechanisms:
A. Feeding on food dissolved (stays in
solution) in the environment –
1. Rare
2. Animal takes up food across body surface
3. No need for a digestive system
(Note that this mechanism is not mentioned in
the textbook.)
Examples a. Tapeworms -
Marine
invertebrates
Fig. 21.1C
B. Feeding on liquids –
Lapping
Nectar as food source (also hummingbirds)
Sucking
1. Requires a
specialized mouthpart to
obtain fluids
2. Mouthparts are
adapted for sucking or
lapping
3. Other examples –
Plant sap – aphids
Blood – mosquitoes and
vampire bats
Milk – young mammals
C. Filter Feeding = feeding on small particles suspended
(will settle out) in the environment
Fig. 21.1A
1. Must be aquatic
2. Requires specialized straining device
3. Continuous feeders
c. Other examples –
baleen whale, mollusks
4. Indiscriminant feeders
a. sponges
D. Feeding on Large Particles –
1. Requires elaborate sensory
systems to find prey
2. Requires elaborate motor
systems to capture prey
3. Discontinuous feeders
4. Highly selective
Ex. Fish to mammals,
insects
Figs. 21.1B (substrate
feeders) and 21.1D
III. Intracellular versus Extracellular Digestion:
A. Intracellular = engulf (phagocytize) food particle
and isolate it in a food vacuole inside a cell; digestive
enzymes are secreted into the vacuole, where chemical
digestion occurs
Problem = food must
be smaller than a cell
Ex. Sponges and
single celled
organisms
III. Intracellular versus Extracellular
Digestion - continued
B. Extracellular = digestive enzymes are
secreted into a cavity where chemical
digestion generates small molecules that
can be absorbed into the body
Advantage = can eat things larger than the
size of a single cell
1. Gastrovascular Cavity = Incomplete Digestive
System
Problem = ingestion and egestion occur at the same site, thus
regional specialization is impossible which reduces
digestive efficiency
Ex. Jellyfish and
their relatives
(shown)
Flatworms (not
shown)
Fig. 21.3A
2. Complete Digestive System = a True Gut,
system with both a mouth and an anus
Advantage = one-way traffic of food bolus,
thus regional specialization is possible which
increases digestive efficiency
Ex. Roundworms
(shown here) through
mammals (not shown)
Fig. 21.3B
IV. Example of a Generalized Complete Digestive Tract:
Salivary
glands
Oral cavity
Mouth
esophagus
Fig. 21.4
liver
Stomach or crop and
gizzard
Gall bladder
pancreas
Large intestine
Small intestine
Anus