Feeding and Digestion – Part 1
Biology 155Spring 2010
B. L. Krilowicz
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 enzymesC. Egestion = elimination of non-digestible
materials
II. Feeding Mechanisms:
A. Feeding on food dissolved (stays in solution) in the environment –
1. Rare2. Animal takes up food across body surface3. No need for a digestive system(Note that this mechanism is not mentioned in
the textbook.)
Examples -
a. Tapeworms -
Marine invertebrates
B. Feeding on liquids –
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
Lapping
Sucking
Nectar as food source (also hummingbirds)
Fig. 21.1C
a. sponges
C. Filter Feeding = feeding on small particles suspended (will settle out) in the environment
1. Must be aquatic
2. Requires specialized straining device
3. Continuous feeders
4. Indiscriminant feedersc. Other examples – baleen whale, mollusks
Fig. 21.1A
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 SystemProblem = 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:
Mouth
Oral cavitySalivary glands
esophagus
Stomach or crop and gizzard
liver
Gall bladder pancreas
Small intestineLarge intestine
Anus
Fig. 21.4
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