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Digestion I

• Functions:

1) food procurement - method of capturing and ingesting food items

2) mechanical (physical) breakdown of food - breaking larger food items into

smaller pieces

to enhance chemical digestion

3) chemical breakdown of food - use of enzymes and other molecules to break

large molecules into smaller molecules

4) absorption - transport of smaller food molecules into body tissues for use

by body cells

5) additional processing - filtering of molecules to remove toxins

• Recall your embryology!

• Cleavage:

• Blastula:

• Blastocoele:

• Blastopore:

• Gastrulation:

• Gastrula:

• Archenteron:

• Protostome:

• Deuterostome:

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• Basic structure of the gut tube (eucoelomates):

• Formation of “accessory organs”:

• Liver

• Gall bladder

• Pancreas

• Cecum

• Gills

• Urinary bladder • Lungs 

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• Basic breakdown of the digestive system (3 regions with different functions):

FUNCTION VERTEBRATE INVERTEBRATE

FOREGUT

MIDGUT

HINDGUT

THE FOREGUT:

• Food procurement – many examples of diversity:

• Molluskan radula for grazing, drilling, or

harpooning:

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• Crustacean modified legs for suspension feeding:

• Fish gill rakers for suspension feeding

• Whale baleen and crabeater seal teeth modified for

suspension feeding

• Fish cranial kinesis for suction feeding

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• Most vertebrates have homodonty for simple prey capture

• Mammals have heterodonty for complex mechanical

breakdown

• Selenodonty:

• Lophodonty:

• Bunodonty:

• Carnassials:

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• Similar diversity in bird beaks:

• FOREGUT (cont.):

1) Components (of tube): pharynx + esophagus

2) Function: transport via flushing with water (aquatic forms) or muscular

peristalsis (terrestrial forms)

3) Specializations in vertebrates:

• salivary glands produce venom (snakes, some shrews)

• salivary glands produce anticoagulant (vampire bats)

• birds with a crop for food storage 

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• MIDGUT:

1) Components: stomach (including pyloris)

2) Function: absorption (proteins), storage, 

transport, chemical breakdown (HCl +pepsinogen), mechanical breakdown

(muscular, rugae, gastroliths)

3) Specializations:

• birds have a proventriculus and gizzard

• ruminant mammals (foregut fermenters)

have a 4-chambered ruminant stomach

1. Food swallowed - passes to rumen

for fermentation

2. Regurgitate large pieces - the cud

3. After fermentation in rumen, 

small pieces pass to

reticulum for absorption

4. Passage to omasum and then to abomasum for absorption

• Digestion slow but efficient - ruminant stomach limits food

intake

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• HINDGUT:

1) Components: intestines, liver, pancreas

2) Function: transport via muscular peristalsis; absorption

(water, sugars, amino acids, fatty acids, ions);fat emulsification (liver bile); starch breakdown

(pancreatic amylase)

3) Specializations:

• plica, villi, + microvilli (tetrapods) 

• typhlosole (annelids)

• spiral intestine (sharks) 

• long intestines (endotherms/herbivores)

• cecum (hind gut fermenting mammals, insects, 

flatworms, … many others)