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Chapter 41Digestion
Digestive SystemsAbsorb nutrients necessary to
maintain life, eliminate wastesIngestionDigestionAbsorptionElimination
Digestion—process by which food is chemically and mechanically broken down for use by the body
Digestive SystemsNo digestive system
Sponges, tapewormsAbsorb only small moleculesDigestion intracellular
Digestive SystemsIncomplete
Cnidarians, planariansSingle body opening
(Gastrovascular cavity)Limited enzymesDigestion begins
extracellularlyDigestion completed
intracellularlyNo regions w/ specialized
function
Digestive SystemsComplete
Annelids, mollusks, arthropods, vertebrates, etc. (most animals)
Two openings (mouth & anus)
Different regions w/ specialized function
Digestion primarily extracellular
Digestive SystemsContinuous feeders
Filter feeders (clams, mussels, barnacles, etc.)
Must complete digestion quickly
Discontinuous feedersMost animalsFeed periodicallyNeed storage area in gut
Complete Digestive SystemFunctions
Mechanical processing & motility
Secretion of digestive enzymesChemical digestionAbsorptionElimination of wastes
Anatomy determined by lifestyleCarnivoreHerbivore Omnivore
MouthInitial digestion beginsMechanical break-down of foodNo teeth
Worms, birds—swallow food whole Grinding done lower in digestive tract
Insects—some chew, some suckArachnids—no chewing
Suck existing fluids Inject digestive fluids into prey
MouthTeeth
Vertebrates onlyIncisors—shearingCanines—puncture,
tearing Capture prey Fighting predators or
within speciesPremolars—grindingMolars—crushing
MouthHerbivores
Plant material toughEmphasis on
grinding, crushingBroad, flat premolars
& molarsLimited or absent
canines
MouthCarnivores
Animal material easier to mechanically tear
Emphasis on piercing, cutting
Long, pointed canines
Molars & premolars modified for shearing or piercing
MouthOmnivores
Combination teethVaries based on amount of
each type of food
MouthSalivary glands
Secrete via ducts into mouth
Lubricate & moisten foodSalivary amylase
Begins to digest starches
TongueGrab foodManipulate foodTaste
EsophagusMuscular tubeLined by stratified
squamous epitheliumMoves food from
mouth to next stepPeristalsis—rhythmic
contractions of gastrointestinal tract
Upper GICrop
InsectsAnnelidsMost birdsTemporary storage
Upper GIProventriculus
Birds, many invertebratesSecretes digestive enzymes
GizzardBirds, many invertebrates,
some reptilesDinosaursThick, muscular wallsGrinding food
Chitinous plates in some insects & mollusks
Birds swallow “grit” to use for grinding
Upper GIStomach (monogastric)
Temporary storageMechanical churning Digestive enzymesCombined functions of
crop, proventriculus, gizzard
Pepsin—breaks down proteins
Hydrochloric acid—slows salivary amylase, activates pepsin
Upper GIStomach (ruminant)
Cattle, goats, llamas, camels, deer, sheep, etc.
Four-chambered stomach Rumen, reticulum, omasum,
abomasumFood swallowed, separated in
reticulorumenSolid (cud) regurgitated from
reticulumCud chewed for further
breakdown, then swallowedMoved to omasum, water &
inorganic minerals absorbedMoves to abomasum, like
monogastric stomach
Lower GISmall intestine
Short in carnivoresLong in herbivores
DuodenumJejunumIleum
Lower GISecretion of enzymes
Intestinal mucosaGall bladderPancreas
AbsorptionVilli—folds of intestinal
liningMicrovilli—folds of
intestinal cells
Lower GICecum
Single in mammalsDouble in most birds &
some reptilesLarge in herbivoresReduced in carnivoresRemove fluids & saltsIn herbivores has large
numbers of bacteria to break down plant material
Lower GIColon (large intestine)
Most food digested & absorbed by this point
Water & wasteFunctions to reabsorb waterDiarrhea—water not
absorbed properlyRectum
Storage of feces ready for elimination
AnusMuscular sphincter
controlling elimination of feces
Other OrgansLiver
Secretes bile (breaks down fats)Many other important functions
Gall bladderStores & concentrates bileNot present in all animals, even
if closely related Absent horse, deer; present cow,
goat, sheep Absent doves; present hawks &
owls Absent rats; present mice
PancreasMany enzymes to break down
foodBuffers against HCl from
stomach
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