Digestion

Digestive Systems Absorb nutrients necessary to maintain life, eliminate wastes Ingestion Digestion Absorption Elimination Digestion—process by which food is chemically and mechanically broken down for use by the body

Digestive Systems No digestive system Sponges, tapeworms Absorb only small molecules Digestion intracellular

Digestive Systems Incomplete Cnidarians, planarians Single body opening (Gastrovascular cavity) Limited enzymes Digestion begins extracellularly Digestion completed intracellularly No regions w/ specialized function

Digestive Systems Complete Annelids, mollusks, arthropods, vertebrates, etc. (most animals) Two openings (mouth & anus) Different regions w/ specialized function Digestion primarily extracellular

Digestive Systems Continuous feeders Filter feeders (clams, mussels, barnacles, etc.) Must complete digestion quickly Discontinuous feeders Most animals Feed periodically Need storage area in gut

Complete Digestive System Functions Mechanical processing & motility Secretion of digestive enzymes Chemical digestion Absorption Elimination of wastes Anatomy determined by lifestyle Carnivore Herbivore Omnivore

Mouth Initial digestion begins Mechanical break-down of food No teeth Worms, birds—swallow food whole Grinding done lower in digestive tract Insects—some chew, some suck Arachnids—no chewing Suck existing fluids Inject digestive fluids into prey

Mouth Teeth Vertebrates only Incisors—shearing Canines—puncture, tearing Capture prey Fighting predators or within species Premolars—grinding Molars—crushing

Mouth Herbivores Plant material tough Emphasis on grinding, crushing Broad, flat premolars & molars Limited or absent canines

Mouth Carnivores Animal material easier to mechanically tear Emphasis on piercing, cutting Long, pointed canines Molars & premolars modified for shearing or piercing

Mouth Omnivores Combination teeth Varies based on amount of each type of food

Mouth Salivary glands Secrete via ducts into mouth Lubricate & moisten food Salivary amylase Begins to digest starches Tongue Grab food Manipulate food Taste

Esophagus Muscular tube Lined by stratified squamous epithelium Moves food from mouth to next step Peristalsis—rhythmic contractions of gastrointestinal tract

Upper GI Crop Insects Annelids Most birds Temporary storage

Upper GI Proventriculus Birds, many invertebrates Secretes digestive enzymes Gizzard Birds, many invertebrates, some reptiles Dinosaurs Thick, muscular walls Grinding food Chitinous plates in some insects & mollusks Birds swallow “grit” to use for grinding

Upper GI Stomach (monogastric) Temporary storage Mechanical churning Digestive enzymes Combined functions of crop, proventriculus, gizzard Pepsin—breaks down proteins Hydrochloric acid—slows salivary amylase, activates pepsin

Upper GI Stomach (ruminant) Cattle, goats, llamas, camels, deer, sheep, etc. Four-chambered stomach Rumen, reticulum, omasum, abomasum Food swallowed, separated in reticulorumen Solid (cud) regurgitated from reticulum Cud chewed for further breakdown, then swallowed Moved to omasum, water & inorganic minerals absorbed Moves to abomasum, like monogastric stomach

Lower GI Small intestine Short in carnivores Long in herbivores Duodenum Jejunum Ileum

Lower GI Secretion of enzymes Intestinal mucosa Gall bladder Pancreas Absorption Villi—folds of intestinal lining Microvilli—folds of intestinal cells

Lower GI Cecum Single in mammals Double in most birds & some reptiles Large in herbivores Reduced in carnivores Remove fluids & salts In herbivores has large numbers of bacteria to break down plant material

Lower GI Colon (large intestine) Most food digested & absorbed by this point Water & waste Functions to reabsorb water Diarrhea—water not absorbed properly Rectum Storage of feces ready for elimination Anus Muscular sphincter controlling elimination of feces

Other Organs Liver Secretes bile (breaks down fats) Many other important functions Gall bladder Stores & concentrates bile Not present in all animals, even if closely related Absent horse, deer; present cow, goat, sheep Absent doves; present hawks & owls Absent rats; present mice Pancreas Many enzymes to break down food Buffers against HCl from stomach