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Digestion
Animals cannot ingest macromolecules made by other animals and use them directly
2 kinds:Mechanical digestionChemical digestion
Vertebrate Digestive System
Food moves through digestive tract by peristalsis:
Mouth pharynx esophagus stomach
small intestine large intestine anus
Accessory organs: Salivary glands Liver Gall bladder Pancreas
Pharynx and Esophagus
Bolus moves through pharynx Epiglottis covers trachea during swallowing
Esophagus leads to stomachPeristalsis moves food along
Stomach
Ring of sphincter muscle relaxes and allows bolus to enter stomach
As stomach fills, rugae of stomach smooth out
Chemical Digestion in Stomach
Gastric glands in mucosa secrete:HCl Intrinsic factorPepsinogen (converts to pepsin in presence of acidic
gastric juice)
Pepsin, main digestive enzyme of stomach, breaks down large proteins into short polypeptides
Food spends 3-4 hours in stomach
Muscle contraction churns food
Salivary amylase works until stomach becomes too acidic
Partly digested food becomes chyme
Peristalsis releases chyme in spurts through pylorus into small intestine
Protection of StomachLining of stomach secretes large amounts of alkaline mucus
Epithelial cells are tightly joined to prevent leaking
Small Intestine
In human, approx. 17 feet long
Divided into 3 regions: duodenum, jejunum, and ileum
Lining is folded and composed of villi and microvilli
Digestion in Small IntestineBile, secreted by liver, mechanically digests fats
Enzymes secreted by small intestine and pancreas:
Polypeptides and peptidesTrypsin, chymotrypsin, peptidases
Completion of carbohydrate digestionPancreatic amylase, maltase, sucrase, lactase
Digestion of fat droplets lipase
Absorption of NutrientsNutrients must pass through epithelium to reach blood or lymph
Simple diffusion, facilitated diffusion, active transport
Large IntestineApprox. 4 feet long
Includes cecum and appendix, a blind pouch
Sections include ascending, transverse, descending & sigmoid colon
Ends at rectum & anus