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Ch. 47

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Rutgers University
Biological Sciences

Ch. 47 Digestion and Nutrition Includes herbivores, carnivores, and omnivores and the feeding, digestion, absorption, and final egestion (elimination) process Gastrovascular Cavities – simple invertebrates a. Hydra: two phases: digestion begins as prey is snared (nematocyst); cells of gut endodermis engulf, digest, and absorb material; waste into gut cavity… b. Flatworm: two phases; extends pharynx through mouth, secretes digestive enzymes onto prey; continued digestion in gastrovascular cavity, absorbed through gut lining For both: body contractions promost egestion of undigested food particles through the mouth; digestive tract has one opening Complex invertebrates and vertebrates with Complete Digestive Tract · Earthworm with two openings to their digestive tract (both a mouth and anus) – food digested in various regions; moved via peristalsis as more food ingested and processed Human Digestion · Mouth  pharynx (throat)  esophagus  stomach  small intestine  large intestine  rectum · Liver, pancreas, salivary glands; accessory glands that secrete digestive juices into digestive tract, and aid in digestion Structure of Digestive Tract · Mucosa: epithelial tissue lines lumen; folded for absorption and secretion · Submucosa: outside mucosa; connective tissue + blood vessels, lymphatics, nerves · Smooth muscle (muscularis externa) o Inner (circular) o Outer (longitudinal) o Oblique layer (to help with churning) · Peritoneum: a layer of serous connective tissue membrane (serosa) o Visceral covers the abdominal organs o Parietal attaches to abdominal cavity o Mesentary attaches to back wall of abdominal cavity to support small intestines Human Digestive System (carnivores) · Mouth: adapted for mechanical (mastication) and enzymatic digestion of carbohydrates: · Teeth: incisors for biting, canines for tearing, premolars, molars for crushing and grinding · Three pairs of salivary glands secrete about 0.5L of saliva · Salivary amylase begins digestion of starch: produces maltose and small polysaccharides Tooth Structure · Enamel: hardest substance in the body · Dentin: resembles bone in composition and hardness · Pulp cavity: contains pulp which produces dentin; and includes connective tissue with blood vessels, lymph, and nerves and… o Contains multipotent stem cells (becoming various cells in the body e.g. osteocytes, myocytes, adipocytes, neuronal and oral cavity bone and tissue) Peristalsis •Pharynx (throat) and esophagus carry food to stomach; epiglottis closes off airway passage •Peristalsis: waves of circular and longitudinal muscular contraction as well as gravity pushes bolus of food along digestive tract •Food passes down esophagus, sphincter muscle relaxes, bolus enters stomach In stomach, mechanical digestion by vigorous churning mixes partially digested food • ← The Stomach Gastric Glands (Fig 47-9) •Rugae (folds) in stomach wall (stretch receptors) expands as stomach fills with food, increasing capacity; indicates fill •Exocrine: secretions from gastric glands (in stomach wall) exit into stomach: • Parietal cells secrete HCL (inactivates salivary amylase) and intrinsic factor (for absorbance of Vit B12) • Chief cells secrete pepsinogen (activated by HCL; convert to pepsin) for proteolysis of proteins ← Stomach Acidity •The acid secreted into the stomach could destroy the stomach lining but: • Alkaline mucus from gastric mucosa columnar cells coats stomach wall, neutralizing effects of acidity on cells •Damaged cells are periodically shed and replaced ← ← NB: H plyori infects mucus secreting cells in stomach lining - envelops itself in mucus (biofilm) avoids cell defenses. Decreases mucus production, causes inflammation peptic ulcers/cancer ← Th Small Intestine •Chyme, which is now a soup of partially digested food, leaves stomach through pylorous and enters small intestine in spurts •Small intestine include duodenum, jejunum, ileum The Duodenum Chyme first moves to the duodenum where most of the enzymatic (chemical) digestion • occurs: • Dipeptidases: digests peptides to amino acids • Maltase: breaks maltose down to glucose • Aided by: liver and pancreatic secretions ← Liver (Fig 47-11) Chyme is mixed with bile from liver: • • Bile: mixture of water, salts, pigments (Hb breakdown of old RBCs), cholesterol and lecithin (fatty substance) which is stored in gallbladder; acts like detergent to help mechanically emulsify fats ← Pancreatic Secretions • HCO 3from pancreas neutralizes stomach acid; provides optimal pH for pancreatic enzymes: • Chymotrypsin and tyrpsin digest peptides to amino acids • Pancreatic lipase degrades fats emulsified by bile salts to FA and glycerol Pancreatic amylase digests complex CHOs with beta links to disaccharides (eg. • Maltose) and trisaccharides • RNAse and DNAse digest DNA and RNA into nucleotides ← Absorption in the Duodenum (Fig 47-10) • Absorption of nutrients into the blood occurs as a combination of i. Simple diffusion ii. Facilitated diffusion (eg. Fructose) iii. Indirect active transport (eg. Glucose and amino acids going against their concentration gradients are coupled with transport of sodium) ← Hepatic Portal Circulation • Excess glucose (for production of glycogen) and amino acids [for production of glucose (gluconeogenesis) and glycogen] etc are then transported to liver via hepatic portal vein; various toxic substances are also removed for detoxification ← Fats in the Villi Lacteal • FA, monoacylglycerols are reassembled as triacylglycerides in smooth ER • Triaclyglycerides combine with cholesterol, phospholipid and protein to form chylomicrons • Enter lacteals, pass into central lymph vessel • Lymphatic system t
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