F1.Food - Diet& Digestion • Macronutrients – – 1 2...

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Unformatted text preview: Diet & Digestion • Macronutrients – – 1. 2. 3. Feeding, digestion & nutrition – Tiny amounts of vital elements & compounds our body cannot adequately synthesize 4. Vitamins 5. Minerals • Hydration 6. Water Hydrolytic Enzymes 1. Ingestion Consuming food 2. Motility – Swallowing, peristalsis, segmentation, defecation 3. Physical disruption – Mastication, dissolution, denaturation , emulsification 4. True digestion – Hydrolysis of polymers into monomers 5. Absorption – Moving nutrients from G.I. Tract into circulatory system 6. Assimilation – Incorporate nutrient molecules into tissues Vertebrate Digestive Tract Energy for all our endergonic processes Monomers to build our polymers Carbohydrates Lipids Protein • Micronutrients Food Processing – Why eat? Hydrolysis reactions catalyzed by enzymes – Remember, enzyme names end with “-ase” • • • • • Amylase: hydrolyze carbohydrates Protease: hydrolyze proteins Peptidase; hydrolyze peptides Lipase: hydrolyze lipids Nuclease: hydrolyze nucleic acids Digestive System • Alimentary Canal (26-foot long tube) – Mouth – Pharynx – Esophagus – Gastrointestinal [G.I.] tract (“gut”) • Stomach • Small Intestine • Large Intestine • Accessory Organs – Salivary Glands – Liver & Gall Bladder – Pancreas Heyer 1 Diet & Digestion Digestion Begins in Mouth Swallowing • Salivary glands – Water & mucus: moisten/lubricate food for swallowing • Pharynx – Initiate swallowing reflex – Divert food bolus from respiratory tract • Esophagus – uses peristaltic action • Salivary glands produce Amylase – Enzyme hydrolyzes starch – Cleans teeth • Saliva also contains mucous, buffers, antibacterials • Slug of mashed food and saliva is a Bolus Peristalsis Gut Motility • Peristalsis — waves of involuntary muscle contraction – Esophagus, stomach, small intestines • Segmentation — churning action – Stomach, small intestines • Sphincters — muscular rings that control entry/exit – Esophageal sphincter – Pyloric sphincter – Anal sphincter Stomach • Distensible: stores swallowed food pending further processing. Sealed by sphincters at either end. • Mucosa wrinkled to form rugae that increase surface area • Highly motile. Regulates rate of passage to intestines. • Many gastric pits leading to gastric glands secreting gastric juice • Masticated food + digestive juices = chyme • Acid in gastric juice disinfects the chyme Heyer Muscular action in the human gut Gastric Glands Gastric Juice: A. Goblet cells ‡ alkaline mucus • Helps protect mucosa from auto-digestion. B. Parietal cells ‡ hydrochloric acid [ HCl ] ‡ pH < 2 • Disinfects chyme ; denatures protein C. Chief cells ‡ pepsinogen ( a weak protease) • Auto-hydrolyzes to form pepsin (stronger protease) D. Other cells produce hormones 2 Diet & Digestion Protein Digestion Begins in the Stomach Regulation of Gastric Activity 1. Acid denatures protein in the chyme – Unfolded protein structure more vulnerable to hydrolysis 2. Pepsin hydrolyzes polypeptides into short peptide fragments 3. Digestion and absorption completed in small intestines Digestion Completed in Small Intestine • Gastric secretion and motility stimulated by parasympathetic neurons, intrinsic stretch reflex, gastrin hormone, and amino acids in chyme (positive feedback amplification). • Inhibited by stretch reflex and hormones from intestines, or chyme pH <1.5. Visceral Digestive Accessory Glands • Chyme enters via the pyloric sphincter. • First segment = duodenum: secretions from accessory glands enter via common bile duct. • First 40% = duodenum + jejunum: most digestion and absorption of macro- & micronutrients. • Final 60% = ileum: most absorption/reabsorption of water and salts. Liver: the biochemistry lab Liver to Gall Bladder: Bile Salts: Fat Emulsifier • Liver produces bile salts & pigments. • Bile is stored & released by Gall Bladder into Small Intestine. • Emulsify: Bile salts break large fat globs into small droplets (detergent action). Heyer 3 Diet & Digestion Small Intestine • Exocrine component — Acini: Pancreas – Secrete pancreatic juice. • Bicarbonate to neutralize acid chyme • Suite of hydrolytic enzymes • Endocrine component — Islets: – Secrete insulin and glucagon. Carbohydrate digestion salivary & pancreatic amylase Absorption enhanced by large surface area & high vascularization: • Lining wrinkled into ridges ( plicae ) and finger-like protrusions ( villi). • Apical surface of mucosal epithelium wrinkled into microvilii (brush border). • Core of each villus bears dense capillary beds surrounding a central lacteal (lymphatic vessel). • Capillaries carry absorbed substances to the hepatic portal vein and then to the liver. Lacteal transports to central veins and general circulation. Carbohydrate Digestion — Summary 1. Hydrolysis of polysaccharides (poly-glucose) starts with salivary amylase. • Minor significance — amylase denatured by gastric acid. brush border enzymes 2. Pancreatic amylase in duodenum & jejunum hydrolyzes polysaccharides into maltose & maltriose. 3. Brush border enzymes hydrolyze tri-/disaccharides into monosaccharides. 4. Brush border transporters absorb monosaccharides by active cotransport into hepatic portal vein. 5. Insulin dependent: monosaccharides taken up by liver for glycogenesis, or circulate as energy substrate for glycolysis or lipogenesis. Protein Digestion — Summary 1. Proteins are denatured by gastric acid. 2. Hydrolysis of exposed polypeptides starts with pepsin in gastric juice under acidic pH. 3. Pancreatic proteases & peptidases, plus brush border peptidases in duodenum & jejunum hydrolyze poly-/oligopeptides into tri-/dipeptides & amino acids. 4. Brush border transporters absorb tri-/dipeptides & amino acids into mucosal epithelia. Intracellular peptidases hydrolyze tri-/dipeptides into amino acids. Amino acids absorbed into hepatic portal vein. 5. Insulin dependent: amino acids taken up by liver for gluconeogenesis, or circulate as substrate for protein synthesis Heyer Lipid Digestion — Summary 1. Fats & oils are emulsified by bile salts (detergent) in duodenum & jejunum . 2. Pancreatic lipases in duodenum & jejunum hydrolyze triglycerides into monoglycerides & fatty acids. 3. Monoglycerides & fatty acids, plus cholesterol, coalesce with bile salts to form micelles. 4. Brush border transporters absorb micelles into mucosal epithelia. Intracellular liposynthases condense monoglycerides & fatty acids into new triglycerides. Triglycerides & cholesterol are bound to carrier proteins to form chylomicron lipoprotein. 5. Chylomicrons are absorbed into the lacteal and systemic circulation. Lipoprotein-lipase of vessel endothelia releases fatty acids & glycerol for tissue lipogenesis. 4 Diet & Digestion Absorption/Assimilation of Lipids Large Intestine • Final water absorption • Compaction, storage, & excretion of feces – 1/3 bacteria by weight (some make K & B vitamins). – Brown color from bile pigments of RBC breakdown. Insert fig. 18.36 Carnivores Have Short Tracts Herbivores Have Long Tracts • Cellulose digestion requires accessory stomachs. WHAT HAPPENS TO IT ALL? • Aided by cellulytic protozoans or bacteria. WHAT HAPPENS TO IT ALL? Fate of Chemical Energy • Stored as – circulating energy substrates – liver & muscle glycogen – body fat • Stored energy used for – 1) metabolism – 2) growth – 3) reproduction Heyer 5 ...
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