Digestive System and Metabolism PDF

1/20/25 The Digestive System and Learning Outcomes Metabolism 1. Functional Anatomy of the Digestive Readings: All of Chapter 18 (this Sys...

1/20/25 The Digestive System and Learning Outcomes Metabolism 1. Functional Anatomy of the Digestive Readings: All of Chapter 18 (this System class, and next one), but… - mouth, esophagus, stomach, small intestine, large intestine, colon (Ignore 18.6 Neural and Endocrine 2. Accessory Organs Regulation of the digestive - liver, gall bladder, pancreas system). 3. Critical thinking - time restricted feeding, microbiota 1 2 Gastrointestinal Gas: How long does it take to digest ½ liter per day, spread out over 14-25 incidents…. food — from the time you eat it to the time you excrete it? Answer from Michael F. Picco, M.D. http://www.mayoclinic.com/health/ digestive-system/an00896 3 4 1  1/20/25 The Digestive System and Structure of the Digestive System Metabolism Digestion Two Main Functional Groups of Organs – Breakdown of ingested food – Absorption of nutrients into the blood – Concentration and removal of waste products Alimentary Canal – Continuous hollow tube Metabolism – Production of cellular energy (ATP) Accessory Digestive Organs – Regulation of cellular activities 5 6 Structures of the Alimentary Canal Human tongue surface, coloured scanning electron micrograph (SEM). The tongue is Mouth covered in many backward facing Pharynx projections called filiform papillae, which sense pressure Esophagus Picture: SCIENCE PHOTO LIBRARY Stomach Small Intestine Large Intestine Anus 7 8 2  1/20/25 Functional anatomy of the digestive tract Oral cavity Salivary glands Liver Esophagus Gall bladder Stomach Duodenum Pancreas Small Large intestine intestine (colon) Photo of a male gardiner's frog (S. gardineri) taken in its natural habitat of the Seychelles Islands. Anus Rectum Credit: R. Boistel/CNRS 9 10 Alimentary Functions of the Salivary Glands Canal and Accessory Organs Lubrication/binding Solubilization of dry food Oral hygiene – flushes away debris Begins starch digestion (salivary amylases) Alkaline buffering Evaporative cooling (ie, important in dogs) 11 12 3  1/20/25 From Mouth to Stomach From Mouth to Stomach Mastication (chewing food) (Add salivary amylases) The Teeth Teeth The teeth are the – Incisors hardest structures in Rip, cut the body – Canines Total of 20 primary Tear, pierce (deciduous, or “baby”) – Premolars teeth. Grind, shear 32 secondary – Molars (permanent) teeth. Grind 13 14 From Mouth to Stomach From Mouth to Stomach Esophagus (connects pharynx to stomach) Deglutition (swallowing) Muscular tube ~ 25 cm long Oral, pharyngeal, esophageal Requires 25 pairs of muscles in the mouth, pharynx, larynx, upper esophagus Peristalsis Food moves by a wave-like muscular contraction Mouth, pharynx, upper esophagus – muscles innervated by somatic motor neurons Middle and lower esophagus – muscles innervated by autonomic neurons. 15 16 4  1/20/25 From Mouth to Stomach From Mouth to Stomach Peristalsis in the Esophagus esophagus. Peristaltic Runs from Pharynx to Stomach contraction and Passes through diaphragm movement of a bolus into the Mobilizes food by peristalsis stomach. 17 18 Peristalsis: Barium Swallow – images only Peristalsis continues in the stomach. This endoscopy shows peristalsis in the antrum (lower stomach). It is a wavelike, constant, rhythmic movement caused by muscular contraction carrying food from the stomach through the pylorus into the duodenum, which is the first part of the small bowel. Images only 19 20 5  1/20/25 The stomach Gross anatomy of the stomach The smooth muscle layers in Circular muscles the wall of the stomach, in addition to running the length of the organ, and around the organ, also run in Longitudinal muscles an oblique direction These muscles act to mix Oblique muscles and mechanically break up food in the stomach 21 22 Musculature of the Stomach The mucosal region of the stomach contains gastric pits and gastric Circular, longitudinal, glands. and oblique fibres arranged perpendicularly to provide complex motility 23 24 6  1/20/25 Secretory Components of the Stomach Secretory Components of the Stomach Gastric pits are the openings of the gastric Mucous Cells: Secrete mucus glands. Parietal Cells: Secrete HCl, intrinsic factor (B12- Gastric glands consist essential for life). of several types of cells (mucous cells, chief Chief (zymogenic) Cells: Secrete pepsinogen cells, parietal cells). Each cell type produces a specific secretion. 25 26 Secretory Components of the Stomach Secretory Components of the Stomach Two questions: Erosions of the mucosa can lead to peptic ulcers (i.e. the stomach digest itself). Caused by Helicobacter pylori. 1. Why doesn’t the stomach digest itself? Infect the GI tract of ~50% of adults worldwide. Nobel prize for recognizing that these relatively common bacteria are responsible, rather than “spicy food”. 27 28 7  1/20/25 Secretory Components of the Stomach Small Intestine 2. What do Regions pepsinogen/HCl – Duodenum do? First 25cm Mucous secretion, receives pancreatic In the presence secretions and bile from liver of HCL, the inactive enzyme – Jejunum pepsinogen is 1m in length activated to the Numerous folds and villi pepsin form, – Ileum which can digest Last 2m proteins into Fewer folds/villi than jejunum smaller Absorbs primarily bile salts, water, electrolytes polypeptides. Contains Peyer’s Patches (Aggregates of lymph nodes) Empties into Large Intestine via Ilieocecal valve 29 30 Small Intestine stomach Esophagus - Anterior sphincter – cardiac stomach – pyloric stomach – pyloric sphincter – duodenum – jejunum – ileum – ileococal valve …. Large intestine… Small intestine Chyme Alkaline bile from liver Regions of the small intestine, and showing the tissue layers, and villi. (to neutralize) The Microvilli are formed by foldings at the apical surface of each epithelial cell membrane. Text discusses the different layers of the GI- tract. 31 32 8  1/20/25 Small Intestine: Villi Small Intestine: Villi are covered with columnar Microvilli epithelial cells. Goblet cells secrete mucous. Epithelial cells at the tip of the villi are continuously sloughed off and replaced by new cells coming from the Intestinal crypts (crypts of Lieberkuhn). Paneth cells at the base of the crypts secrete antibacterial molecules (lysozyme, antimicrobial peptides) to protect the intestine from inflammation. *Microvilli are NOT villi – zooming in! 33 34 Small Intestine: Microvilli Brush Border Enzymes Attached to the Cell Membrane of Microvilli in the Small Intestine 35 36 9  1/20/25 Time Restricted Eating / Feeding / Critical Thinking Intermittent Fasting 37 38 Time Restricted Eating Time Restricted Eating 39 40 10  1/20/25 Dr. Satchin Panda Distinguished Public Lecture UofG Timing of diet, sleep, and exercise to optimize heart health stomach (link provided - watch outside of class if interested) Esophagus - Anterior sphincter – cardiac stomach – pyloric stomach – pyloric sphincter – duodenum – jejunum – ileum – ileococal valve …. Large intestine… Small intestine Chyme Alkaline bile from liver (to neutralize) https://www.youtube.com/watch?v=x5eZuJuo8iI 41 42 Large Intestine The colon (large intestine) The bacterial colony is made up of many species of bacteria and plays an essential role in digestive processes Valve The “good” bacteria out compete pathogenic bacteria When pathogenic bacteria take over the colon reacts by eliminating colon content and sloughing off the colon epithelium - diarrhea Rectum Anus It is smoother… 43 44 11  1/20/25 The colon (large intestine) The colon (large intestine) The appendix is a small The appendix does not component of the colon function in digestion, but Until recently, no role had like the tonsil it contains lymph vessels. been identified for the appendix It is subject to inflammation It is now thought that the – called appendicites. Causes pain in the lower right appendix contains a reservoir of “good bacteria” quadrant of the abdomen. that can re-colonize the Ruptured appendix can cause colon following diarrhea, and inflammation in the expulsion of the colon peritoneal cavity – content peritonitis. Appendix Appendix 45 46 The colon (large intestine) Critical Thinking Intestinal microbiota are about 10x more than # of human cells in the body. “Commensal”, “mutualism” Valve Rectum Anus 47 48 12  1/20/25 The colon (large intestine) The colon (large intestine) Intestinal microbiota How Gut Bacteria Regulate Happiness in Human originates at birth. What grows in the gut, for animals and people, is affected by diet. Beginning at birth, gut microbial colonies, known as gut microbiome, start forming. Serotonin is altered with stress, anxiety, depression. Major target of clinical antidepressants. Cat Microbiomes Stem From Diet - Science News - Study used a “germ-free mouse model to show that the redOrbit absence of bacteria during early life significantly affected serotonin concentrations in the brain in adulthood. “ 49 50 The colon (large intestine) The colon (large intestine) “What grows in the gut, for animals and people, is affected by diet. Beginning at birth, gut microbial colonies, known as gut microbiome, start forming. The composition of these colonies plays a big part in how the immune system develops and is linked to the later onset of metabolic diseases such as obesity.” Dysbiotic microbiota – cystic fibrosis, autism, obesity? changing diet? fecal transplants? 51 52 13  1/20/25 Digestion and Absorption in the G.I. Tract Accessory Organs of Digestion Pancreas Liver Gallbladder 53 54 Accessory Digestive Organs: Liver Liver, with 2 blood inputs – - Made up of hepatic cells lining portal vein (coming from large capillaries called sinusoids. intestines, major source of blood supply to the liver) and also - Sinusoids also lined by hepatic artery (blood from heart endothelial cells. to liver). - Sinusoids also contain Kupffer Endocrine function is where the cells (phagocytes). enzymes & hormones of the liver - Amazing regenerative do their thing, and output gets capabilities – if 2/3rds of a sent into the hepatic vein and that goes on back to the heart, rodent’s liver are surgically so the nutrients loaded into removed, the remaining tissue there can get pumped through will regenerate to it’s original the whole body. mass in 1 week. 55 56 14  1/20/25 Accessory Digestive Organs: Also 2 exocrine regions – right Architecture of the Liver and left hepatic ducts that come out from liver, which makes bile. Bile is stored in the gallbladder. The R/L hepatic ducts meet up with the cystic duct from the gall bladder which stores bile, and together they form the common bile duct which then goes into the intestines. Hepatic portal system page 629, also Figure 18.20 57 58 Accessory Digestive Organs: Liver A derivative of the Heme Major Categories of Liver Function group (minus the iron) is converted into bilirubin, carried in the blood on albumin proteins, taken up by the liver, mixed with glucoronic acid, and now is water soluble and can be secreted into bile, to intestine, converted by bacteria into urobilinogen, and removed in feces. Some urobilinogen can re- enter circulation and be excreted by kidneys. 59 60 15  1/20/25 Accessory Digestive Organs: Accessory Digestive Organs: Gallbladder Gallbladder Common complication (20 million Americans) are Sac-like organ attached to the inferior surface of the gallstones. Mineral deposits that produce painful liver. symptoms by obstructing the bile ducts. Stores and concentrates bile from the liver. Usually removed by surgery, sometimes oral ingestion of bile acids, or fragmentation by high energy shock waves. 61 62 Accessory Digestive Organs: Gallbladder Gallbladder Removal Surgery Animation – animation, video not tested. Link active to watch later. A. Radiograph of gallbladder with gallstones. B. Gallstones – note size relative to dime. https://www.youtube.com/watch?v=X0unDF8dFr0 63 64 16  1/20/25 Accessory Digestive Organs: Pancreas Accessory Digestive Organs: Pancreas Pancreatic juice contains ~20 different digestive enzymes including: Amylase – digests starch Trypsin – digests proteins Lipase – digests triglycerides PD to CBD 65 66 Accessory Digestive Organs: Pancreas Accessory Digestive Organs: Pancreas ** Digestion requires pancreatic enzymes PLUS brush border enzymes. The inactive form of the pancreatic enzyme trypsin is activated by the brush border enzymes; Both an … trypsin is a protease that can then activate other 1. Endocrine gland making pancreatic enzymes. hormones (insulin, glucagon, somatostatin, etc)… 2. Digestive organ secreting digestive enzymes destined for the small intestine. Enzymes help digest chyme carbs, proteins, lipids. 67 68 17  1/20/25 Learning Outcomes Learning Outcomes 1. Carbohydrate metabolism - Critical thinking – sugar, insulin 1. Functional Anatomy of the Digestive System - mouth, esophagus, stomach, small intestine, large intestine, colon 2. Cholesterol metabolism - Critical thinking – genetics vs. diet 2. Accessory Organs - liver, gall bladder, pancreas 3. Lipid metabolism - Critical thinking – fats are good for you 3. Critical thinking - time restricted feeding, microbiota 4. Protein metabolism 5. Absorption of Ions and Vitamins 69 70 Digestion: the need for processing Foods used by the body: 1. Carbohydrates (50% of total American dietary Introduction to Comparative calories) Physiology of Digestive 2. Proteins (11-14%) 3. Lipids (most of the rest) System 4. Cholesterol Reza Fatemi DVM from the Shahid Chamran University of Ahvaz, Iran PhD from the University of Tehran, Iran Typically cannot be absorbed through the intestinal mucosa in their native state (except cholesterol). 71 72 18  1/20/25 Main Functions of The Digestive System Classification of Animals Based on Food Habits Motility: Moves through the digestive tract Secretion: Release of enzymes, hormones, and other substances that help the body to digest food Digestion: Breaking down of food into components small enough to cross the plasma membrane Absorption: Transfer of single nutrient molecules to the blood or lymph 73 74 Carnivore Omnivore Herbivore Frugivorous Carnivore Omnivore Herbivore Frugivorous Physiologic food: Meat PF: Meat & vegetables PF: Herbs PF: Fruits, vegetables & nuts Strong acidity of Strong acidity of Weak acidity of Weak acidity of Great sharp fang Great sharp fang Rudimentary, blunt Canine for defense stomach stomach stomach stomach canines --- --- Require fiber to Require fiber to Short and pointed Short and pointed Big & flattened incisors Short and pointed stimulate peristalsis stimulate peristalsis incisors incisors incisors Intestine 1.5 to 3 times Intestine 3 times body Intestine 20 times Intestine 9 times body Blade shaped molars Blade shaped/crushing Flattened & strong Flattened molars body length length body length length molars molars No lateral or forward No lateral or forward Great lateral & Great lateral & Not metabolize Not metabolize Metabolize cellulose Not metabolize motility of jaws motility of jaws forward motility of forward motility of cellulose cellulose cellulose jaws jaws Complete digestion 2 Complete digestion 6 Complete digestion 24 Complete digestion 12 Shear & swallow w/o Shear & No shear, chew much No shear, chew their to 4 hs to 10 hs to 48 hs to 18 hs chewing swallow/crushing foods 75 76 19  1/20/25 Types of Digestion in Animals Herbivores 1. Enzymatic digestion Ruminants Simple stomach herbivores 2. Microbial Digestion: Fermentation Digestion in carnivores is mostly enzymatic Digestion in herbivores is microbial and enzymatic The consequence of microbial and enzymatic digestion is reversed in ruminant and simple stomach herbivores 77 78 The number and diversity of ruminants is large 79 80 20  1/20/25 Carbohydrates Classes of carbohydrates The major carbohydrate groups in our diet are: Polysaccharides (starches; complex carbohydrates) Disaccharides (sucrose, maltose, lactose) Monosaccharides (glucose, fructose, galactose) Most carbs are ingested as starches. 81 82 Classes of carbohydrates Most carbs are ingested as starches. The most commonly ingested sugar is sucrose (table sugar) and lactose (from milk). 83 84 21  1/20/25 Chemical digestion of carbohydrates Chemical digestion of carbohydrates polysaccharides The major steps in digestion of carbohydrates are summarized below: Starch (polysaccharides) Amylases Amylases Disaccharides Brush border enzymes: Brush border enzymes: Disaccharides Dextrinase Dextrinase Glucoamylase Glucoamylase Monosaccharides Monosaccharides Lactase Lactase (simple sugars) Maltase Maltase Sucrase Sucrase 85 86 Enzyme Target Monosaccharide Lactase Lactose Galactose Glucose Sucrase Sucrose Fructose Glucose Maltase Maltose Multiple molecules of glucose a-dextrinase Small glucose Multiple polymers molecules of glucose 87 88 22  1/20/25 Digestion of Carbs in the Mouth & Stomach Chewing mixes food with salivary secretions containing Digestion of Carbs in the Mouth and ptyalin (an a-amylase) Stomach Ptyalin hydrolyzes starch into maltose and other small People don’t chew their food very long, so the glucose polymers salivary amylase is inactivated by the stomach pH, Up to 30-40% of the starches can be hydrolyzed to and the rest of the complex carbs get digested maltose in the mouth and stomach. from pancreatic amylase. 89 90 Digestion of carbohydrates Digestion of Carbs in the Small Intestine Salivary amylase Digestion by Pancreatic Amylase – Pancreatic secretions contain a large quantity of amylase Pancreatic amylase – Pancreatic amylase is more potent than salivary source Dextrinase Glucoamylase – Within 30 mins. almost all of the carbohydrates are hydrolyzed to maltose (and other small glucose Lactase polymers) in the duodenum and jejunum Maltase Sucrase 91 92 23  1/20/25 Absorbing the products of carbohydrate digestion Hydrolysis of Disaccharides and Disaccharide sugars, lactose, sucrose, maltose are Small Glucose Polymers digested by brush border enzymes, lactase, sucrase The final products of CHO hydrolysis are all and maltase, respectively monosaccharides The monosaccharides are water soluble and are absorbed immediately into the blood stream Lactase breaks lactose into galactose and glucose. Maltase breaks maltose into 2 molecules of glucose. Typical Breakdown Products: Sucrase breaks sucrose into glucose and fructose. 80% glucose Monosaccharides formed are galactose, glucose 10% galactose fructose; these are internalized by the absorptive cell 10% fructose and diffuse out into caps of the intestinal villi 93 94 Absorbing the products of Post-absorption events carbohydrate digestion Disaccharides: Blood leaving the villi of the small intestine drain into blood Lactose vessels that lead directly to the Sucrose (Brush liver – the hepatic portal system Maltose border Blood leaving the villi is rich in enzymes nutrients; is this nutrient load Absorptive cell are, in no were allowed to enter the Capillary Monosaccharides: order, general circulation blood Lactase viscosity would be increased Galactose with possible adverse affects on Sucrase blood pressure Glucose Maltase) DIFFUSION Fructose 95 96 24  1/20/25 Post-absorption events Post-absorption events As the blood passes through the blood vessels of the liver, much of the nutrient load is removed – this is referred to as the first pass effect 97 98 Post-absorptive events: simple sugars Post-absorptive events: simple sugars Glucose: essential nutrient for energy Later the glycogen is broken back down production by many cells to simple sugars (or liver can make glucose from non carb substances like fats and proteins) Glucose in excess of immediate needs can be converted into a polysaccharide Glucose monomers Glycogen polymer (storage) compound, glycogen, and stored in liver and muscle 99 100 25  1/20/25 Post-absorptive events: simple sugars Insulin helps bring glucose into cells Critical Thinking - Glucose up in bloodstream, insulin released by pancreas, insulin binds to cell surface receptors, triggers glucose uptake by facilitated diffusion. -explore…. 101 102 Simple sugars and insulin Glycemic Index Chart Simple sugars like glucose are taken into cells along with insulin. Some foods have a high “Glycemic Index” – that is, they can make your blood sugars rise quickly after a meal. http://buckwheathealth.wordpress.com/tag/glycemic-index/ 103 104 26  1/20/25 Glycemic Index & Insulin 100 calories from complex sugars (+AA, electrolytes, caffeine) NYC Marathon 105 106 Dr. David Jenkins UofT: Two problems: 1. Mice on the high-GI diet had ~2X the body fat of those on Diabetes Patients the low-GI diet after 9 weeks. Rats fed the high-GI diet were 71% fatter and had 8% less lean body mass than the low-GI group. Canada Research Chair in Nutrition and Metabolism Director, Clinical Nutrition and Risk Factor Modification Centre Lancet 2004:364:778-85 2. Which foods are best for people with diabetes – insulin loss (or resistance) affects blood glucose levels. 107 108 27  1/20/25 Insulin and Glucose Google for any video online if you want a general summary of how they work. Khan Academy is usually pretty good. 109 110 e.g. Atkins, Keto, Bernstein, NSNG (no sugar no grains) 111 112 28  1/20/25 GLP1 receptor agonists & type 2 diabetes 113 114 115 116 29  1/20/25 Cholesterol ?? POTENTIAL BENEFITS Type 2 diabetes Weight Loss Reducing CV risk Better insulin & glucose control Meats, esp. organ meats BUT POTENTIAL DOWNSIDES & RISKS GI side effects - lifelong thyroid tumors pancreatitis hypoglycemia Esp. yolks Butter, oils, cream 117 118 Post-absorptive events: cholesterol Cholesterol is a small molecule that is absorbed from food without being digested Most blood cholesterol (85%) is produced by the liver; only 15% comes from the diet ** Cholesterol is absorbed without digestion 85% 15% 119 120 30  1/20/25 Post-absorptive events: Post-absorptive events: cholesterol cholesterol Cholesterol is transported in the blood Cholesterol is a major component of cell associated with lipoproteins membranes, and it is the base substrate for the synthesis of steroid hormones (HDL/VDL/etc) 121 122 Classes of lipids Lipids - fats The major lipid groups in our diet are: Triglycerides Phospholipids Sterols (such as cholesterol) 123 124 31  1/20/25 Chemical digestion of lipids Digestion of Fats in the Stomach Because lipids are not water soluble, there A very small amount of fat digestion occurs are problems associated with lipid in the stomach digestion that are not found for protein and carb digestion. Triglycerides are digested by lingual lipase swallowed with saliva Basically, the enzymes can’t get to them! Accounts for < 10% of fat digestion 125 126 Chemical digestion of lipids Chemical digestion of lipids Enzymes in solution have only limited access to the molecules of insoluble lipid The enzymes that digest triglycerides and phospholipids (called lipases) are only Bile salts help lipid digestion by emulsifying present in pancreatic juices; thus lipid lipids – they are broken into small droplets; digestion can only occur in the small this has the affect of greatly enhancing intestine (following emulsification by bile access of the lipases to lipid molecules salts) Gall bladder/ Pancrea bile s/ lipases Sm. Intestine duodenum 127 128 32  1/20/25 Digestion of Triglycerides by Chemical digestion of lipids Pancreatic Enzymes The most important enzyme responsible for digesting triglycerides is pancreatic lipase Pancreatic juice contains an enormous amount of lipase - within 1 minute there is enough pancreatic lipase secreted to digest all of the triglycerides that it reaches Pancreatic lipase splits triglycerides to free fatty acids and monoglycerides 129 130 Chemical digestion of lipids Chemical digestion of lipids The major steps in digestion of lipids are shown below: Triglycerides Pancreatic lipase Monoglycerides Fatty acids 131 132 33  1/20/25 Digestion of lipids Absorbing the products of lipid digestion The products of triglyceride and phospholipid digestion (by lipase) are monoglycerides, fatty acids and Pancreatic lipase glycerol; these diffuse into the (active through absorptive cells small intestine) The absorptive cells re-synthesize triglycerides from monoglycerides and fatty acids, and droplets of triglyceride leave the cells by exocytosis; these enter the lacteales of the villi 133 134 Absorbing lipid digestion products Chemical digestion of lipids Lacteals Monoglycerides Fatty acids Absorptive cell Fatty acids Triglycerides Monoglycerides Chylomicrons are secreted into central lacteals of the intestinal EXOCYTOSIS villi, passing via the lymphatic system until eventually they reach the blood through the TD. 135 136 34  1/20/25 In the blood, the chylomicrons add ApoE protein, which Proteins helps it to bind to capillaries in the target muscles and adipose tissue. Digested in target tissues by lipoprotein lipase, releasing the fatty acids - used by the skeletal muscles for energy, adipose for synthesis of stored fat. After the triglycerides are broken up, the depleted chylomicron particle back through circulation to the liver. 137 138 Proteins Proteins are polymers of amino acids Protein digestion (by proteases) produces large polypeptides (shorter chains of amino acids) Digestion of these large polypeptides (by peptidases) produces smaller amino acid chains (peptides) Peptides are the digested (by peptidases) into amino acids Proteases Peptidases Peptidases Peptides: Ala-Arg & Asp-Asn-Cys Amino Acids: “Ala” “Arg” “Asp” “Asn” “Cys” 139 140 35  1/20/25 Chemical digestion of proteins Digestion of proteins The major steps in digestion of proteins are shown below: Protease (pepsin) Protein (A-A-A-A…..) Proteases Pancreatic Large polypeptides (A-A-A..) proteases and Peptidases peptidases Small polypeptides and Small peptides (A-A…) intestine Brush border brush border peptidases peptidases Amino acids (A) 141 142 Digestion of proteins Digestion of Proteins in the Stomach Protease Pepsin - the important proteinase in the (pepsin) stomach – Functions at a pH of 2.0-3.0 – Inhibited at pH > 5.0 Pancreatic proteases and – One of the only enzymes capable of digesting collagen peptidases Collagen is a main component of connective tissue found in meats In order for meat digestion to occur, collagen must first be Small broken down by pepsin intestine peptidases 143 144 36  1/20/25 Digestion of Proteins by the Stomach Digestion of Proteins by Pancreatic and Pancreatic Secretions Secretions Pancreatic secretions contain a number of proteolytic enzymes that act in the duodenum Pepsin and upper jejunum Partial breakdown products generated from the stomach stimulate the release of pancreatic Protein proteolytic enzymes: – Trypsin Proteases Peptones Trypsin – Chymotrypsin Polypeptides Chymotrypsin – Carboxypeptidase Carboxypolypeptidase – Proelastase ProElastase 145 146 Digestion of proteins Digestion of Proteins in Small Intestinal Villi Enterocytes contain other peptidases that are Protease specific for linkages between amino acids. (pepsin) Endopeptidases (like trypsin) cleave internal bonds). Exopeptidases cleave off the ends. Pancreatic proteases and Digest remaining Di- and Tri-peptides into peptidases single amino acids Small > 99% of final protein product are single amino intestine acids that pass into the blood stream peptidases 147 148 37  1/20/25 Absorbing the products of protein Absorbing the products of digestion protein digestion AAs transported into absorptive cells by Transporter transport proteins (carrier-mediated transport) Peptidase Small peptides Peptides are digested by brush border Amino acids peptidases; the AAs formed are transported by a transporter associated Absorptive cell with the enzyme into the absorptive cells Transport proteins carry the AAs out of Amino acids the absorptive cells to the capillaries of Capillary the villi Amino acids 149 150 Post-absorptive events: amino acids Absorption of Other Ions Calcium Ions (Ca2+) AAs cannot be stored, except as Active absorption into the blood components of cell protein Controlled stringently Consequently, AAs in excess of immediate – Parathyroid Hormone - activates Vit. D - enhances calcium absorption needs are immediately metabolized by liver cells Iron Ions (Fe+) The nitrogen (amine) unit of the AA is Particularly important for the formation of Hb removed to make urea; the remaining part of the molecule is converted into lipid K+, Mg2+, phosphates Urea is excreted as a component of urine Direct absorption through the intestinal mucosa 151 152 38  1/20/25 Vitamin Absorption Absorption of Vitamins Water-soluble vitamins (B1 thiamine; B2 riboflavin; B3 niacin; B6 pyridoxine; pantothenic Vitamin absorption occurs mainly by the passive acid; biotin; folic acid, B12; Vit C ascorbic acid) process of diffusion in the jejunum and ileum diffuse into the blood, except for vitamin B12. This vitamin combines with intrinsic factor produced Fat-soluble vitamins are absorbed with dietary by the stomach, which the intestine absorbs by lipids (Vitamins A, E, D, K) endocytosis. Water-soluble vitamins pass into the urine when their concentration in plasma exceeds the renal capacity for reabsorption. 153 154 Learning Outcomes 1. Carbohydrate metabolism - Critical thinking – sugar, insulin 2. Cholesterol metabolism - Critical thinking – genetics vs. diet 3. Lipid metabolism - Critical thinking – fats are good for you 4. Protein metabolism 5. Absorption of Ions and Vitamins 155 39