NFS284 Chapter 3 Review Notes

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University of Toronto St. George
Nutritional Science
Tom Wolever

Nutrition: Science and Applications Chapter 3 – Diet, Absorption, and Metabolism 3.1 Cells and tissues - There are four major types of tissue o Muscle o Nerve o Epithelial o Connective - The stomach contains many types of tissues o Nervous tissue and connective tissue which signal the brain when it is empty or full o Muscle tissue which churn ingested food o Specialized epithelial cells which produce acids and enzymes 3.2 An overview of the digestive system - The structure of the digestive system o Gastrointestinal tract  Mouth  Chews food and mixes it with saliva  Pharynx  Swallows chewed food mixed with saliva  Esophagus  Moves food to the stomach  Stomach  Churns and mixes food; Secretes acid and a protein-digesting enzyme  Small intestine  Completes digestion; Absorbs nutrients into blood or lymph  Large intestine  Absorbs water and some vitamins and minerals; Home to intestinal bacterial; Passes waste material  Anus  Opens to allow waste to leave the body o Accessory organs  Salivary glands  Produce saliva, which contains a starch-digesting enzyme  Liver  Makes bile, which aids in digestion and absorption of fat  Pancreas  Releases bicarbonate to neutralize intestinal contents; Produces enzymes that digest carbohydrate, protein, and fat  Gall bladder  Stores bile and releases it into the small intestine when needed - Transit time  The time it takes food to pass the length of the GI tract, typically 24-72 hours in a healthy adult o Indigestible food components will travel the full length digestive tract to be eliminated in the feces o Digestible food components will enter the cells of the small intestine, where absorption takes place o To measure transit time, a non-absorbable dye is added to a meal and the time it takes to appear in the feces is recorded o Affected by dietary fibre - The structure of the small intestine wall o Four layers of tissue  Mucosa  Layer of tissue lining the lumen  Function to absorb nutrients  Secrete mucus  Connective tissue  Smooth muscle layers  Outer connective tissue layer o Lumen is the space inside of the tube of GI tract - Digestive secretions  Aids digestion inside the lumen of the GI tract o Mucus  To protect the GI tract from the action of enzymes  To protect cells from autodigestion  Cells in the mucosa layer are relatively short-lived (2-5 days)  Continually dying off and re-growing  Intestinal cells need high levels of nutrients  One of the first parts of the body affected by nutrient deficiency o Digestive enzymes  Amylase breaks down starch (e.g., break large carbohydrate molecules into smaller ones) - Enzyme functions  Different enzymes are needed to breakdown different nutrients o Located in mouth  Salivary amylase  Breaks starch into smaller carbohydrate molecules o Located in stomach  Rennin  Causes the milk protein casein to curdle  Pepsin  Breaks proteins into polypeptides and amino acids o Located in pancreas  Trypsin  Breaks proteins and polypeptides into shorter polypeptides  Chymotrypsin  Breaks proteins and polypeptides into shorter polypeptides  Carboxypeptidase  Breaks polypeptides into amino acids  Pancreatic lipase  Breaks triglycerides into monoglycerides, fatty acids, and glycerol  Pancreatic amylase  Breaks starch into shorter glucose chains and maltose o Located in small intestine  Carboxypeptidase, aminopeptidase, dipeptidase  Breaks polypeptides into amino acids  Lipase  Breaks monoglycerides into fatty acids and glycerol  Sucrose  Breaks sucrose into glucose and fructose  Lactase  Breaks lactose into glucose and galactose  Maltase  Breaks maltose into glucose  Dextrinase  Breaks short chains of glucose into individual glucose molecules - The regulation of gastrointestinal function o Nerve signals from the brain to the GI tract and from the GI tract to the brain help to regulate the function of the digestive system o Nerve signals are also sent between different parts of the GI tract o Hormones are produced by accessory organs and cells in the gut  Released into the blood stream  Help prepare different parts of the gut for arrival of food  Regulate the digestion of nutrients  Regulate the rate that foods move through the system - The gastrointestinal tract and immune function o The mucus secreted by cells of the digestive tract  Acts as a barrier to infection from harmful micro-organisms that we ingest along with our food  Limits the absorption of toxins and disease-causing organisms  If this barrier is unsuccessful, the immune system is able to mount a response  White blood cells of the immune system help to fight infection  Phagocytes (e.g., macrophages)  Lymphocytes (e.g., B cells, helper T-cells, cytotoxic T-cells)  The immune system is also vulnerable to nutrient deficiency o Mechanism  Phagocytes  The first type of cells to respond to infection  Engulfs the harmful organism, break it down, and present antigens  An antigen is a protein found in a micro-organism that promotes an immune response  Lymphocytes  The presentation of antigens stimulates the production of antibodies  An antibody will bind to any microorganism containing that specific antigen  Makes it easier for phagocytes to detect micro-organisms  Certain types of lymphocytes can also bind to body cells that are infected  An immune response to proteins present in food (e.g., allergens) results in food allergies 3.3 Digestion and absorption - Digestion  The process of breaking food into component parts small enough to be absorbed into the body - Absorption  The process of taking substances into the interior of the body - Mouth  The entry point for food into the digestive tract o Digestion begins in the mouth o Food is mixed with saliva called a bolus o Lysozyme  An enzyme that kills the bacteria responsible for decay o Salivary amylase  An enzyme that begins the breakdown of starch - Pharynx  Connects the nasal passages and mouth to the respiratory passages and esophagus o During swallowing, a piece of elastic tissue called the epiglottis covers the opening to the lungs, to ensure that the bolus of food passes to the stomach and not the lungs - Esophagus  Connects the pharynx to the stomach o Peristalsis  Alternating circular and longitudinal contraction that move food through the GI tract o Sphincters  Muscular valves that separate organs in the GI tract  Esophagus and stomach are separated by the gastroesophageal sphincter  Stomach and small intestine are separated by pyloric sphincter  Small intestine and large intestine are separated by ileocecal valve - Stomach  Muscular structure that contains specialized cells to produce acid for mixing o Bolus is mixed with acidic gastric juices to form a semi-liquid mixture called a chyme o Gastric pits  Contain specialized cells that secrete various products (e.g., gastric juice) o The composition of gastric juice  Parietal cells  Produces hydrochloric acid  Produces intrinsic factor, which is needed for vitamin B12 absorption  Chief cells  Produces pepsinogen, which is an inactive form of pepsin  When pepsinogen + HCl  pepsin, which is an active enzyme that breaks proteins into shorter chains of amino acids called polypeptides (e.g., protein  polypeptides) o The regulation of stomach motility and secretion by nerves and hormones  Thought, sight, smell, or taste of food  brain signals the stomach to prepare the stomach  Food entering the stomach  stretching (distension) of the stomach muscles  stomach signals the brain  increased secretion of gastrin  increased secretion of gastric juice and stomach motility  Chyme moving out of the stomach  pass through the pyloric sphincter  chyme empties the stomach at a controlled rate (2-6 hours to empty the stomach)  Food entering the small intestine  stretching (distension) of the intestine  nerve signals and hormonal signals the stomach  reduced gastric secretion and stomach motility  chyme enters the small intestine at a controlled rate - Small intestine  Major site for digestion and absorption o Maximizes the absorption of nutrients  Surface area of intestinal wall is increased by large surface folds  Surface folds are covered with finger-like projections called villi  Contains a blood vessel and a lymph vessel (also called a lacteal)  Nutrients pass through mucosal cell layer on the surface of the villi o Enter either the blood or the lymph o Transported to the tissues of the body  Each villi is covered with microvilli (also called the brush border)  Located on the mucosal cell layer that face the lumen  Enterocytes  Mucosal cells with microvilli - The enzymes and secretions in the small intestine and accessory organs o Pancreas  An accessory organ required for normal digestion and absorption in the small intestine  Secretes bicarbonate ions which neutralize the HCl in chyme  Secretes digestive enzymes  Amylase  Breakdown starch into sugars that was started by salivary amylase  Lipases  Breakdown triglycerides into fatty acids  Proteases (e.g., trypsin, chymotrypsin)  Breakdown proteins o Gall bladder  An accessory organ required for normal digestion and absorption in the small intestine  Stores bile, which is produced in the liver  Bile  Emulsifies fat, or breaks it into smaller droplets, so it can be absorbed o Small intestine  Intestinal juice  A watery and mucus-containing mixture that helps to mix chyme with digestive juices  Brush border enzymes  Breakdown small polypeptides into amino acids  Dige
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