Digestions of Food: The Alimentary Canal (Gut) 1) MOUTH where food is chewed and swallowed Hydrolysis of starch to maltose by salivary amylase Alkaline conditions assist to break the glycosidic bonds in starch 2) Food travels down the OESOPHAGUS by peristalsis 3) To the STOMACH Acidity kills bacteria / Inhibits salivary amylase Gastric glands are stimulated by gastrin to secrete gastric juice Contains HCl and pepsinogen (inactive pepsin) Active pepsin digests proteins → would damage glandular tissue Damage to stomach wall by acidic gastric juice is prevented by mucus Stomach digests proteins by hydrolytic endopeptidase pepsinogen + HCl + pepsin → pepsin Endopeptidases (pepsin/trypsin) break peptide bonds in the middle of polypeptides SUBTRATE PRODUCTS ENZYME PRODUCED BY Proteins Smaller polypeptides Endopeptidase: Pepsin Gastric glands 4) To the SMALL INTESTINE (duodenum → ileum) Liver in the upper abdomen secretes bile Liver detoxifies blood by removing poisonous substances / destroys old red blood cells / converts Hb to bilirubin (present in bile) / produces bile / produces urea from amino groups and ammonia Gall bladder stores bile Secreted into small intestine by bile duct Muscles in wall of small intestine mix H2O and oil forming small droplets/emulsion Larger surface area / higher lipase activity Bile prevents droplets from running together Exocrine gland of the pancreas secretes pancreatic juice into the duodenum Contains amylase, lipase, exopeptidase, trypsinogen (inactive trypsin) Exopeptidases break peptide bonds at the end of smaller polypeptide chains Intestinal brush border contains enterokinase Trypsinogen + enterokinase → trypsin Intestinal brush border contains peptidase Intestinal brush border contains maltase Maltose in lumen of small intestine binds to maltase Resulting glucose diffuses into the cytoplasm of epithelial cells Glucoseis also released back into the intestinal lumen and absorbed further down Thus, duodenum digests food by HYDROLYTIC enzymes (→H2O) Ileum absorbs food Last and longest part of small intestine Microvilli (brush border) increase surface area SUBTRATE PRODUCTS ENZYME PRODUCED BY Starch Maltose Pancreatic amylase Pancreas Maltose Glucose Maltase Intestinal cells Proteins Smaller polypeptides Endopeptidase: trypsin Pancreas Smaller polypeptides Amino acids; Dipeptides Exopeptidase Pancreas Dipeptides Amino acids Peptidase Intestinal cells Triglycerides Glycerol; Monoglycerides; Fatty Acids Lipase Pancreas 5) To the LARGE INTESTINE (cecum → colon → rectum) to anus Larger in diameter than small intestine but shorter in length Stores undigested food before it is egested as faeces Lactose and Lactose Intolerance Lactase splits lactose into β-glucose and galactose Lactose intolerant person lacks lactase → lactose is neither digested nor absorbed High levels of soluble lactose remain in small intestine Supports large populations of bacteria / ferment lactose producing gas / causing discomfort Water potential becomes more negative / H2O moves into small intestine / not reabsorbed / diarrhoea Adults rarely produce lactase / gene is switched off in adulthood Absorption of Products of Digestion Histology of the Ileum in Relation to its Secretory and Absorptive Functions Na+, Cl-, digestive juice secreted into duodenum → LOWERS water potential Thus, H2O moves from epithelial cells into lumen by osmosis Increases efficiency of digestion (hydrolytic reactions) and absorption Ileum absorbs ions by active transport → INCREASES water potential Thus, H2O moves back into epithelial cells The Layers of the Gut Wall and the Ultrastructure of the Epithelium Hollow organs with a layer of epithelial cells surrounding the lumen Walls of the lumen contain muscles and blood vessels Small change in structure has a specific function Small intestine Tube with a thick wall surrounding a hollow lumen Epithelial cells have microvilli on their surface Epithelial cells secretes mucus Absorption And Active Uptake Of The Products Of Digestion In Small Intestine Glucose Absorbed by epithelial cells using a protein carrier This protein carrier works by secondary active transport (co-transport system) Glucose and Na+ are transported across the membrane into the intestinal cell Further transport of glucose into blood capillaries by facilitated diffusion NOTE: Fructose moves entirely by facilitated diffusion! Amino Acids Absorbed by epithelial cells by secondary active transport Co-transport carrier proteins absorb amino acids and Na+ across the membrane Different carrier molecules transport different types of amino acids Carriers are associated with peptidase Passes from the epithelial cells into capillaries by facilitated diffusion Newborns don't produce trypsin, HCl → proteins are not digested before small intestine is reached Whole proteins may be transported by endocytosis and exocytosis Uptake by endocytosis, release into blood by exocytosis Often occurs in newborns due to their immature mucosa Allows passage of antibodies from mother's milk - provides passive immunity for the infant Accounts for many early food allergies as the protein is recognized as "foreign" Lipids Triglycerides digested into monogylcerate + glycerol + fatty acids by lipase Monoglycerides combine with bile to form micelles 5mm in diameter / forms an emulsion / contains fatty acids and glycerol Micelles move to membrane of epithelial cells Monoglycerides + glycerol + fatty acids dissolve in bilayer Triglycerides re-synthesise in cytoplasm / move into lymph capillaries (→lacteals) Bile stays in small intestine Oral Rehydration Therapy In The Control Of Gastro-Intestinal Infections High amounts of semi-liquid faeces result form toxins produced by microorganisms Toxins block Na+ channels in cells lining small intestine Stop reabsorption / conc. of Na+ ions in small intestine increases Water potential gradient is in the opposite way (into small intestine) Water is drawn out of epithelial cells and added to the contents of the gut This results in diarrhoea Toxins have little effect on glucose co-transport carrier proteins TREATMENT: measured amounts of glucose and mineral salts are mixed with H2O Drinking the solution stimulates Na+ and glucose uptake by co-transport proteins H2O is absorbed from small intestine ORT increases performance of co-transport proteins / adequate amounts of glucose and Na+ pass into intestinal cells / clears up attack of diarrhoea Thus, Na+ is absorbed by Na+ channels AND mainly by glucose-Na+-co-transport carrier proteins Control Of Digestive Secretions Nervous And Hormonal Control Of Salivary, Gastric And Pancreatic Secretions Mammal has 2 communication systems → nervous and endocrine system Nervous system is based on electrical impulses passing along nerve cells Short-lasting effects, can be switched on or off rapidly Secretes salvia when food enters mouth Endocrine system is based on hormones Travel in blood to target organ Produce long-lasting effects Trigger secretion of bile and pancreatic juice Endocrine system is only activated with large amounts of food intake Food takes a long time to reach small intestine Food stays there for a long time Digestive juice can be secreted as large amounts of food are present Digestive juice contains trypsin and pepsin → both enzymes are peptidases which damage proteins → they would damage epithelial cells if only small amounts of food would be present Importance Of Simple And Conditioned Reflexes And The Hormones Nervous reflexes Nerve pathway involving small number of nerve cells (2/3) → rapid response Automatic response → particular stimulus has same effect Condition reflex Salvia and gastric juice are secreted By various stimuli associated with food (smell/sight/sound) By contact of substances in food with taste buds on tongue Hormones Secreted in response to presence of food in particular region of gut Hormones travel in blood to glands / in glands, stimulate secretion of digestive juices GASTRIN stimulates exocrine glands in stomach to release gastric juice Acids (chyme) from stomach, fatty acids in duodenum stimulate release of SECRETIN Stimulates secretion of alkali (bicarbonate ions) from pancreas Neutralises acidity from intestinal contents When pH reaches neutrality, secretion of secretin is inhibited Inhibits gastric gland secretion Acidic chyme from stomach, fat, amino acids in duodenum stimulate release of CHOLECYSTOKININ-PANCREOZYMIN CCK-PZ Activates smooth muscle contraction/emptying of gall bladder (to release bile) Triggers secretion of enzymes from pancreas Stimulates Medulla oblongata which give a satiety signal Once molecules stimulating CCK are digested → CCK inhibited again SOMATOSTATIN Acts on stomach, duodenum, pancreas Inhibits release of gastrin, secretin, and CCK-PZ