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Digestion This demo only contains a sample of the full content Chemical digestion The hydrolysis of large polymers e.g. proteins/starch into smaller, soluble, diffusible monomers e.g. amino acids and glucose. Starch CH2OH O OH CH2OH O OH CH2OH O OH O OH O OH OH n Glucose amylose OH CH2OH O H OH OH CH2OH O OH O CH2OH O OH OH amylopectin O OH n OH CH2 O OH O OH OH H OH b-glucose Monomers can then be absorbed across the gut lining and assimilated into polymers e.g. glycogen again. OH O a-glucose CH2OH O OH OH Digestion Food moves through the digestive tract by contractile waves of smooth muscle – peristalsis. This muscle is myogenic but the parasympathetic nervous system stimulates activity, the sympathetic inhibits it. There are a number of different juices involved in the digestion process. Mouth • Moistening and physical breakdown of food • Very little enzymatic digestion by amylase before food is swallowed Stomach • Hydrochloric acid kills micro-organisms • Physical churning • Peptidases begin initial protein digestion Small intestine Entry of acidic stomach fluids (chyme) triggers hormonal feedback loops which cause the release of pancreatic juices: • Sodium bicarbonate neutralizes the acidic chyme • Lipase digests fats • Amylases digest carbohydrates • RNase/DNase digest nucleotides • Trypsin and chymotrypsin digest protein • Bile from the gall bladder emulsifies/increases surface area of fats so that they can be attacked by the lipase Large intestine Water re-absorption and compaction and storage of faeces. Extracellular digestion : Stomach Protein digestion • The enzyme pepsin begins protein digestion in the stomach. • Pepsin is an endopeptidase which attacks peptide bonds inside the molecules. Produces peptides of varying length. What is produced What the juice does Hydrochloric acid • • • • Pepsinogen Activated to pepsin which digests proteins to peptides Rennin Only in babies where, together with Ca2+, it coagulates milk proteins to curd making it more digestible Lipase Digests fats to fatty acids and monoglycerides Intrinsic factor Promotes vitamin B12 absorption which stimulates red blood cell formation Provides an optimum pH for stomach enzymes. Disinfects the food Changes inactive pepsinogen to pepsin Promotes absorption of ions Stomach structure & adaptations Gastric pits Gastric pits Mucous cell Mucosa Gastric glands parietal cell peptic cell submucosa • • • Gastric pits contain gastric glands that produce HCl. Intrinsic factor and pepsinogen Mucous cells produce mucus to prevent auto-digestion Muscle layers to churn food Pancreactic and liver functions Bile duct from liver Duodenum Stomach blood Endocrine portion of pancreas (Islets of Langerhans) Duct cells Secrete aqueous NaHCO3 solution Acinar cells Secrete digestive enzymes including amylase Exocrine portion of pancreas Duodenum & ileum adaptations Duodenum and ileum have adaptations to maximise absorption: • • • • Large size: 6m long, 350m2. Folded inner wall increases surface area. Villi –projections of inner wall – move and increase surface area. Surface epithelial cells have microvilli which hugely increase the surface area. Mucosa Submucosa Muscle layers Capillary bed microvilli lacteal Duct Lumen Villi Villi Structure to function: absorption in the ileum Pancreas, liver and other functions Pancreas Involved in carbohydrate & protein digestion Bile from the liver What is produced What the juice does Amylase Digests starch & glycogen to maltose Lipase Digests fats to fatty acids and monoglycerides Trypsinogen Activated to trypsin which digests proteins to peptides and which activates chymotrypsinogen Chymotrypsinogen Forms chymotrypsin which digests proteins to peptides Carboxypeptidase Digests peptides to amino acids What is produced What the juice does Bile salts Emulsifies fats into 1 micrometer droplets which increase the surface area for lipase action. Also aids the absorption of fats (Bile pigments) e.g. bilirubin Breakdown products of haemoglobin which will be excreted in the faeces Succus entericus (small intestine) & Brunner’s gland (Duodenum) What is produced What the juice does Aminopeptidase Digests peptides to amino acids Maltase Digests maltose to glucose Sucrase Digests sucrose to glucose and fructose Lactase Digests lactose to glucose and galactose Enterokinase Activates trypsinogen to trypsin Protein Digestion : Small intestine Digestion of proteins • • Pancreatic secretions are a mixture of peptide-digesting enzymes. Trypsin and chymotrypsin endopeptidases release short chain dipeptides and tripeptides (2 or 3 amino acids). • • Dipeptides and tripeptides may be absorbed directly. Alternatively exopeptidases complete peptide digestion. Dipeptides/ tripeptides exopeptidases Amino acids Exopeptidases cleave amino acids from the edges of the peptide chain, hydrolysing inwards. Digestion: Summary Intermediate Digestion Nutrient Final Digestion Carbohydrate Sugar polymer Amylase Protein Amylases Pepsin, Trypsin Proteases Triglyceride Monosaccharides (glucose, galactose, fructose) Maltase peptide fragments Protein Chymotrypsin, Carboxypeptidase Fats disaccharide Amino acids Aminopeptidase Monoglyceride Lipase Lipases Free fatty acids
