0212 - Exocrine Pancreas and Digestion Flashcards
Where are carbohydrates digested?
5% via salivary amylase, initially in mouth and until inactivated by stomach acid.
Most in small intestine (D&J) by pancreatic amylase.
Disaccharides digested by enzymes in microvilli.
Where are fats digested? What is required?
Mostly in small intestine with very small amount in stomach (lingual lipase). Requires bile salts to emulsify fat into micelles for water-soluble digestive enzymes to work.
Where is protein digested? What is required?
Mostly in the duodenum, by pancreatic proteases, though some in stomach with pepsin. This leaves di- and tri-peptides, which are digested by peptidases in the microvilli. Some final digestion by peptidases inside enterocytes before individual AA’s are absorbed.
How are carbohydrates digested?
Amylases (salivary and pancreas) hydrolise the glycosidic linkages, leaving oligo- and disaccharides. These are metabolised into monosaccharides (80% glucose) by disaccharidases in the borders of enterocytes.
How are carbohydrates absorbed?
Glucose and galactose are absorbed by Na-driven transport (SGLT), fructose by facilitated diffusion (Na-independent).
How are lipids digested?
90% dietary lipids are TAGs (remainder cholesterol, cholesterol ester, phospholipid and FFA). Emulsified with bile salts and lecithin in the duodenum (micelles, increase surface area). Pancreatic lipase hydrolises TAGs to mono-glycerides and FFA. Co-lipase (cofactor) is required both to bind to water-lipid interface on micelle and activate lipase.
Where and how are lipids absorbed?
100% in jejunum, via microvilli. FFAs and MAGs diffuse into enterocytes, where they are converted to triglycerides, made into chylomicrons, and released into lymph (enter blood at thoracic duct). Bile salts resorbed by active processes in ileum.
How are proteins digested?
2 phases of proteolysis - proteins ->polypeptides by trypsin and chymotrypsin, whereas carboxypeptidase cleaves proteins->amino acids. Sites of action are very specific.
Peptidases are inactive when secreted, and activated by pH change or HCl itself in the stomach (pepsinogens), or by duodenal enteropeptidase (pancreatic trypsinogen).
Trypsinogen then cascades to activate other pancreatic peptidases (trypsin inhibitor also secreted in pancreas in concert).
HCO3- essential for optimal protease activity (145vs25mmol/L).
How are proteins absorbed?
Membrane-bound endopeptidases, dipeptidases and amninopeptidases cleave into AAs, di and tri-peptides. AA’s absorbed by very specific carriers.
What causes the release of Gastrin? What does it do?
Stomach stretch and presence of protein causes gastrin release. Gastrin stimulates the release of gastric juices (acid and pepsinogen).
What is the enterogastric inhibitory reflex?
Presence of food in duodenum causes a reflex for the stomach to slow chyme delivery to a rate at which it can be absorbed.
What are the two major signals for exocrine pancreatic release? What happens next?
Secretin (from D and J mucosa) - when acid enters duodenum. Stimulates Water and Bicarb release (neutralise acid)
Cholecystokinin (CCK) - from D and J - when fat or aa enter duodenum. Stimulates pancreatic acinar cells to produce digestive enzymes.
Most digestion occurs via hydrolysis. What is hydrolysis?
Reaction that uses water to break a molecular bond.
Why does the stomach produce HCl?
Start to denature protein, and provide the pH for most digestive enzymes to work optimally.
