GI tract absorption

• duodenum • jejunum • ileum

• it has circular folds which increase the surface area and allow the spiralling of chyme (this allows chyme to come into contact with membrane bound enzymes) • it contains lymph nodules known as Peyer’s patches

• 20 - 25 cm long • absorbs 9 litres of water per day

• 1 m long • absorbs 3 - 4 litres of water per day

• 2 - 3 m long • absorbs 1 litre of water per day

• enterocytes • goblet cells • paneth cells • I cells • S cells • D cells

• columnar epithelial cells • have microvilli on their ends which creates a brush border

they secrete mucus to reduce friction

they have an immunological function and produce lysozymes

release CCK

release secretin

release somatostatin

• they have microvilli creating a brush border • contain lacteals which are involved in the absorption of fats • there is a capillary network which has a high blood flow and a countercurrent flow which maximises absorption • found over intestinal crypts • brush border enzymes are anchored on the apical membrane of enterocytes

• carbohydrates have been partly converted by salivary amylase • proteins have been hydrolysed by pepsin • fats are now mostly in the form of triglycerides

• fats have an important role in the regulation of food release from the stomach • they increase the motility of the stomach • fats are the last to leave the stomach

• carbohydrates are hydrolysed by salivary and pancreatic amylase into disaccharides • brush border enzymes hydrolyse the disaccharides into monosaccharides

• maltase hydrolyses maltose into 2 glucose molecules • lactase hydrolyses lactose into glucose and galactose • sucrase hydrolyses sucrose into glucose and fructose

a mutation in the lactase enzyme means there will be no production of glucose and galactose and so the person will be lactose intolerant

1) glucose and galactose will enter the enterocyte via SGLT1 2) Na+ will enter simultaneously 3) the Na+ concentration is maintained via Na+/K+ ATPase 4) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane 5) GLUT 2 is also present on the basolateral surface 6) glucose will diffuse into the blood

1) fructose will enter the enterocyte via a GLUT 5 carrier - this is a Na+ independent transporter 2) a high glucose concentration will result in the insertion of GLUT 2 into the brush border membrane and fructose can also enter the enterocyte via GLUT 2

• pancreatic enzymes : trypsin, chymotrypsin and carboxypeptidase • brush border enzymes : aminopeptidases • stomach : pepsin

• pancreatic enzymes break down proteins into small peptides • aminopeptidases break down proteins into amino acids

1) small peptides enter enterocytes via a H+ dependent carrier 2) amino acids enter via a Na+ dependent carrier 3) small peptides are then hydrolysed into amino acids by intracellular peptidases 4) amino acids exit the cells via imports and Na+ linked antiports 5) they diffuse into the blood

1) fats form large fat globules 2) they get coated with bile salts to break them into smaller globules 3) phospholipids bind to smaller globules 4) this forms an emulsion 5) pancreatic lipase will act on this emulsion and form micelles 6) the monoglycerides and fatty acids within the micelles will enter the enterocytes via diffusion 7) the smooth endoplasmic reticulum will resynthesise the fats 8) this forms chylomicrons which diffuse into lacteals