Digestion and absorption Flashcards
Surface of SI mucosa
Circular folds of Keckring with vili projecting from folds (covered with epithelial cells with goblet cells - mucous secreting)
Carbohydrate digestion summary - what products broken into and what mechanism?
Into glucose, galactose, fructose into SI
Mech is Na-dependent cotransport and fac diff (for fructose)
Digestion of carbohydrate sites
Mouth - salivary alpha amylase (in response to sight and smell of food)
Stomach - amylase continues to act
Duodenum - pancreatic amylase (secreted into duodenum from pancreas), brush border enzymes (maltase, sucrase and lactase convert disaccharides into monosaccharides)
Products of amylase
Now digested by oligosaccharides (attached to enterocyte mucosal membrane of brush border of epithelial cells)
- alpha-glucosidase - cleaves a-1,4 glycosidic bonds to remove single glucose units from the non reducing end of polymer
- isomaltase - cleaves a-1,6 glycosidic bonds in a-limit dextrin oligosaccharides
Then hydrolysed by disaccharidases (maltase, sucrase, lactase)
Absorption of carbohydrates
Secondary active transport - SGLT2 transporters located on apical membrane transports glucose and galactose
Fac diff - GLUT 5 transports fructose across apical membrane
Summarise protein digestion
Begins with pepsin in stomach, completed in SI with pancreatic and brush-border proteases
Endopeptidases hydrolyse interior peptide bonds of proteins - pepsin in stomach (hydrolyses links with tyrosine, D-alanine and leucine), trypsin (SI) hydrolyses links with arginine and lysine, chymotrypsin (SI) hydrolyses links with tyrosine, tryptophan, D alanine and leucine, elastase (SI) degrades elastin
Exopeptidases hydrolyse one amino acid at a time from end of protein - carboxypeptidases (C-terminal), aminopeptidases (N terminal)
Separate protein digestion into gastric, pancreatic and mucosal phases
Gastric phase - denaturation by HCl, pepsin
Pancreatic phase - trypsin, chymotrypsin, carboxypeptidase
Mucosal phase - oligopeptidase, aminopeptidase
Protein absorption
All in SI
Amino acids absorbed by Na-dependent cotransport
Dipeptides absorbed by H-dipeptide cotransport
Tripeptides absorbed by H-tripeptide cotransport
What are the four main types of lipids?
4 main types of lipids
- Fats/oils - triacylglycerols
- Phospholipids
- Cholesterol and cholesterol esters
- Fatty acids
Three main types of enzymes in lipid digestion?
Lipases
Phospholipases
Cholesterol esterases
Lipid digestion of TAG by lipases
In mouth - Salivary/gastric lipases - salivary/gastric lipases (relatively unimportant - 10% of ingested lipids hydrolysed) - heat and movements in stomach mix food with lipases, hydrolysis initially slow due to largely separate aqueous/lipid interface, as hydrolysis proceeds rate increases due to fatty acids produced acting as surfactants
In duodenum - panceratic lipase aided by bile salts from gallbladder
What does emulsification do?
Done by bile salts, lysolecithin and products of lipid digestion
Produces small droplets of lipids dispersed in aqueous solution cretaing large SA for pancreatic enz digestion
What is the end product of lipid digestion and where does it go?
Chylomicrons
Goes from SI cell to lymphatics
They have a core of TGs and cholesterol ester - phospholipds and apoprotines on outside
What happens to bile salts after use?
Most reabsorbed from terminal ileum into liver via hepatic portal vein because there aren’t enough bile salts to deal with the average meal
Absorption of water in LI
Na actively absorbed (in exchange for K), K reabsorbed in exchange for H, Cl absorbed (in exchange for HCO3), H20 follows by osmosis
