Digestion and absorption Flashcards
what promotes pancreatic alpha-amylase secretion
CCK
what emulsifies fat
bile salts and lecithin (but need vigorous mixing)
how does pancreatic alpha-amylase get into the duodenum
via the pancreatic duct
names of the transporters for glucose and fructose
SGLT1 - glucose GLUT5 and GLUT2 - fructose
how is pepsin inactivated
neutral pH (in the duodenum)
what happens to lipids when absorbed into epithelial cells
the FAs and monoglycerides reform triglycerides in the SER and are then coated with apolipoproteins –> chylomicrons
action of salivary amylase
hydrolyses 1:4 alpha linkages between glucose molecules (linear or branched chain)
pepsin is important for
breaking down cell-cell adhesion
what is the next carbohydrate digestive enzyme to act after salivary amylase
pancreatic alpha-amylase in the duodenal mucosa
what are the 4 main enzymes of the pancreatic proteases after action by enterokinase
- trypsin - elastase - chymotrypsins - carboxypeptidases
how are micelles formed
bile salts + lecithin + monoglycerides + FAs + cholesterol
how does digestion of fat start
lingual lipase - very minor effect (probably more involved with taste)
where is glucose and fructose absorbed?
in the epithelium of duodenum and jejunum - predominatly at the tips of villi
how is pancreatic lipase activated?
CCK –> trypsin –> colipase –> lipase
what enzymes are found at the brush border of enterocytes for digestion of carbohydrates
isomaltase sucrase maltase
where and how are the FAs absorbed that are made by the microbiota
in the distal small bowel and proximal part of the colon via H+ dependent mechanism
what happens to some di and tri-peptides that are transported directly into epithelial cells
they are broken down to free amino acids by enterocyte small peptidases
first enzyme of carbohydrate digestion
salivary amylase
what size peptides end up at the brush border membrane
di and tri peptides and free amino acides
action of pancreatic alpha-amylase
hydrolyses 1:4 alpha linkages between glucose molecules (linear or branched)
where do pancreatic carboxypeptidases act? and what are their function
at the carboxy terminals of peptides producing free amino acids
what is the action of sucrase
- breaks sucrose into glucose and fructose - acts with isomaltase and maltase to break down maltotriose and maltose
action of enterokinase
activates trypsinogen to trypsin –> positive feedback mechanism –> more trypsin and activates chymotrypsin
how is cholesterol esterase activated
by bile
how are pancreatic pro-enzymes activated
first by enterokinase in the epithelial membrane
what is the next step in carbohydrate digestion after pancreatic amylase
digestion at the brush border membrane of mucosal enterocytes
which enzymes act at the brush border for protein digestion
mix of: - aminopeptidases - carboxypeptidases - endopeptidases (trypsin, chymotrypsin and elastase) - dipeptidases
where do endopeptidases act and what are their function
- act at interior bonds of peptides - produce short polypeptides
what is special about maltase and sucrase on the brush border of enterocytes
they are synthesised as a single large glycoprotein which is the inserted into the brush border membrane and then separated and activated by separate pancreatic proteases
main disacchardies of diet
lactose and sucrose
major sites of digestion
duodenum and upper jejunum
what happens to the monosaccharides and amino acids that enter the capillaries from the small intestine
go into the circulation –> portal vein –> liver
action of cholesterol esterase
hydrolysis of: - cholesterol esters - esters of fat - soluble vitamins - phospholipids
function of pepsin
hydolyses bonds between aromatic amino acids (phenylalanine, tyrosine) and a second amino acid –> producing polypeptides of diverse sizes
which enzymes are endopeptidases
trypsin elastase chymotrypsin
what happens to the chylomicrons formed in epithelial cells
they are exocytosed –> enter the lymphatics
how is fat absorbed
lipids inside micelles bought to apical surface of epithelial cells at tips of villi - when in contact with the membrane, the lipids dissolve in the membrane and enter the cells
2 types of pepsinogen and where are they secreted
- pepsinogen 1 - secreted in acid secreting regions - pepsinogen 2 - secreted close to pylorus
how are free amino acids transported into capillaries
by 7 or more different transport systems
what happens to salivary amylase over time
- inactivated at acid pH in the stomach - reactivated at neutral pH in the duodenum after acid neutralization by bicarbonate in pancreatic juice
what causes the secretion of pancreatic proteases
CCK triggered by amino acids
even after salivary and pancreatic amylase acts on carbohydrates, we have a variety of oligosaccharides intact. Why is that?
because they are both ineffective at hydrolysing 1:6 alpha linkages
how is fructose absorbed
- transported by GLUT5 into epithelial cells - transported by GLUT2 into interstitium
action of gastric lipase
again - minor role - produces enough FA to activate FA receptors –> release CCK from duodenum
dietary monosaccharides
glucose and fructose and galactose
what is the action of isomaltase
- hydrolyses 1:6alpha linkages - acts with sucrase and maltase to break down maltotriose and maltose
what causes the secretion of pancreatic lipolytic enzymes
CCK
what is the accessory mechanism for the production of FAs
fermentation of dietary fibre by microbiota
how is glucose absorbed
cotransported with Na by a sodium-dependent glucose transporter (SGLT1)
how does the digestion of proteins start
pepsinogen –> pepsin (from chief cells) in the stomach - activated by gastric acid
