1.4 Digestion and absorption Flashcards
Define digestion
Mechanical, chemical and microbial breakdown of large insoluble food molecules into simple absorbable compounds
Define absorption
Process by which these simple compounds are taken across the intestinal membranes into the bloodstream
What are 𝛂-amylases
Enzyme that catalyses hydrolysis of starch into sugars
Salivary = pH optimum 6.6-6.8, inactivates at stomach pH 3
Pancreatic = digests most carbs in small intestine, secreted from exocrine pancreas
What are the biological functions of the 𝛂-amylases?
- Attack 𝛂-1,4 glycosidic links only
2. Attack only in the middle of the chain: endoglycosidases
What is an enterocyte?
Absorptive functional unit of the digestive tract
Describe the life cycle of an enterocyte
Lumen is toxic so instead of repairing every insult, damaged enterocytes die by apoptosis and are removed from the epithelial layer and continuously replaced by new ones
Describe the progression of nutrients over the brush border into cells
Cross over the baso-latera membrane and enter the bloodstream
Directed to the hepatic portal vein to the liver and beyond or indirectly via the lymphatic system
What characteristics of glucose transport allow the mechanism to be discovered?
- saturable
- competitively inhibited by galactose
- inhibited by glucose analogues e.g., phlorizin
- sodium dependant
- inhibited by sodium/potassium ATPase inhibitor: ouabain
- requires energy
Define proteases
Enzymes that break down proteins and peptides
- cleavage of peptide bonds within proteins by hydrolysis
What is a zymogens?
Secreted as inactive precursors - activated immediately before use
What are the differences between endo and exopeptidases?
Endopeptidases attack and cleave in the middle of an amino acid chain unlike exopeptidases which spilt off amino acids from the end of the chain
Endopeptidases are secreted by the stomach & pancreas whereas exopeptidases are secreted by the pancreas and the small intestine glands
What are pepsins?
Gastric proteases
- pepsin secreted as pepsinogen by chief cells
What ion is always produced simultaneously with pepsinogen?
H+, secreted by parietal cells
What are pancreatic endopeptidases?
Produced from pepsin digestion in the stomach pass into the small intestine
- zymogen
- trypsinogen
- chymotrypsinogen
- pro-elastase
What are the two subtypes of pancreatic exopeptidases?
Carboxypeptidases + Aminopeptidases
What are carboxypeptidases?
Split as aas from the carboxy terminal are secreted by the pancreas as zymogens
What are aminopeptidases?
Spilt as aas from the N terminal, are made in the SI glands and are found on the intestinal cell membrane
What are lipases?
Digest lipases, removing fatty acids from TAG
What is the main fat digesting enzyme?
Pancreatic lipase - active in the small intestine
What is bile?
Greenish-brown alkaline fluid which aids digestion and is secreted by the liver and stored in the gall bladder
What is the function of bile?
Bile acids aid in the digestion and absorption of fat
Also excretes excess cholesterol and the toxic breakdown products of haem (bile pigments)
What hormone initiates bile release?
Cholecystokinin
What does emulsification entail?
- Forcing (spraying) the stomachs contents through the pyloric sphincter into duedenum
- Bile salts stabilise/coat the resulting lipid droplets (emulsification) and prevent them from coalescing
- Peristalsis helps further stabilise the emulsion droplets and keep them in suspension
What are the products of fat digestion?
Free fatty acids
2-monoacylglycerol
1-monoacylglycerol
Glycerol
What are chylomicrons?
Small structures containing triglycerides Surface proteins = apolipoprotein C Surface lipids Core containing mostly TAG Intergral protein apolipoprotein B
How are chylomicron remnants formed?
Some TAG is broken down by lipoprotein lipase on the surface of capillary endothelial cells which are then released from the endothelium and taken up by the liver and further metabolised to other lipid particles e.g., HDL, LDL, VLDL
