Digestion Flashcards
Digestion is when components of meals must be degraded into small molecules for
absorption by epithelial cells of the intestine and for transport in the blood
Proteins are digested to _____ by _____, which is secreted by _____
Amino acids, proteolytic enzymes (proteases), stomach and pancras
Polysaccharides (ie starch) are cleaved into ______ by ______, which is from _____
Monosaccaharides, alpha-amylase, pancreas and saliva (tiny bit)
Lipids are converted into _______ by ______, which are secreted from ______
Fatty acids, lipases, pancreas
All digestive enzymes are
hydrolases (cleave their substrates by addition of H2O molecule)
All digestive enzymes (except alpha-amylase) are secreted as inactive forms called
Zymogens or proenzymes, which are activated by proteolytic cleavage of part of the inactive precursor
Before secretion of zymogens, they exist in
granules near cell membrane
In response to signals: granules
fuse with the cell membrane and expel their contents into the lumen of the intestine
What is Pepsin?
Stomach enzyme secreted as zymogen = pepsinogen (can activate itself to some degree in acidic environment from its little enzymatic activity), active pepsin activates remaining pepsinogen
Digestion begins in mouth with process of chewing
- Teeth/tongue/saliva make food = homogenous aqueous slurry that is easily attacked by digestive enzymes
- Saliva (from salivary glands): Aq soln of Na+, K+, Cl-, and HCO3- that contains mucoproteins, facilitates homogenization of food, lubricates slurry for swallowing
Saliva also contains alpha-amylase which
cleaves alpha-1,4-glycosidic bonds, but short time in mouth = minimal polysaccharide breakdown in mouth
Protein digestion begins in the stomach:
1) Proteins are denatured by acidic environment, caused by breakage of ionic and H-bonds due to low pH, renders protein better substrate for degradation
2) Protein degradation begins via action of pepsin -> yields protein fragments that are further degraded in intestine
Protein digestion continues in the intestine
1) partly digested food enters small intestine from stomach = low pH -> causes secretion of hormone secretin
2) Secretin promotes release of NAHCO3 from pancreas to neutralize pH of food
3) Polypeptide products of pepsin digestion = release of cholecystokinin (CCK) by intestine = pancreas then releases tons of digestive enzymes
4) Pancreatic proteases hydrolyze proteins into small fragments = oligopeptides
5) Digestion completed by peptidases, which cleave oligopeptides into A.As and di/tripeptides that can be conveyed into intestinal cell by transporters
6) Absorbed A.As are released into blood by antiporters for use by other tissues
Dietary carbs are digested by alpha-amylase
Most common end products are monosaccharides: glucose, fructose, and galactose. Primary source if complex carb: starch.
1) Starch digested by pancreatic enzyme alpha-amylase, which cleavy alpha-1,4 bonds but not alpha-1,6 = di/trisaccharides maltose and maltotriose and dextrin
2) alpha-glucosidase digests maltose, maltotriose, and any other ogliosaccharides that may have escaped alpha-amylase
3) alpha-dextrinase further digests dextrin into simple sugars
Digestion of disaccharides are much simpler than complex carbs
1) Sucrose digested by sucrase
2) Lactase degrades lactose into glucose and galactose
3) Glucose and galactose transported into intestinal cells by secondary transport process carried out by Na-glucose linked transporter
4) Fructose diffuses across cell membrane through transport: GLUT5
5) GLUT2 transporter releases all 3 monosaccharides into bloodstream, travel to tissues for fuel usage
Digestion of lipids is complicated by their hydrophobicity
1) After leaving stomach, emulsification is enhanced with aid of bile salts (amphiphatic molecules from cholesterol in liver, secreted by gallbladder in response to CCK)
2) Bile salts insert into lipid droplets, making triacylglycerols more digestable
3) Triacylglycerols are degraded into FFAs and monoacylglycerol by lipases (pancreas), which attach to surface of a lipid droplet
4) FFAs and monosacylglycerols carried in micelles to plasma membrane where they are absorbed, transported into cells via FABP (fatty-acid-binding-protein)
5) FABPs inside the cell transport them to cytoplasmic face of SER -> resynthesized into triacylglycerols
6) Triacylglycerols associate with proteins and small amount of phospholipid and cholesterol to form lipoprotein transport particles = chylomicrons
7) Chylomicrons released into lymph system then into blood
8) Chylomicrons bind to membrane-bound lipoprotein lipases (at adipose and muscle tissues) where triacylglycerols are degraded into FFAs for transport into tissue then resynthesizes and stored
