Digestion and Absorption: Carbs, Proteins, Nucleotides, and Lipids Flashcards
What is the length of the duodenum and jejuum combined?
3 meters
Cause a 5x increase in surface area of small intestine
Valvulae conniventes
Cause a 10x increase in surface area of small intestine
Villi
Cause a 20x increase in surface area of small intestine
Microvilli
The entire small intestine absorbs carbohydrates, proteins, and lipids, but absorption is greatest in the
Duodenum (then jejunum and then ileum)
What is a mineral absorbed in the duodenum
Iron (as Fe2+)
Folate is a vitamin absorbed in the
Small intestine
Absorbed in the terminal ileum along with bile salts
-requires intrinsic factor
B12
Total sugars on a food label is referring to
Sucrose, lactose, glucose, and fructose
Total carbohydrates minus total sugars and dietary fiber gives you
Complex carbohydrates like starch or sugar alcohol
Of the digestible carbohydrates
- ) 50% =
- ) 40% =
- ) 10% =
- ) Polysaccharides (starch and glycogen)
- ) Disaccharides (Sucrose and lactose)
- ) Monosaccharides (Glucose and fructose)
The small intestine can only absorb
Monosaccharides
Can not absorb monosaccharides
Colon
Can not be digested
Dietary fiber
Important sources of starch include
Corn, potato, and rice
Exists in the form of granules, each of which consist of several million amylopectin molecules together with an even larger number of amylose molecules
Starch
Glycogen from animals is a more highly branched form of
Amylopectin
In the mouth, starch is attacked by salivary
α-amylase
The optimal pH for salivary α-amylase is
7 (i.e. it is inactivated in the stomach)
In the duodenum, the acted polysaccharides are then acted on by
Pancreatic α-amylase
The main enzyme that breaks the α-1,4 linkage
Glucoamylase
Also have activity in breaking the α-1,4 linkage, even though their names suggest otherwise
Sucrase and isomaltase
Glucoamylase removes one glucose from the α-limit dextrin formed by
Amylase
Then isomaltase acts as the only enzyme that can break the
α-1,6 linkage
A More Alkaline Environment in the Duodenum is Required for function of
Pancreatic enzymes
Brush border digestion of sucrose and lactose results in glucose and galactose. The uptake of glucose and galactose is then dependent on
SGLT-1
Uptake of glucose and galactose is then dependent on sodiumglucose cotransporter-1 (SGLT-1) and driven secondarily by the low intracellular sodium concentration established by the
Basolateral Na+/K+ ATPase
Specifically inhibit glucose reabsorption in the kidney tubule
SGLT2 inhibitors
The only diabetes-specific drug class that has been show to reduce CV morbidity and death
Gliflozins
At about age 5, normal lactase expression in the small intestine is suppressed by the
MCM6 gene
This results in about a 70% world-wide prevalence of
Lactose intolerance
Has allowed for the continued consumption of milk and milk products in the other 30% of the worlds population
MCM6 mutation
To prevent autodigestion, gastric and pancreatic proteases are secreted as
Proenzymes
Secrete pepsinogen, which is activated to pepsin after exposure to HCL
Gastric Chief Cells
Peptides in the stomach stimulate gastrin secretion –> gastric acid secretion increases –> increased
Protein digestion
Because pepsin has a pH optimum around 2.0, it is inactivated in the
Duodenum (pH = 6)
In infants IgA antibodies in maternal colostrum enter the circulation of the infant from the intestine, providing
Passive immunity against infection
Tri- and dipeptides and amino acids can be absorbed from the
GI tract
Causes secretion of pancreatic proenzymes
-Stimulated by peptides in the duodenum
CCK
Trypsinogen is cleaved to trypsin by the enzyme
Enterokinase
Amplifies its own activation leading to activation of the remaining proteolytic enzymes from the exocrine pancreas
Trypsin
Peptides are absorbed together with a proton supplied by an apical
Sodium/Hydrogen exchanger (NHE)
Peptides are absorbed together with a proton supplied by an apical sodium/hydrogen exchanger (NHE) using
Peptide transporter 1 (PepT1)
Absorbed peptides are digested by cytosolic proteases, and amino acids are transported into the bloodstream at the
Basolateral membrane
Some amino acids are absorbed directly from the lumen via specific
Transporters
Made in the exocrine pancreas and secreted into the duodenum via the pancreatic juice
RNAse and DNAse
At the brush border, phosphatase and nucleosidase convert nucleotides to
Pentose sugar, nitrogenous base, and phosphate
Pentose, nitrogenous base and phosphate are absorbed in the
Duodenum and Jejunum
These products enter the blood capillaries of the intestinal villi and travel to the
-For further metabolism
Liver
A gastric acid lipase is responsible for about
-activated in the duodenum
15% of fat digestion
Fatty acids enter the duodenum, triggering the release of
CCK
Contracts the gallbladder and relaxes the sphincter of Oddi resulting in secretion of bile and stimulation of pancreatic enzyme secretion
CCK
Emulsify fat and increase micelle formation
Bile Acids
Further digest TAG (90% of fat in the diet)
Pancreatic lipase and Colipase
Need the higher pH in the duodenum (pH = 6) to act
Pancreatic lipase and colipase
Packaged into chylomicrons and travel in thoracic duct to the venous circulation
TAG and Cholesterol
Produced in the colon and absorbed from it
Short-chain fatty acids (acetate, propionate, butyrate)
Formed by the action of colonic bacteria on complex carbohydrates, and resistant starches
Short-chain fatty acids (acetate, propionate, butyrate)
Synthesized in the intestine, travels in thoracic duct to the L subclavian vein, then to the liver
Chylomicron
The apolipoprotein of the chylomicrons
ApoB-48
Carry TAG, phospholipid, CE, and fat soluble vitamins (ADEK)
Chylomicrons
Transfers an acyl CoA residue to cholesterol forming a cholesterol ester (CE)
ACAT
Most of the lipid in the diet is in the form of
TAGs
