GI Digestion and absorption Flashcards
Protein Digestion
Stomach: Pepsin breaks down about 15% of proteins to small peptides
Small Intestine (lumen): Pancreatic proteases like trypsin, chymotrypsin, carboxypeptidase & elastase break down proteins to oligopeptides, di/tri-peptides and amino acids
Brush Border: Peptidases break down oligopeptides into amino acids, dipeptides, tripeptides
Intracellular peptidases: Peptidases in the enterocyte can break down di/tri-peptides into amino acids
Endo vs exo pepsidases
exo- target bonds on outside
endo- target internal bonds within those proteins
Protein Uptake Pathways
Sodium dependent co-transporters that utilize the N+/K+ ATPase gradient are the major route for the different classes of amino acids. Water follows.
Sodium independent transporters of amino acids
Specific carriers for small peptides (di- and tri-) linked to H+ uptake (co-transporter; example is PEP T1)
Pinocytosis of small peptides by enterocytes (infants)
Dietary fat (percent of our caloric intake, what they’re used for, and how digested)
Fats provide 30-40% of our caloric intake
Fats are essential for building cell membranes, hormones, bile acids, etc.
The body can make most lipids EXCEPT linoleic (an omega 6 fatty acid) acid converted to arachidonic acid and alpha-linolenic (omega-3)acid “essential fatty acids”
Triglycerides are the most abundant fat in our diet
Our GI tract is water based so there are challenges to fat absorption
Bile Acids
Primary bile acids are produced in the liver from cholesterol – cholic acid & chenodeoxycholic acid
Secondary bile acids are formed by bacteria in the intestines & colon
Bile acids are complexed with glycine or taurine to make bile salts
Bile is recycled during a meal by uptake in the distal ileum – enterohepatic circulation
Bile salts
Have hydrophobic end and hydrophilic end
Rips fat apart into micelles which helps make digestion at the brush border much more efficient
Lipid digestion summary
Lingual and gastric lipase
Pancreatic lipase- hydrolyzes triglycerides into FFAs
Bile Salts solubilize fats into micelles. FFAs are transported to enterocytes.
Trigycerides are resynthesized and chylomicrons form
Vitamin Absorption (which are fat and water soluble and how does that change their absorption?)
Fat soluble vitamins (A, E, D, K) are absorbed along the length of the small intestines and are carried in micelles and form chylomicrons similar to dietary lipids.
Water soluble vitamins either enter the enterocyte by simple diffusion (biotin, folic acid) or via specific transporters (e.g. Vit B12)
Secretion & Absorption of Fluids
About 9 L of fluid is put into the gut each day and of this, only about 100-200 mls are lost
Movement of water follows the movement of solutes (that requires a solute gradient or active transport)
After the stomach, the small intestinal contents become iso-osmotic with respect to the blood
Water and ions can move paracellularly and/or transcellularly, depending on location
There is a net fluid secretion from cells in the intestinal crypts and a net fluid absorption from enterocytes on the villi. Villi surface area > crypt surface area
Sodium Absorption
Absorbed all along the intestine, with most absorption in the jejunum (60-80%).
Dependent on a gradient established by the Na+/K+ ATPase.
Water absorption is critically linked to Na+ absorption.
Mechanism is via Na+/glucose & galactose or
Na+/amino acid cotransport, NaCl cotransport,
Na+/H+ exchange or passive diffusion
Chloride Absorption
Passive in proximal intestines (due to loose TJs). Offsets Na+ charge in the intercellular space
In the distal ileum & colon, with less leaky TJs, Cl- is exchanged for HCO3- that is offsetting the acids produced by bacteria (HCO3- is generated by the action of carbonic anhydrase)
Potassium Absorption
Important!! Also has effects on Na.
Passive process
Paracellular movement in jejunum (due to low concentration of K+ in the intercellular space from the N+/K+ ATPase) but transcellular in colon
K+ normally high in cells due to Na+/K+ ATPase. A gradient is established as luminal water decrease on approach to the colon, with passive flux of K+ into the cells
Severe diarrhea can cause significant loss of K+ and hypokalemia
Calcium and Magnesium Absorption
Ca++ and Mg++ compete for uptake by the cells
Ca++ enters enterocyte passively down its electrochemical gradient in proximal intestines
Uptake of Ca++ in intracellular calcium stores maintains the gradient
Ca++ ATPase pumps calcium out to the blood
Remember Vit D important for Ca
Iron Absorption
Regulated absorption in the proximal intestines
Transported across apical membrane as either heme or Fe++ (receptor mediated)
Two possible fates:
- binds to apoferritin to form ferritin that
stays in the cell and is lost when the cell dies
- binds to transferrin (carrier protein), leaves
the cell and goes into the blood
Movement of Water in the Intestines
Paracellular water permeability decreases from proximal to distal in the small intestines
The colon has the lowest paracellular permeability to water because it’s trying to solidify waste and it needs to link water movement to transcellular ion movement
