Digestion and Abs 1 (Johnson) Flashcards
Where are digestive enzymes found?
Salivary, gastric, pancreatic secretions, and apical membrane of intestinal epithelial cells
Describe the cellular path of absorption
Particle crosses apical membrane to enter the intestinal epithelial cell and is then exported on the basolateral side of the cell and into the blood
Describe the paracellular path of absorption
Substances move across tight junctions between intestinal epithelial cells and into the blood
increase the surface area of the small intestine, maximizing the exposure of nutrients to digestive enzymes and creating a large absorptive surface
Villi and microvilli and folds of Kerckring
What are folds of Kerckring? What projects from them?
Longitudinal folds of small intestine with villi coming off of them
Where are the villi the longest?
Duodenum
**where most of the abs and digestion occurs
Where are the villi the shortest?
Terminal ileum
The villi are covered by
Epithelial cells (enterocytes) interspersed with goblet cells
Aka the brush boarder
Microvilli on apical surface of enterocytes
What are the carbohydrates that are “absorbable” by the intestinal epithelial cells
Glucose, galactose, or fructose
Describe the digestion of starch
Starch is broken down by salivary amylase. Then once in the stomach/duodenum pancreatic alpha amylase digests starch into disaccharides: alpha-dectrins, maltose, and maltotriose. These disaccharides are broken down by alpha dextrinase, maltase, sucrose into glucose.
What monosaccharide does trehalose get broken down into? Via what enzyme?
Glucose via trehalase
What monosaccharide does lactose get broken down into? Via what enzyme?
Glucose and galactose via lactase
What monosaccharide does sucrose get broken down into? Via what enzyme?
Glucose and fructose via sucrose
How is glucose abs across the epithelial intestinal cells?
Active transport: Na cotransport via SGLT1 on apical side
Facilitated Diffusion: GLUT2 on basolateral side
How is galactose abs across the epithelial intestinal cells?
Active transport: Na cotransport via SGLT1 on apical side
Facilitated Diffusion: GLUT2 on basolateral side
How is fructose abs across the epithelial intestinal cells?
Facilitated diffusion via GLUT5 on apical side and GLUT 2 on basolateral side
**dependent on electrochemical gradient to move it across the epithelim
What is the source of the energy needed to move glc against its electrochemical gradient via active transport on apial membrane of intestinal epithelial cells?
Na gradient NOT directly ATP
**this gradient is maintained by Na/K ATPase on basolateral side (= indirectly)
Difference between the endopeptidases and exopeptidases
Endo hydrolyze the inferior peptide bond in proteins
Exo hydrolyze one amino acid at a time from the C term
What are the endopetidases and the expopeptidases?
Endo: pepsin, trypsin, chymotrypsin, and elastase
Exo: carboxypeptidases A and B
Describe the process of protein digestion
Begins in the stomach via action of pepsin and continues in the small intestine via action of pancreatic and brush boarder peptidases.
Trypsinogen is activated by enterokinase into trypsin. Trypsin then activates all the other pancreatic proteases (including more trypsin). These proteases hydrolyze proteins into amino acids, dipeptides, tripeptides, and oligiopeptides. The oligiopeptides are then further digested by brush boarder peptides
What form of proteins is absorbable?
Amino acids, di, and tri peptides
Where does pepsin act? Why does it not continue to work farther down the GI tract?
Stomach—it has a narrow range of pH in which it is active, in the duodenum bicarb is secreted into it from the pancreas, binging the pH out of the range in which it is active
Digestion of proteins is dependent on peptidase. How do we know?
F: in pts whose stomach is removed or does not secrete H+ protein digestion and abs is normal
**pancreatic and brush-border proteases alone can adequately digest ingested protein.
The L or D form of amino acids is absorbed by intestinal epithelial cells
L
How are amino acids absorbed by the intestinal epithelial cells
Na cotransport on apical membrane. There are separate transporters for neutral, basic, acidic, and imino amino acids. They are then transported across the basolateral side via facilitated transport
How are di and tri peptides absorbed by the intestinal epithelial cells
H+ cotransport on apical side. Once they are inside they are hydrolyzed into amino acids by cytosolic peptidases and then transported across the basolateral side via facilitated transport. Remaining di and tri peptides are absorbed unchanged
What drives the active transport of amino acids across the apical membrane of intestinal epithelial cells
Na gradient (established by Na/K ATPase on basolateral membrane)
What drives the active transport of di and tri peptides across the apical membrane of intestinal epithelial cells
H+ gradient (established by Na/H exchanger on apical membrane)
a genetic disorder in which the transporter for the dibasic amino acids are absent in both the small intestine and the kidney
What are these amino acids?
Cystinuria: cystine, lysine, arginine, and ornithine
→ increased excretion of these amino acids in the feces and the urine bc they cannot be absorbed by either the kidneys or the intestine
What is the role of the stomach in the digestion of lipids
Churn and mix the lipids to break them into small droplets (increases SA) for digestive enzymes.
Peptides present in the stomach emulsify the lipid droplets.
Enzymes that initiate lipid digestion in the stomach
Lingual an gastric lipases (Hydrolyze ~10% of ingested triglycerides to glycerol and free fatty acids)
still have to
finish
