Gut Absorption Physiology Flashcards
T or F. The epithelium of the intestine is polarized, charac- terized by having tight junctions, an apical membrane, and a basolateral membrane
T. Thus, the epithelial cells control the passage of sub- stances between the intestinal lumen and the milieu interieur because only very small molecules are able to pass through tight junction
Which end of intestine junctions, distal or proximal is tighter?
distal. “The “tightness” of these junctions changes along the length of the intestine, generally with the distal epithelium being tighter than the proximal epithelium
Why can passive transport mechanisms can be coupled with active transport mechanisms in intestinal epithelium?
By virtue of cell sidedness (polarized), apical and basolateral, transport processes that are passive on one side can be coupled to active processes on the other side.
What facilitates passive secretion of water into the digestive tract?
Passive mechanisms are osmotically driven (recall that water moves down its concentration gradient like any other diffusible substance). The digestion of ingested food creates the osmotic gradient that facilitates the movement of water into the lumen. For example, digestion of starch to maltose and protein to peptides dramatically increases the number of dissolved particles, increasing the osmolarity of the luminal contents.
What facilitates active secretion of water into the digestive tract?
Active mechanisms involve the secretion of electrolytes into the lumen, with water following passively. For example, secondary active transport mechanisms on the basolateral membrane bring Na+, K+, and Cl- into the cell. Na+ is pumped back out into the interstitial space by ATPase pump, and excess K+ likewise moves out of the cell through K-channels. The excess Cl-, on the other hand, moves out of the cell into the intestinal lumen through the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) Cl-channel. This does two things: puts an osmotically active ion in the lumen, and creates a greater negative charge in the lumen relative to the interstitium. This negative charge attracts another osmotically active ion, Na+, to move through the tight junctions. The increase in osmotically active molecules in the lumen attracts water to also move through the tight junctions.
Coupling the movement of a substance to Na+ moving down its electrochemical gradient (cotransport) is called what?
secondary active transport mechanism
Can complex sugars be transported in the body?
No. Secreted enzymes and epithelium attached enzymes hydrolyze the complex sugars into mono- and di-saccharides. Specific transporters then move these simple sugars into the body.
How does transport of glucose and galactose work? What proteins are involved?
Glucose and galactose transport utilizes a cotransport mechanism. The protein coded for by the solute carrier family 5 member 1 (SLC5A1, previously known as SGLT1) transports one molecule of sugar with two Na+, coupling the movement of the sugar to the Na+ electrochemical gradient. As a secondary active transport mechanism, the sugar can be moved up its concentration gradient. (The Na+-K+ ATPase then pumps Na+ out of the epithelial cell) The sugar diffuses down its concentration gradient through a facilitated transport mechanism mediated by the protein coded for by the solute carrier family 2 member 2 gene (SLC2A2, formerly known as GLUT2) that resides in the basolateral membrane.
How does transport of fructose work? What proteins are involved?
Fructose also uses a facilitated transport mechanism, moving from the lumen into the epithelial cell by action of the solute carrier family 2 member 5 coded protein (SLC2A5, formerly known as GLUT5). Fructose also diffuses out of the cell through the SLC2A2 transporter.
Are proteins typically absorbed intact?
No. The ingested proteins are broken down by the action of secreted peptidases, resulting in amino acids, di-, and tri-peptides.
T or F. Individual amino acids are absorbed in a Na+-independent mechanism
F. Amino acids are absorbed in a Na+-dependent mechanism (secondary active transport)There are specific apical membrane co-transporters for the acidic, neutral, and basic amino acids.
Di- and tripeptide transport is dependent on what?
Hydrogen
How does Di- and tripeptide transport work? What proteins are involved?
Di- and tri-peptides are transported in a H+-dependent cotransport mechanism by the peptide transporter coded for by the solute carrier family 15 member 1 gene (SLC15A1, also known previously as PEPT1). Once the peptides are in the epithelial cell, most of the peptides are degraded to amino acids by intracellular proteases.
Intact proteins are not normally absorbed, except under a very important condition. What is this condition?
In non-human mammals, and to some extent in humans, neonates can absorb immunoglobins secreted into the mother’s milk (in humans, antibodies can also cross the placental barrier). This improves immunity in the early days after birth, but the mechanism will disappear rapidly if the antibody containing milk is absent for several days.
What role does bile acid play in lipid absorption?
Ingested triglycerides, the primary chemical form of ingested fats, would clump together in the digestive tract without emulsification by secreted bile acids. Not only does emulsification disperse triglycerides in the aqueous chyme, but it also makes the triglyceride more readily accessible for hydrolysis (hydrolytic action converts triglycerides to monoglycerides)
