Lecture 33 - Monogastric Lipid digestion and absorption Flashcards
what is the most important enzymatic reaction in digestion?
hydrolysis: the breaking of a chemical bond by the addition of a water molecule
protein digestion begins in the:
Stomach
describe how proteins are digested in the stomach
Pepsin is converted from pepsinogen with the help of HCl from parietal cells. Pepsin then breaks down proteins into smaller peptide fragments, so that when they leave the stomach and enter the SI, they are 25-100 aa long
what is the enzyme produced by chief cells that is especially important for digestion of casein in neonates?
Rennin
to aid in protein digestion in the lumen of the SI, the pancreas secretes:
proteases, including trypsin
what facilitates the conversion of peptides into smaller peptides and free amino acids?
peptidases on the brush border of the small intestine
are the brush border peptidases integral membrane proteins, or soluble enzymes?
integral membrane proteins
Pancreatic exocrine secretion is strongly stimulated by:
food entering the stomach and small intestine
CCK and vagal stimulation increase the production and secretion of what organ’s enzymes?
the pancreas
CCK acts on villous enterocytes and causes them to secrete ______, which converts trypsinogen to the active trypsin
enteropeptidase
food entering the small intestine will result in the release of _____, which increases alkaline (bicarbonate) secretion to neutralize the low pH contents entering the lumen from the stomach
Secretin
What is the function of trypsin in protein digestion?
trypsin cleaves off portions of all other inactive pancreatic enzymes, thus activating them in the intestinal lumen
what do single amino acids use to cross the apical membrane of the lumen into the enterocyte?
Sodium dependent amino acid transporters
Once inside the enterocyte, dipeptides and tripeptides are converted into single amino acids by:
intracellular peptidases
what do dipeptides and tripeptides use to be transported across the apical membrane?
Special peptide proteins that do not require sodium (ie. PEPT1)
what happens after cytosolic peptidases in the enterocyte have converted all of the peptides into free amino acids?
transporters on the basolateral membrane facilitate the diffusion of the amino acids into the extracellular fluid
Why can some drugs be taken orally, while others can’t?
Some drugs cannot survive the enzymes or acid in the stomach, or are broken down before being able to enter the extracellular fluid. However, there is potential to make any drug a substrate for PEPT1, the peptide transporter on the apical membrane of the enterocytes.
How are neonates able to acquire passive immunity?
For a short time after birth, neonates are able to absorb intact proteins (ie immunoglobulins) in colostral milk. However, the small intestine rapidly loses this ability.
what are the postabsorptive endstates for amino acids?
- protein synthesis
- making nonessential amino acids
- making other nitrogen-containing compounds
- converted to fatty acids
- converted to glucose
- metabolized as a source of ATP
true or false: the amino acids that are essential (aka needed from dietary sources) are the same for all species
false. although all animals share the same set of amino acids, which are essential vary among species
what are two clinically relevant essential amino acids for cats?
Arginine and taurine. Cats are particularly sensitive to dietary deficiencies in these two aas.
The digestion of lipids is hindered by the fact that lipids are hydrophobic, and lipases are hydrophilic. What does this mean?
it means that the lipids form globules, and the lipases can only act upon the lipid on the globules surface.
where and how are bile salts formed?
within hepatocytes, by conjugating an amino acid with cholesterol
what is one of the most commonly used amino acids for creating bile salt?
taurine, which binds to cholesterol to form taurocholic acid
bile salts are amphipathic. the cholesterol end is hydro____ while the amino acid end is hydro____
cholesterol - hydrophobic
amino acid - hydrophilic
the breakdown of fat globules into smaller droplets is called:
emulsification
when a bile salt comes into contact with a triglyceride, which domain (hydrophilic/hydrophobic) stays on the surface, and which intercalates into the lipid?
the hydrophilic stays on the surface, the hydrophobic enters the lipid and facilitates the breakdown of the lipid into smaller droplets
how do bile salts increase lipase action?
they increase the surface area where the lipases can act
hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by:
pancreatic lipase
as monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called:
micelles
What are micelles?
small aggregates of mixed lipids and bile acids suspended within the ingesta
what happens to the micelles when the ingesta is mixed?
they bump into the brush border of small intestinal enterocytes, and the lipids (monoglycerides and fatty acids) are taken up into the epithelial cells
as fats enter the duodenum, they stimulate enteroendocrine cells to secrete __, which stimulates the pancreas to release ____ and _______, as well as bile production and secretion
CCK, lipase and pro-colipase
what cleaves pro-colipase, so that is can be activated for activity with lipase?
trypsin
what three things work together on the emulsified fat to convert the triglycerides to monoglycerides and free fatty acids?
lipase, colipase, and bile salts
what are differences between fat droplets and micelles?
- fat droplets are much larger
- micelles are only monoglycerides and fatty acids, while fat droplets can have larger lipids
- fat droplets are often associated with enzymes for digestion
True or False: Fatty acids, monoglycerides, and cholesterol can readily diffuse across the apical membrane to the cytosol
true!
what happens to the empty micelle bile salts after they release their FFA and MGLYC to the apical membrane?
they return to the lumen to collect another load
the bulk of the bile salts are absorbed in the ____, and then recycled by the _____
ileum, liver
once inside the enterocyte, what happens to the FFA and monoglycerides?
they are transported to the ER, where they are synthesized into triglycerides.
once the FFA and the monoglycerides are converted back into triglycerides by the enterocyte ER, what happens?
they move on to the Golgi, where they are packaged with cholesterol, lipoproteins, and other lipids into particles called Chylomicrons.
how do chylomicrons get transported out of the basolateral membrane of the enterocyte?
through exocytosis
once in the interstitial fluid, chylomicrons are transported where?
into the lymphatic vessel called the ‘central lacteal’, which will eventually drain to the systemic circulation
which have longer digestive systems: herbivores or carnivores?
Herbivores. The increased length allows for digestion of cellulose from plant cell walls.
