Lecture 33 - Monogastric Lipid digestion and absorption

Question 1

what is the most important enzymatic reaction in digestion?

Answer 1

hydrolysis: the breaking of a chemical bond by the addition of a water molecule

Question 2

protein digestion begins in the:

Answer 2

Stomach

Question 3

describe how proteins are digested in the stomach

Answer 3

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

Question 4

what is the enzyme produced by chief cells that is especially important for digestion of casein in neonates?

Answer 4

Rennin

Question 5

to aid in protein digestion in the lumen of the SI, the pancreas secretes:

Answer 5

proteases, including trypsin

Question 6

what facilitates the conversion of peptides into smaller peptides and free amino acids?

Answer 6

peptidases on the brush border of the small intestine

Question 7

are the brush border peptidases integral membrane proteins, or soluble enzymes?

Answer 7

integral membrane proteins

Question 8

Pancreatic exocrine secretion is strongly stimulated by:

Answer 8

food entering the stomach and small intestine

Question 9

CCK and vagal stimulation increase the production and secretion of what organ’s enzymes?

Answer 9

the pancreas

Question 10

CCK acts on villous enterocytes and causes them to secrete ______, which converts trypsinogen to the active trypsin

Answer 10

enteropeptidase

Question 11

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

Answer 11

Secretin

Question 12

What is the function of trypsin in protein digestion?

Answer 12

trypsin cleaves off portions of all other inactive pancreatic enzymes, thus activating them in the intestinal lumen

Question 13

what do single amino acids use to cross the apical membrane of the lumen into the enterocyte?

Answer 13

Sodium dependent amino acid transporters

Question 14

what do dipeptides and tripeptides use to be transported across the apical membrane?

Answer 14

Special peptide proteins that do not require sodium (ie. PEPT1)

Question 15

what happens after cytosolic peptidases in the enterocyte have converted all of the peptides into free amino acids?

Answer 15

transporters on the basolateral membrane facilitate the diffusion of the amino acids into the extracellular fluid

Question 16

Why can some drugs be taken orally, while others can’t?

Answer 16

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.

Question 17

How are neonates able to acquire passive immunity?

Answer 17

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.

Question 18

what are the postabsorptive endstates for amino acids?

Answer 18

• protein synthesis
• making nonessential amino acids
• making other nitrogen-containing compounds
• converted to fatty acids
• converted to glucose
• metabolized as a source of ATP

Question 19

true or false: the amino acids that are essential (aka needed from dietary sources) are the same for all species

Answer 19

false. although all animals share the same set of amino acids, which are essential vary among species

Question 20

what are two clinically relevant essential amino acids for cats?

Answer 20

Arginine and taurine. Cats are particularly sensitive to dietary deficiencies in these two aas.

Question 21

The digestion of lipids is hindered by the fact that lipids are hydrophobic, and lipases are hydrophilic. What does this mean?

Answer 21

it means that the lipids form globules, and the lipases can only act upon the lipid on the globules surface.

Question 22

where and how are bile salts formed?

Answer 22

within hepatocytes, by conjugating an amino acid with cholesterol

Question 23

what is one of the most commonly used amino acids for creating bile salt?

Answer 23

taurine, which binds to cholesterol to form taurocholic acid

Question 24

bile salts are amphipathic. the cholesterol end is hydro____ while the amino acid end is hydro____

Answer 24

cholesterol - hydrophobic
amino acid - hydrophilic

Question 25

the breakdown of fat globules into smaller droplets is called:

Answer 25

emulsification

Question 26

when a bile salt comes into contact with a triglyceride, which domain (hydrophilic/hydrophobic) stays on the surface, and which intercalates into the lipid?

Answer 26

the hydrophilic stays on the surface, the hydrophobic enters the lipid and facilitates the breakdown of the lipid into smaller droplets

Question 27

how do bile salts increase lipase action?

Answer 27

they increase the surface area where the lipases can act

Question 28

hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by:

Answer 28

pancreatic lipase

Question 29

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:

Answer 29

micelles

Question 30

What are micelles?

Answer 30

small aggregates of mixed lipids and bile acids suspended within the ingesta

Question 31

what happens to the micelles when the ingesta is mixed?

Answer 31

they bump into the brush border of small intestinal enterocytes, and the lipids (monoglycerides and fatty acids) are taken up into the epithelial cells

Question 32

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

Answer 32

CCK, lipase and pro-colipase

Question 33

what cleaves pro-colipase, so that is can be activated for activity with lipase?

Answer 33

trypsin

Question 34

what three things work together on the emulsified fat to convert the triglycerides to monoglycerides and free fatty acids?

Answer 34

lipase, colipase, and bile salts

Question 35

what are differences between fat droplets and micelles?

Answer 35

• 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

Question 36

True or False: Fatty acids, monoglycerides, and cholesterol can readily diffuse across the apical membrane to the cytosol

Answer 36

true!

Question 37

what happens to the empty micelle bile salts after they release their FFA and MGLYC to the apical membrane?

Answer 37

they return to the lumen to collect another load

Question 38

the bulk of the bile salts are absorbed in the ____, and then recycled by the _____

Answer 38

ileum, liver

Question 39

once inside the enterocyte, what happens to the FFA and monoglycerides?

Answer 39

they are transported to the ER, where they are synthesized into triglycerides.

Question 40

once the FFA and the monoglycerides are converted back into triglycerides by the enterocyte ER, what happens?

Answer 40

they move on to the Golgi, where they are packaged with cholesterol, lipoproteins, and other lipids into particles called Chylomicrons.

Question 41

how do chylomicrons get transported out of the basolateral membrane of the enterocyte?

Answer 41

through exocytosis

Question 42

once in the interstitial fluid, chylomicrons are transported where?

Answer 42

into the lymphatic vessel called the ‘central lacteal’, which will eventually drain to the systemic circulation

Question 43

which have longer digestive systems: herbivores or carnivores?

Answer 43

Herbivores. The increased length allows for digestion of cellulose from plant cell walls.

Question 44

Once inside the enterocyte, dipeptides and tripeptides are converted into single amino acids by:

Answer 44

intracellular peptidases