MP - Hu - 2/23 - Intestinal Transport II

Question 1

Pepsin from the stomach and the five pancreatic proteases hydrolyze proteins (both dietary and endogenous) to: ___, or ___

Answer 1

single amino acids, AA

oligopeptides, (AA)n

Question 2

Where do pepsin from the stomach and the five pancreatic proteases hydrolyze proteins?

Answer 2

Lumen of stomach or small intestine

Question 3

What do various peptidases at the brush borders of enterocytes progressively do?

Answer 3

Hydrolyze oligopeptides to amino acids

Question 4

The enterocyte directly absorbs some of the small oligopeptides through the action of the ___ across the apical membrane

Answer 4

H+/oligopeptide cotransporter (PepT1)

Question 5

What happens to small peptides once in the enterocyte?

Answer 5

They are digested to amino acids by peptidases in the cytoplasm of the enterocyte and then moved out of the cell across the basolateral membrane by Na+-independent amino acid transporters

Question 6

Amino acid transport is __1__ with __2__ preferentially absorbed

Answer 6

1. stereospecific

2. L-isomers (over D-isomers)

Question 7

T/F: Most carriers transport amino acids against concentration gradient

Answer 7

True

Question 8

T/F: The mechanism of transport for sugars—facilitated diffusion or sodium-coupled cotransport–is kinetically the same as for the transport of amino acids

Answer 8

True

Question 9

___ is the Na+-coupled transporter that mediates the uptake of glucose or galactose from the lumen of the small intestine into the enterocyte

Answer 9

SGLT1

Question 10

___ mediates the facilitated diffusion of fructose into the enterocyte

Answer 10

GLUT5

Question 11

Once the monosaccharides are inside the enterocyte, ___ mediates their efflux across the basolateral membrane into the interstitial space

Answer 11

GLUT2

Question 12

T/F Lipase is water soluble like all luminal enzymes of digestion

Answer 12

True

Question 13

T/F Colipase anchors lipase to micelles and provides access to neutral triglycerides inside micelles

Answer 13

True

Question 14

T/F Pancreatic lipase hydrolyzes only at 1 and 3 position of triglycerides

Answer 14

True

Question 15

What are micelles?

Answer 15

Aggregates of amphipathic molecules including:
Long chain fatty acids
Fat soluble vitamines (A, D, E, K) Cholesterol
Monoglycerides
Phospholipids
Bile salts

Question 16

For absorption of cholesterol, ___ serves as a transporter and is required for cholesterol uptake by the enterocyte

Answer 16

Niemann-Pick C1-like 1 (NPC1L1)

Question 17

Digestive enzymes can be secreted by the pancreas in active or inactive forms. Enzymes capable of digesting which of the following nutrients are only secreted as inactive precursors?

a) starch
b) nucleic acids
c) proteins
d) triglyceride
e) cholesterol esters

Answer 17

c) proteins

Question 18

Glucose-galactose malabsorption is a rare disorder caused by mutations in SGLT-1. Infants with this disorder develop severe osmotic diarrhea if they consume certain carbohydrates. Of the following, which would not be expected to cause symptoms in these patients?

a) sucrose
b) glucose
c) amylopectin
d) lactose
e) fructose

Answer 18

e) fructose

Question 19

A child is brought to the pediatrician because of severe failure to thrive, diarrhea, and edema of the extremities. Blood tests reveal that he has low plasma protein concentration (hypoproteinemia). Duodenal aspirates are obtained at endoscopy after intravenous administration of cholecystokinin, and are found to be incapable of protein hydrolysis at neutral pH unless a small amount of trypsin is added. The patient is likely suffering from a congenital lack of which of the following?

a) pepsinogen
b) PEPT1
c) trypsinogen
d) carboxypeptidases
e) enterokinase

Answer 19

e) enterokinase

Question 20

A mouse was constructed in which the expression of NPC1L1 was knocked out by genetic targeting. Assimilation of which of the following substances from the diet would be expected to be abnormal in this animal?

a) triglyceride
b) vitamin D
c) vitamin E
d) cholesterol
e) phospholipids

Answer 20

d) cholesterol

Question 21

Which of the following is the main digestible carbohydrate normally consumed in the human diet?

a) Amylose
b) Cellulose
c) Maltose
d) Starch

Answer 21

d) Starch

The three major sources of carbohydrates in the normal human diet include sucrose mainly from sugarcane and sugar beets, lactose from milk, and a wide variety of large polysaccharides collectively known as starches. Although some diets may contain a large quantity of cellulose, this substance cannot be digested by the human gut and is not considered a food. Maltose is a product of the digestion of starch but is not consumed in large quantities in the human diet.

Question 22

Which of the following is not normally found in abundance in the portal blood?

a) Amino acids
b) Glucose
c) Short-chain fatty acids
d) Triglycerides

Answer 22

d) Triglycerides

Triglycerides are digested within the intestinal lumen to monoglycerides and free fatty acids, which are then absorbed directly through the membrane of the intestinal epithelial cells. After entering the epithelial cell, the fatty acids and monoglycerides are taken up by the cell’s smooth endoplasmic reticulum where they are mainly used to form new triglycerides that are subsequently released in the form of chylomicrons through the base of the epithelial cell. The chylomicrons are absorbed by the central lacteal in the villus and transported in lymph through the thoracic lymph duct to the circulating blood. Thus, most triglycerides bypass the portal circulation.

Question 23

Various proteolytic enzymes are secreted in an inactive form into the lumen of the gastrointestinal tract. Which of the following substances is/are important for activating one or more proteolytic enzymes, converting them to an active form?

Trypsin Enterokinase Pepsin

Answer 23

Essentially all proteolytic enzymes are secreted in an inactive form, which prevents autodigestion of the secreting organ. Enterokinase is physically attached to the brush border of the enterocytes which line the inner surface of the small intestine. Enterokinase activates trypsinogen to become trypsin in the gut lumen. The trypsin then catalyzes the formation of additional trypsin from trypsinogen as well as several other proenzymes (e.g., chymotrypsinogen, procarboxypeptidase, proelastase, and others). Pepsin is first secreted as pepsinogen, which has no proteolytic activity. However, as soon as it comes into contact with hydrochloric acid, and especially in contact with previously formed pepsin plus hydrochloride acid, it is activated to form pepsin.