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FHB Exam 3: GI & Metabolism > GI VIII: Digestion & Absorption > Flashcards

GI VIII: Digestion & Absorption Flashcards

1
Q

Digestive enzymes are secreted where?

A

in salivary, gastric, and pancreatic juices

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2
Q

What are the two paths for absorption of nutrients, water, electrolytes, etc.?

A

cellular and paracellular pathways

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3
Q

How are carbohydrates digested and absorbed?

A

carbohydrates must be digested into their monosaccharide form before they can be absorbed through intestinal epithelial cells

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4
Q

What is the major dietary carbohydrate, and what does it contain?

A

starch; contains amylose (straight-chain polymers) and amylopectin (branched-chain polymers)

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5
Q

What are the disaccharides in food?

A

trehalose (2 glucose), sucrose (glucose + fructose) and lactose (glucose + galactose)

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6
Q

What are the monosaccharides in food?

A

glucose, fructose, galactose

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7
Q

Why is cellulose unable to be digested by the human body?

A

it contains a beta-1,4 linkage, and the body has no enzymes available to hydrolyze this linkage; thus, cellulose is excreted instead

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8
Q

What is the main enzyme used to digest carbohydrates?

A

alpha amylase (in saliva and pancreas) - only hydrolyzes alpha-1,4 linkages

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9
Q

amylases are inactivated by ______

A

low gastric pH

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10
Q

Do the disaccharides (trehalose, lactose, and sucrose) require amylase for digestion?

A

No, they require on the brush border enzymes trehalase, lactase, and sucrase.

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11
Q

What is alpha-dextrinase?

A

it is the debranching enzyme responsible for digesting alpha-1,6 linkages

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12
Q

What are some of the disorders associated with carbohydrate digestion and absorption?

A
  • lactose intolerance: lack/deficiency of lactase in the brush border; lactose remains undigested/absorbed in intestine and retains water, causing osmotic diarrhea; represents a normal developmental decline in expression of lactase by enterocytes; present in 50% of adults;
  • congenital lactose intolerance: lack of jejunal lactase; rare and very serious
  • glucose-galactose malabsorption: mutation of SGLT1; very rare; fructose diet recommended
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13
Q

What is the SGLT1 symporter responsible for?

A

transporting glucose and galactose across the apical membrane against their concentration gradients by coupling their transport to Na+

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14
Q

What is the GLUT5 transporter responsible for?

A

transporting fructose across apical membrane by facilitated diffusion

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15
Q

What is the GLUT2 transporter responsible for?

A

transporting glucose, galactose, and fructose across basolateral membrane by facilitated diffusion

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16
Q

What are essential amino acids?

A

those that cannot be synthesized by the body and must be obtained from the diet

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17
Q

Where does protein digestion begin and finish?

A
  • begins in the stomach with activity of pepsin

- finishes in small intestine with pancreatic and brush-border proteases

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18
Q

What are the 2 classes of peptidases?

A

endopeptidases (hydrolyze interior peptide bonds) and exopeptidases (hydrolyze one aa at a time from C-terminal ends)

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19
Q

Where is pepsin normally inactivated and why?

A

in the duodenum due to pancreatic bicarbonate

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20
Q

For patients who have had their stomach removed, is protein digestion and absorption normal?

A

yes, as pepsin is actually NOT essential for normal protein digestion

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21
Q

What is more important than pepsin in protein digestion?

A

pancreatic and brush border proteases in the small intestine

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22
Q

__________ cleaves trypsinogen to yield active trypsin.

A

Enterokinase

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23
Q

What is the function of trypsin?

A

It works on all proenzymes in the small intestine to convert them into their active form

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24
Q

Trypsin can even ______ the remaining trypsin to ________.

A

autocatalyze; active trypsin

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25
Q

Trypsin activation yields which five active enzymes?

A
  • trypsin
  • chymotrypsin
  • elastase
  • carboxypeptidase A
  • carboxypeptidase B
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26
Q

In regards to protein digestion, what is absorbable?

A

amino acids, dipeptides, and tripeptides (oligopeptides and polypeptides must be further broken down by brush border proteases)

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27
Q

Peptide transporters uptake some _________.

A

drugs (this is clinically relevant)

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28
Q

Describe the transporters relevant to protein absorption.

A

There is a wide number of amino acid transporters, including neutral, acidic, basic, and imino ones.

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29
Q

Describe the way in which some amino acid transporters transport amino acids.

A

some symport amino acids and Na+

30
Q

How do amino acids exit epithelial cells of the small intestine?

A

they exit via facilitated diffusion

31
Q

What are some disorders related to protein digestion and absorption?

A
  • single aa absorption diseases: rare; can be partially or completely compensated by absorption of di and tripeptides that will have their aa’s hydrolyzed in the cytoplasm and released in the blood
  • trypsinogen deficiency: rare and serious; diet composed of partially hydrolyzed proteins
  • cystinuria: transporter for dibasic amino acids absent in small intestine and kidney; defect results in failure to absorb aa’s, causing them to be excreted in the feces or urine
32
Q

What are the major categories of lipid in the diet?

A
  • triglycerides (major pool!)
  • phospholipids
  • cholesterol
  • vitamins A, D, E, and K
33
Q

Describes a process in which the mixing action of the stomach churns the dietary lipids into a suspension of fine droplets, which greatly increases SA for digestive enzymes:

A

emulsification

34
Q

How are lipids emulsified in the stomach vs. small intestine?

A
  • stomach: via dietary proteins

- small intestine: via bile acids

35
Q

Where does lipid digestion begin?

A

in the oral cavity with lingual lipase

36
Q

What is the stomach’s role in lipid digestion?

A

it empties chyme slowly into the small intestine, allowing adequate time for pancreatic enzyme action

37
Q

What role does CCK play in lipid digestion?

A

it is released when dietary lipids enter the small intestine, and its purpose is to reduce gastric emptying

38
Q

Where does most lipid digestion occur?

A

small intestine

39
Q

How much does gastric lipase contribute to lipid digestion?

A

it hydrolyzes only ~10% of dietary triglycerides to glycerol and free fatty acids

40
Q

Pancreatic juice contains 3 important lipolytic enzymes that can work at neutral pH:

A
  1. pancreatic lipase
  2. phospholipase A2
  3. cholesterol ester hydrolase
41
Q

Describe the relationship between bile acids, pancreatic lipase, and colipase.

A

bile acids inactivate pancreatic lipase, but this effect is overcome by an important factor, colipase

42
Q

An important molecule that binds to both bile acids and lipase, anchoring lipase to the fat droplet even in the presence of bile acids.

A

colipase

43
Q

What are the products of lipid digestion, and where are they solubilized?

A
  • products=monoglycerides, fatty acids, cholesterol, and lysolecithin
  • solubilized in intestinal lumen in mixed micelles
44
Q

What is glycerol? Is it solubilized in mixed micelles? Why or why not?

A

Glycerol is another of the major products of lipid digestion, but it is NOT solubilized in mixed micelles because it is water soluble on its own.

45
Q

Describe the structure of a micelle.

A

core contains the products of lipid digestion (hydrophobic interior), while the exterior is lined with amphipathic bile salts

46
Q

Inside the cell, lipid digestion products are _________ with free fatty acids on the smooth ER to form ______, ________, and _______.

A

re-esterified; triglycerides; cholesterol ester; phospholipids

47
Q

What is a chylomicron composed of?

A

triglycerides and cholesterol at the core; phospholipids and apoproteins on the outside

48
Q

Do chylomicrons enter blood vessels?

A

No, due to their large size; instead, they enter the lymphatic capillaries.

49
Q

Describe pancreatic insufficiency.

A
  • disease of exocrine pancreas (chronic pancreatitis, cystic fibrosis)
  • results in defects of pancreatic enzyme secretion
  • abnormality of lipid digestion/absorption, resulting in steatorrhea
50
Q

What happens if there is a deficiency of bile salts?

A

Micelles do not form, which causes interference of lipid digestion/absorption. In ileal resection, enterohepatic circulation of bile is interrupted, causing bile to be lost in feces.

51
Q

Describe abetalipoproteinemia.

A
  • failure to synthesize apolipoprotein B (including B-48 and B-100, which are impt. in synthesis/exportation of chylomicrons and VLDL)
  • chylomicrons are either not formed or are unable to be transported into lymph
  • interferes w/ normal absorption of fats, cholesterol, and fat-soluble vitamins from the diet
52
Q

What happens to the majority of our fluid intake?

A

it is absorbed back into the small intestine and colon (only a small portion is excreted in stool)

53
Q

The movement of water follows what? Why is this important?

A

The movement of water follows the absorption/secretion of ions and other substances (osmotic flow). This is important because as soon as the osmotic balance is disrupted, certain things (like diarrhea) can result.

54
Q

There is net absorption of ____ in the ileum and ____ in the jejunum.

A

NaCl; NaHCO3

55
Q

The _______ is the major site of Na+ absorption in the small intestine.

A

jejunum

56
Q

The epithelial cells of intestinal crypts _______ fluid and electrolytes vs. those lining the villi, which mostly ______ fluid and electrolytes.

A

secrete; absorb

57
Q

What are Cl- channels of the apical membrane activated by? Are they normally opened or closed?

A

they are normally closed, but they are activated by hormones and NTs (ACh, VIP) - these bind to basolateral receptors and activate adenylyl cyclase, which generates cAMP, which opens Cl- channels and causes Cl- secretion

58
Q

Why does cholera cause such life-threatening diarrhea?

A

Adenylyl cyclase is highly activated by cholera toxin, so fluid secretion by the crypt overwhelms the absorptive capacity of villi cells, causing this dangerous diarrhea.

59
Q

What is the difference between osmotic and secretory diarrhea?

A
  • osmotic: caused by presence of non-absorbable solutes in intestinal lumen, which end up retaining water
  • secretory: caused by excessive secretion of fluid by crypt cells, usually provoked by overgrowth of enterohepatic bacteria (ex: Vibrio cholerae or E. coli)
60
Q

Where is calcium absorbed, and what does it depend on?

A

It is absorbed in the small intestine, and it depends on the presence of the active form of Vitamin D (1,25-dihydroxycholecalciferol), which induces synthesis of calbindin D-28K, which binds Ca2+ and allows it to be pumped across the basolateral membrane by a Ca2+ ATPase.

61
Q

What can inadequate calcium absorption cause?

A

rickets (in kids) and osteomalacia (in adults)

62
Q

How can a gastrectomy lead to pernicious anemia?

A

In a gastrectomy, parietal cells are lost, IF is not released, and vitamin B12 is not absorbed. Since vitamin B12 helps the body make healthy RBCs, an insufficiency of vitamin B12 can cause pernicious anemia.

63
Q

Do the disaccharides require amylase digestion?

A

No, only starch does. Disaccharides require the brush border enzymes (trehalase, lactase, sucrase, maltase) for digestion.

64
Q

Which enzyme is required for digesting alpha-1,6 linkages?

A

alpha-dextrinase (a debranching enzyme)

65
Q

What is an example of an exopeptidase?

A

carboxypeptidase (A & B)

66
Q

Is pepsin essential for normal protein digestion?

A

No; in people whose stomach has been removed or who do not secrete gastric H+, protein digestion/absorption is still normal.

67
Q

What is the first step in carbohydrate digestion vs. protein digestion in the intestine? (note that this is different from the mouth/stomach)

A
  • carbohydrate: breakdown of starch into oligosaccharides, trisacch., and disacch. via pancreatic amylase
  • protein: activation of trypsinogen to trypsin by enterokinase
68
Q

True or false: oligo-peptides are absorbable.

A

False - they must be further hydrolyzed by brush border proteases into smaller, more absorbable molecules.

69
Q

Why is emulsification important?

A

Lipid in a fatty meal floats on the surface of gastric contents, limiting the area of interface b/t aqueous component (where lipolytic enzymes are) and lipid component. Emulsification churns dietary lipids into a suspension of fine droplets, which greatly increases SA for digestive enzymes.

70
Q

What are two possible causes of increased acidity of duodenal contents?

A

1) Gastric parietal cells secrete excess H+ (Zollinger-Ellison Syndrome)
2) Pancreas does not secrete enough HCO3- to neutralize acidic chyme

FHB Exam 3: GI & Metabolism (17 decks)

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