52 Digestion/Absorption Flashcards
1
Q
Define digestion.
Where does most of it occur in the small intestine?
A
The process by which ingested food is broken down chemically into absorbable molecules.
- Duodenum
2
Q
Define absorption.
Where does most of it occur in the small intestine?
A
The movement of nutrients, water and electrolytes from the lumen of the intestine into blood.
- Jejunum
3
Q
What are the 2 general cellular pw’s of absorption?
A
Cellular and paracellular (tight junctions) paths
4
Q
Where does most carbohydrate absorption occur?
A
Most in duodenum, followed by jejunum and then ileum.
5
Q
Where does most Ca2+ absorption occur in the GI tract?
Folate?
Iron?
A
- Ca2+: Duodenum, but also some in jejunum and ileum.
- Folate: Duodenum
- Iron: Duodenum
6
Q
Where does most bile acid recycling occur in the GI tract?
A
Distal ileum, some in colon and jejunum, and least in duodenum.
7
Q
Carbs constitute __% of the typical american diet.
A
50%
8
Q
What types of carbs are typically ingested?
A
Polysaccharides, disaccharides, and very little amount of monosaccharides.
9
Q
Which of the following can be absorbed through intestinal epithelial cells? Polysacchs, disacchs, monosacchs.
A
Monosaccharides only (Thus all ingested carbs must be digested to monosacchs to be absorbed thru the intestine)
10
Q
A major dietary carbohydrate is starch – a mixture of both straight and branched-chain polymers of ________.
A
glucose
11
Q
What are the straight chain polymers of glucose (found in starch) called?
A
amylose
12
Q
What are the branched chain polymers of glucose (found in starch) called?
A
amylopectin
13
Q
What 3 disaccharides are found in our food?
A
Trehalose, sucrose, lactose
14
Q
What 2 molecules are linked together in trehalose?
What breaks down trehalose?
A
2x glucose
- Trehalase
15
Q
What 2 molecules are linked together in sucrose?
What breaks down sucrose?
A
glucose + fructose
- Sucrase
16
Q
What 2 molecules are linked together in lactose?
What breaks down lactose?
A
glucose + galactose
- Lactase
17
Q
What type of linkage is found in cellulose? Why is this important?
A
Beta-1,4 (we can’t break it down)
18
Q
Where is alpha amylase found?
What does it hydrolyze? (what alpha bonds remain?)
A
- Saliva and pancreatic secretions
- Breaks down alpha-1,4 (alpha-1,6 bonds remain)
19
Q
How far down the GI tract does salivary alpha amylase go before being inactivated?
A
Esophagus (inactivated at low pH of stomach)
20
Q
Pancreatic amylase (most significant) digests internal alpha-1,4-bonds in starch, yielding a mixture of mainly what 3 lengths of sugars?
A
Disaccharides, trisaccharides and oligosaccharides.
21
Q
Give an e.g. of an oligosaccharide breakdown product of amylopectin.
E.g of a trisaccharide? Disaccharide?
A
- Oligo: alpha-limit dextrin (from amylopectin)
- Tri: maltotriose (3x glucose)
- Di: maltose (2x glucose)
22
Q
After starch digestion into alpha-limit dextrins, maltotriose, and maltose by alpha amylase (mostly via pancreas), these disaccharides are further digested to monosaccharides by what 3 enzymes? Where are they found?
A
- Alpha-dextrinase AKA isomaltase (breaks down alpha-1,6 branches), maltase and sucrase. (break down to glucose)
- Brush border
23
Q
What are the 3 end products of carb digestion that can be absorbed by intestinal epithelial cells?
A
Glucose, galactose, fructose
24
Q
*How is glucose absorbed across the apical intestinal epithelial cell membrane?
What other molecule is transported this way?
A
SGLT1, a sodium/glc active transporter (against both gradients)
- Galactose as well
25
*In what way are glucose and galactose transported across the basilar membranes of intestinal epithelial cells?
Facilitated diffusion via GLUT2
26
*How does fructose get across the intestinal epithelial cell apical membrane?
Basilar membrane?
- Apical: Facilitated diffusion (GLUT5)
| - Basilar: Facilitated diffusion (GLUT2)
27
Define lactose intolerance.
How prevalent is it?
What is the major symptom?
Tx?
Definition: Lack or deficiency of lactase in the brush border- lactose is not digested to glucose and galactose. Reflects a nl developmental decline in the expression of lactase by enterocytes.
- Present in 50% of adult and higher in some pops (Asians).
- Sx: Diarrhea (ingested lactose remains undigested and unabsorbed in the intestinal lumen, leading to water retention and thus *osmotic diarrhea)
- Tx: Some benefit from taking bacterially derived lactase enzyme in a tablet form.
28
What is congenital lactose intolerance?
| Tx?
Lack of jejunal lactase (rare, serious).
| - Tx: Replace lactose with a sucrose or fructose diet to avoid diarrhea.
29
A defect in the SGLT1 transporter would cause what molecules to not be absorbed?
What is the name of the condition?
Sx?
Tx?
- glucose, galactose
- Glucose-galactose mal-absorption
- Sx: diarrhea
- Tx: Fructose diet
30
What are the 7 essential AA's that must be obtained from the diet? (do we have to memorize?)
Val, Leu, Ile, Phe, Try, Thr, Met
"Valerie Lu is phoning translations through Mexico"
31
Protein digestion starts in the stomach with the action of ______________.
Pepsin
32
Protein digestion is completed (after the stomach) via proteases released from ________________ and ________________.
- Pancreas
| - Brush border of small intestine
33
What are some e.g.'s of endopeptidases?
Pepsin, trypsin, chymotrypsin, elastase
34
What is a common e.g. of an exopeptidases?
| What end of the polypeptide does it break down?
Carboxypeptidase A and B
| - C-terminal end
35
Recall: what cells secrete pepsinogen in response to protein?
Gastric chief cells
36
*What activates pepsinogen to pepsin?
Low pH
37
*How and where is pepsin inactivated?
Once it reaches high pH of duodenum (via pancreatic HCO3-)
38
What are the protein breakdown products of pepsin?
| Is pepsin essential for protein digestion?
- A mixture of intact ptn, large peptides, and very little free AAs
- No
39
Name the 5 major pancreatic proteases (in their inactive precursor form).
1. Trypsinogen
2. Chymotrypsinogen
3. Proelastase
4. Procarboxypeptidase A
5. Procarboxypeptidase B
40
*Pancreatic trypsinogen is converted to trypsin via what enzyme?
Where is this enzyme found?
- Enterokinase
| - Brush border epithelial cells
41
*Chymotrypsinogen, proelastase, and procarboxypeptidase A + B are conveted to their active form via what molecule?
What else does this molecule convert?
Trypsin
| - Also converts some trypsinogen to trypsin
42
The 5 major proteases break down proteins into...
AAs, di- and tri-peptides and oligo-peptides.
43
How large can peptides be and still be absorbable?
Tripeptides or smaller are absorbable
44
*What 2 transport proteins move AA's (and di/tri-peptides) transported across the apical membrane of intestinal epithelial cells?
- Cotransported passively w/Na+
| - Cotransported passively w/H+
45
*How do AA's leave through the basilar membrane of intestinal epithelial cells?
Facilitated diffusion (like carbs)
46
*Inside the cell, most of the peptides are hydrolyzed to amino acids by ________________.
Cytosolic peptidases
47
What is clinically relevant regarding peptide xport into epithelial cells?
Peptide transporters uptake some drugs
48
If someone has a disorder where they can't absorb single AA's, are they doomed?
No, because can partially compensate w/di-/tripeptides + cytosolic peptidase.
- Trypsinogen deficiency: rare and serious. Tx: Diet of partially hydrolyzed proteins.
49
What is cystinuria?
Transporter for dibasic amino acids (cystine, lysine, arginine, ornithine) is absent in small intestine and kidney, so there's low or no absorption of these in intestine or kidney.
- The intestinal defect results in failure to absorb amino acids – excreted in feces. The renal defect results in increased excretion (hence cystinuria)
50
*Name the 4 major categories of lipids in the diet?
1. Triglycerides (major pool) – most of these have long-chain FAs esterified to glycerol backbone.
2. Phospholipids
3. Cholesterol
4. Vitamins A, D, E and K (not lipids but fat soluble)
51
Why can't lipids be digested similarly to carbs and proteins?
Lipid in a fatty meal floats on the surface of gastric contents, limiting the area of interface b/w aqueous and lipid phases. Thus access to lipolytic enzymes are restricted (enzymes are soluble in aqueous phase).
52
How does emulsification help in the digestion of lipids?
Mixing/churning into fine droplets greatly increases SA for digestive enzymes to act
53
In the stomach, lipid droplets are emulsified by _________________.
In the small intestine, the primary emulsifying agent is _________________.
- dietary proteins (lipid digestion begins here)
| - bile
54
Recall: what cells release gastric lipase?
Chief cells (as well as pepsinogen)
55
What % of lipid digestion occurs in the stomach?
| Is it required?
10%
| - no, *more important that the stomach empty chyme slowly to allow for pancreatic enzyme action (recall: CCK)
56
*Bile acids together with ____________ and products of lipid digestion surround and emulsify dietary lipids.
Lysolecithin
57
Name the 3 three important lipolytic enzymes (that can work at neutral pH) in pancreatic juice.
- Pancreatic lipase
- Phospholipase A2
- Cholesterol ester hydrolase
58
*Describe the action of pancreatic lipase.
Hydrolyzes triglycerides to monoglyceride + fatty acids.
59
*In the presence of bile acids, pancreatic lipase is inactivated unless this cofactor binds to both ____________ (name the enzyme/cofactor) and the bile acid.
Colipase
60
*Describe the action of phospholipase A2.
| What activates it?
Hydrolyzes phospholipids such as those present in cell membranes to lysolecithin + fatty acids.
- Trypsin activates it
61
*Describe the action of cholesterol ester hydrolase.
Hydrolyzes cholesterol ester to free cholesterol + fatty acids. (Also hydrolyzes ester linkages of triglycerides, producing glycerol).
62
Which products of lipid digestion are soluble in the intestinal lumen in mixed micelles?
What is the water-soluble exception?
Monoglycerides, fatty acids, cholesterol, lysolecithin
| - Exception: glycerol
63
Core of a micelle contains the products of lipid digestion – the exterior is lined with ___________________.
Amphipathic (both hydrophilic and hydrophobic face) bile salts
- The hydrophilic portion of the bile salts dissolves in the aqueous solution of intestinal lumen, shielding the hydrophobic regions of lipolytic products, solubilizing the lipids in micellar core.
64
*What form does digested lipid have to be in to traverse the apical intestinal epithelial membrane?
How does it get across?
Must be released from micelles (micelle doesn't enter cell)
| - Diffusion
65
What happens to lipid digestion products once they enter the epithelial cell?
They are re-esterified with free FAs on the sER to form triglycerides, cholesterol esters, and phospholipids.
66
*The re-esterified lipids are packaged with apoproteins to form ______________.
Chylomicrons
67
Describe the structure of a chylomicron. (not sure if important)
Have triglycerides and cholesterol at the core, phospholipids and apoproteins on the outside.
68
How do chylomicrons get back to the bloodstream?
Exocytosis, but so large that they travel in lymphatics (rather than bv's) to get to thoracic duct and into bloodstream
69
Is pancreatic insufficiency related to exocrine or endocrine dysfunction?
What are some dz's that fall under the umbrella of pancreatic insufficiency?
What's a fatty symptom of this defect?
- Exocrine
- Chronic pancreatitis; CF
- Steatorrhea (fat in stool)
70
*What are the 2 ways pH can become to low in the small intestine, causing pancreatic enzymes to be disabled?
1. Gastric parietal cells secrete excessive H+ (Zollinger-Ellison syndrome)
2. Pancreas doesn’t secrete enough HCO3- to neutralize acidic chyme.
71
What is a possible dangerous problem during ileal resection, w/r/t bile salts?
Enterohepatic circulation of bile is interrupted – bile is lost in feces.
72
What is abetalipoproteinemia?
| What is the problem on the cellular level?
Failure to synthesize Apo B – chylomicrons are either not formed or are unable to be transported into lymph.
73
Name all of the ways that fluid can get into the GI tract.
Small intestine, pancreatic juice/bile, gastric juice, saliva, and diet (all relatively even amounts)
74
What are the 2 parts of the GI tract that absorb water?
| Which does more?
```
Small intestine (majority) and colon.
(only 100-200 mL excreted in stool)
```
75
What are the 2 broad cellular mechanisms by which water can be reabsorbed in the small intestine?
What about the colon?
Paracellularly and transcellularly
| - Colon: only absorbs transcellularly (due to tighter junctions)
76
What part of the small intestine is the major site of Na+ reabsorption?
Jejunum
77
Recall: What are the 3 ways by which Na+ enters the apical side of GI epithelial cells?
1. Na+/monosaccharide cotransporters
2. Na+/AA cotransporters
3. Na+/H+ exchanger
78
What additional transporters does the ileum have that the jejunum does not?
Cl-/HCO3- exchanger in apical membrane
| - Also has a Cl- xporter (instead of HCO3- xporter) in the basolateral membrane
79
*In the ________ (J/I), there is net absorption of NaCl and in the ________ (J/I) there is net absorption of NaHCO3.
Ileum
Jejunum
(both absorb Na+)
80
Generally, how do the crypt epithelial cells of the intestines differ from the villi cells in terms of secretion/absorption?
- Crypt cells: mostly secrete fluid and electrolytes
| - Villi cells: mostly absorb fluid and electrolytes
81
*Besides NKP, what other major transporter is found on the basolateral membrane of intestinal epithelial cells?
What Cl- protein is found in the apical membrane?
Na+ -K+ -2Cl- cotransporter (*brings them all into cell from basolateral side)
- Cl- channel
82
In the small intestine, the Cl- channels of the apical membrane are usually closed. How are these channels activated?
What occurs when the channels open, secretion of Cl- or absorption?
Activated by hormones and NTs such as ACh, VIP (-> AC, cAMP second msger)
- *Cl- secretion
83
*How does Cholera lead to severe diarrhea?
Cholera toxin comes in from gut, activates adenylyl cyclase to high degree (normally activated via hormones/ACh/VIP) - fluid (via Cl-) secretion by the crypts overwhelms the absorptive capacity of villus cells, causing life-threatening diarrhea.
84
Differentiate osmotic diarrhea from secretory diarrhea.
- Osmotic Diarrhea: Caused by the presence of non-absorbable solutes in the intestinal lumen.
- Secretory Diarrhea: Caused by excessive secretion of fluid by crypt cells.
85
Would cholera cause osmotic or secretory diarrhea?
Secretory (fluid excessively secreted due to overactive Cl- excretion
86
Would lactase deficiency cause osmotic or secretory diarrhea?
Osmotic (lactose not digested to glucose and galactose, thus retaining water; bacteria can make it worse by digesting the lactose into more osmotically active forms)
87
*Ca2+, absorbed in small intestine, depends on the active form of what vitamin for its absorption?
How does this help Ca2+ get into cell?
Active vitamin D (1,25-dihydroxycholecalciferol, 1,25-DHCC)
| - Induces formation of calbindin D-28K
88
How does Ca2+ get across apical intestinal cell membrane?
| Basolateral?
- Diffuses in w/calbindin D-28K (requires 1,25-DHCC for its formation)
- Ca2+ ATPase
89
Inadequate Ca2+ absorption causes what dz in children?
| What about adults?
- Rickets (kids)
| - Osteomalacia (adults)
90
How is vit B12 released from food in the stomach?
| What then binds the B12?
Pepsin
| - R proteins
91
*How what happens to vit B12 in the small intestine (what 2 things bind to it)?
Which part of the small intestine absorbed it?
1. R proteins degraded by pancreatic proteases
2. B12 xfered to IF (secreted by gastric parietal cells)
- Ileum
92
*During a gastrectomy, parietal cells are lost. How could this lead to a dz of vit B12 absorption?
What is the dz called?
No parietal cells -> no IF -> no B12 absorption
| - Pernicious anemia
