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Digestive Systems, Nutrition, and Metabolism > Digestion and Absorption > Flashcards

Digestion and Absorption Flashcards

1
Q

Describe the structure of small intestine. note that the BULK of absorption happens here.

A 
Designed to reabsorb small nutrients and max surface area:
1. Plicae circulares.
2. Vili
3. microvilli
IN THIS ORDER
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2
Q

Carbs make up what percent of US diet?

A

50%

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3
Q

Monomer vs polymer. What’s the difference and which one is the intestine able to absorb?

A

Monomer = single carb unit, cyclic form
polymer = multiple carb unit, either oligo or polysaccharide
Intestine can only absorb monomer.

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4
Q

non-digestible carb

A

fiber

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5
Q

digestible carbs (there are 3 kinds)

A

50% polysachs (starch, glycogen (obtained from animal muscle…can get carbs from this))
40% disachs (sucrose, lactose)
10% monosachs. (glucose, fructose)

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6
Q

Describe the structure of a alpha-amylose (polysach)

A

alpha-1,4 links. straight chain. makes up plant carbs

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7
Q

Describe the structure of a amylopectin. Why is this important?

A

alpha-1,4 and alpha-1,6 links. the 1,6 links are the branching structures. Denotes breakdown location for specific enzymes.

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8
Q

Why is starch easier to digest than glycogen and cellulose (fiber, not digestible at all)?

A

Starch is not as packed and has less branches.

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9
Q

describe fiber. Where can fiber be acted on, and what is the gain from this?

A

non-digestible (IN THE SMALL INTESTINE) carb. comes from plants. Can still be acted on in large intestine, but it only makes short chain FAs for the bacteria present there. does not help human.

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10
Q

What is a disach? which ones are examples?

A

carb molecule made of 2 different kinds of carbs. examples are lactose and sucrose

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11
Q

sucrose makeup

A

1 of each, fructose and glucose

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12
Q

lactose makeup

A

1 of each, galactose and glucose

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13
Q

What is the only kind of carb that can be absorbed by small intestine? what are examples of monosach?

A

monosach such as glucose, fructose and galactose (GFG). these are the final products after the brushborder digestion enzymes

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14
Q

Describe the digestion of carbs. What is the first enzyme? What composes the second? What is the final fate of the digested carb?

A
  1. large carbs like starch go through intraluminal hydrolysis to become oligosaccarides. done with Amylase.
  2. Oligosachs are broken down nto monosachs by Brush-border enzymes?
  3. monosach is then transported into cell.
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15
Q

Where is amylase? What does it do? Where does it come from?

A

Amylase, found in intestinal lumen, is major carb digestion enzyme. Produced in salivary glands and in pancreas. Note that the amylase secreted in both spaces is different, but close enough. Also note that it is secreted in ACTIVE form in both pancreas and mouthm

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16
Q

Where does digestion of carbs begin? what hinders mouth carb digestion enzyme?

A

Begins in mouth since amylase is secreted there. This enzyme is hindered by fact that amylase is wrecked by stomach acid as soon as it gets there. So it is not as important as amylase secreted into lumen.

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17
Q

What makes pancreatic amylase more important to carb digestion? what causes its secretion?

A

It is not digested by impending acid. CCK causes its secretion. Amylase here completes the carb digestion in small intestine.

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18
Q

Describe amylase bond selectivity. Be specific. Why is this important? How does the intestine overcome this issue?

A

Amylase = endoenzyme, meaning it only targets bonds within the starch. can’t target the bonds at the ends.
Specifically, alpha-amylase can’t cut:
1. terminal alpha-amylase
2. alpha-1,6 linkage branching
3. alpha-1,4 link adjacent to alpha-1,6.
Significant since it means that the products of amylase CANNOT be used (absorbed) by the small intestine. Can only take monsachs, remember? brushborder enzymes take care of the the large molecules remaining after amylose degredation

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19
Q

True/false. amylose breaks starch into glucose

A

false.

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20
Q

describe enzyme digestion,.

A

Integral enzymes bound to brush border that further digests molecules broken down by amylose. They digest oligosachs.

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21
Q

List functions of the brush border enzymes:
lactase
maltase (glucoamylase)
trehalase
sucrase-isomaltase (isomaltae = alpha-dextrinase)

A

lactase: lactose digestion
maltase (glucoamylase): glucose di or trisacharide digestion
trehalase:
sucrase-isomaltase (isomaltae = alpha-dextrinase): this is one polypeptide with 2 enzymes together. Has sucrase and isomaltase, responsible for breaking up the branches left behind by amylase

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22
Q

List products of maltase digestion

A

breaks maltase into glucose

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23
Q

list products of lactase digestion

A

breaks lactose into glucose and galactose

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24
Q

List products of sucrase-isomaltase. What is significant about this enzyme

A

sucrease component breaks sucrose into glucose and fructose. isomaltase (alpha-dextrase) component breaks dextran branches and breaks it all to glucose.

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25
Q

T/f rate of hydrolysis exceeds rate of absorption. What is the rate limiting step of digestion? hydrolysis or absorption?

A

T. makes process very efficient. absorption of monosachs is the rate limiting step. not the hydrolysis step.

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26
Q

Lactase deficiency. What gene screws up lactase presence?

A

Caused by brush border damage in some way (like disease), screwing up lactase enzyme. So, you can’t digestion lactose. that become one less thing you can digest, so one less thing the intestine can absorb (remember, monosachs only). Everything is then sent to colon, leading to diarrhea (caused by osmotic effects of the products the bacteria created, drawing in more water) and bloating as a result of the colon microorganism eating these sacharides instead of them being digested by brushborder. Lactase deficiency is a major cause of malabsorption. Screwed up gene = MCM6. note Celiac disease is another brush border disease with similar complications.

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27
Q

correlelation between H2 in breath and lactase activeity

A

increases h2 in breath means lactase is NOT working. also means not many glucose molecules are available for absorption since lactase is not breaking down lactose into glucose and galactose.

28
Q

What happens in sucrase-isomaltase deficiency?

A
  1. Can’t hydrolyze either sucrose or alpha-limit dextranns or any branched carbs
  2. has undigested carbs present in colon
  3. chronic osmotic diarrhea once they stop breastfeeding (lactase is still gucci)
  4. failure to thrive, screwing up growth curve.
  5. irritability and vomiting after consuming sucrose.
29
Q

Describe 2 step process to get carb into bloodstream

A
  1. Carb monosach uptaken into APICAL membrane of enterocyte.
  2. Exits basolateral side
    (From A to B)
30
Q

Describe how monosach gets from apical side to basolateral side1.

A

SGLT transporter on apical side accepts glucose and galactose (needs 2 Na to work). GLUT5 takes sucrose specifically. All to the sugars get to the basolateral side through GLUT2

31
Q

Explain importance of Na/K pump in enterocytes with monosach transport.

A

Na/K pumps 2 K into the enterocyte and 3 Na out into the basolateral side, which creates a more nagative resting membrane potential. This allows glucose, fructose, and gaalctose to enter with their Na due to the SGLT channel. Also note that the decrease in Na in the enterocyte because of the Na/K fosters Na entry by SGLT.

32
Q

Why is SGLT a target for DM2?

A

Drugs can target and disrupt SGLT, preventing sugar absorption, both in the enterocytes and the renal proximal tubules.

33
Q

describe the 4 general processes of protein digestion

A
  1. Enzymes hydrolyze proteins to peptides and
    to amino acids (aa) which are then absorbed
  2. Enzymes hydrolyze proteins to peptides.
    Brush border enzymes digest peptides to aa
    which are then absorbed
  3. Enzymes hydrolyze proteins to peptides
    which are absorbed. Oligopeptides are
    digested within the cells to aa which are
    transported from the cell.
  4. Digested oligopeptides can be taken up by enterocytes via endocytosis
34
Q

What can enterocytes absorb when it comes to proteins?

A

BOTH oligopeptides and AA.

35
Q

List the 9 ESSENTIAL (get from animal/plant or suffer) proteins

A

Histadine, isoleucine, leucine, methionine, phenylalanini, threonin, tryptophan, valine, and lysine.

36
Q

Where does protein digestion begin?

A

Stomach

37
Q

Explain luminal protein digestion

A
  1. Pepsinogen (zymogen secreted from GASTRIC chief cells) is activated by acid (pH below 3) in the stomach. Becomes pepsin (active)
  2. Pepsin hydrolyzes interior peptide bonds. Protein becomes smaller fragments
  3. Pepsin is inactivated at pH greater than 5 (inactivated by duodenum due to crap ton of HCO3)
38
Q

List 5 key pancreatic proteases secreted in pro form. Note that they all deal with protein digestion in the intestin…no more pepsinogen. Explain importance of these enzymes being secreted in inactive form. Provide reason for pancreantitis.

A
  • Trypsinogen
  • Chymotrypsinogen
  • Proelastase
  • Procarboxypeptidase A
  • Procarboxypeptidase B

These guys start of inactive to avoid damage to organs. Pancreantits = activation of these protein zymogens while still in pancreas, injurying pancrease.

39
Q

Explain activation of protein enzymes

A
  1. Brush border enzyme enterokinase in duodenum
  2. Converts trypsinogen to trypsin
  3. Trypsin activates other proenzymes to their active state. (• Trypsinogen
    • Chymotrypsinogen
    • Proelastase
    • Procarboxypeptidase A
    • Procarboxypeptidase B)
40
Q

explain job of carboxypepsidases A and B

A

exopeptidase (cuts off peptides at the ends of polypeptides)

41
Q

explain job of trypsin, chymotrypsin, and elastase

A

endopeptidase which cuts bonds in the middles of polypeptides to make oligopeptides

42
Q

Job of brushborder peptidases

A

Fully digests oligopeptidases to AA. Resulting tri and di peptides are taken up by cell.

43
Q

Where do you find processing of entire proteins? what cells do this? why?

A

terminal ileum with M cells. Reason for doing this is to initiate immune response.

44
Q

Describe absorption of oligopeptides

A
  1. Oligopeptides enter apical side through H driven cotransporter, PepT1. Note that NHE must first take in Na and spit H into lumen so that the H can come back in from apical side with oligopeptide with PepT1. Also not that Na/K pump on basolateral side still fosters charge and gradient environment for apical side.
45
Q

Explain AA absorption from apical side of small intestine.

A

Needs Na/K transporter for same reasons as sachs and oligopeps. However, there are several different transporters used to absorbe the different kinds of AA (neutral, basic, acidic, etc). Na is also the cation that flows in with intended AA.

46
Q

Describe predominant apical AA trasnporter

A
  1. Na dependent SLCA19 (found in intestine and proximal tubule of kidney)
  2. Na dependent
  3. Neurtral AA transporter
47
Q

Hartnup disease

A

AR disorder in which there’s a mutation of trasnporter handling neutral AA. Affects transport in both kidney and intestine. Affects L-phenylalanine and tryptophan absorption. Upon absorption of these guys, a toxic intermediate is produced, screwin up enterocytes. NOTE: if its still in di/tri peptide form, the person is gucci. just can’t be taken up by itself. Symptoms show themselves in patients with LOW protein diets. Symptoms, if any (90% assymptomatic for reason I just described) = mainly issue with tryptophan uptake, which is needed for niacin. pellagra-like skin rash, photosensitivity, and psychiatroc symps.

48
Q

Predominant dietary lipid

A

TAGs, commonly long chain FAs (16-18 carbon groups) + glycerol backbone. Dietary fat = essential FA source.

49
Q

Unsaturated vs saturated FAs

A

Saturated: No double bonds. bad
Unsaturated: Kinks. At least one double bond. Good.

50
Q

What’s point of Glycophospholipids

A

Endogenous lipid. Has phosphate group added to glycerol backbone. Polar head is added to phosphate group.

51
Q

Major events of lipid digestion

A
  1. Secretion of lipase and bile i order to hydrolyses these things.
  2. Emulsyfication of dietary fats by this bile. makes the fat globules into smaller particles.
  3. Enzymatic hydrolysis of ester linkages, thanks to lipase. allows them to be taken up by enterocyte.
  4. solubilization of lipolytic products within bile salt micelles for presentation to apical surface.
52
Q

Describe job of gastric lipases.

A

Gastric lipases, secreted from gastric chief and stimulated by gastrin hormone. Churning of stomach helps with this. Lipases are active in acidic pH. The lipases specifically chop of a FA chain from the TAG, creating 1,2-diacylglycerol. note that short FA’s can just flow through enterocytes. Long chain FAs are NOT absorbed in stomach. needs further digestion. Note: lipase is important for newborns and patients with pancreatic insufficiency.

53
Q

What happens when 1,2-diacylglycerol makes way to duodenum?

A

More lipases and co-lipases are secreted to complete the lipid hydrolysis. FAs from stomach enter dudenum, triggering CCK and GIP, but CCK slows gastric emptying to allow more time for lipid digestion.

54
Q

What does CCK stimulate?

A
  1. flow of bile salts into duodenum

2. secretion of lipase and colipase in addition to other pancreatic enzymes.

55
Q

Describe FA digestion in the duodenum/small intestine

A
  1. BS first emulsify the FA (surrounds the FAs and breaks them into smaller components by increasing surface tension between the fats.)
  2. Pancreatic lipase is secreted in active form (like amylase). works best in alkaline environments. when alone, lipase action is inhibited by BS. Also produced 1000x more than what we need. Note: lipase will not work unless co-lipase is present, because bile salts keep kicking off the lipases.
  3. Note that lipase generates FAs and monoglycerides.
  4. FAs and monoglycerides are stored in miscelles (100-500x smaller than og status). These guys are completely in solution and soluble in water. this allows enterocyte to penetrate outer aqueous layer to present the FA to the apical side.
56
Q

What activates co-lipase? What happens after its activation?

A

activated by trypsin. Co-lipase activation allows colipase to anchor lipase to lipid particles. Note: Colipase has zero enzymatic activity. that’s all lipase.

57
Q

Job of phospholipase 2

A
  1. Secreted from pancreas as zymogen (trypsin activation)

2. Hydrolyzes phospholipids

58
Q

Job of cholesteral esterase

A
  1. Secreted from pancreas
  2. Hydrolyzes cholesteral esters and esters of ADEK.
  3. Hydrolyzes esters of ALL TAG links.
59
Q

What happens once FAs enter cell?

A
  1. monoglycerid acylation, which occurs in SER. needs acyl-CoA synthase, and mono/diglyceride acyl transferases. This allows for reattachment of FA to the glycerol backbone. Note that binding proteins inside enterocytes grad the FA and drive this attachment process
  2. Reassembled TAGs are shipped in chylomicrons.
60
Q

Describe cholesterol handeling

A
  1. Dietary cholesterol is absorbed in free form.

2. also re-esterified and packages into chylomicrons.

61
Q

Explain chylomicron synth

A

apoB in RER works with microsomal triglyceride transfer protein (MTP) to add lipids to apoB. The TAG rich particles from SER then fuse with MTP-formed particles.
Note: Chylomicrons hold 80-90% TAG. not much else. Works as TAG trasnporter.
Note: Packaging works through apoB48 and apoA1. apoCII and apoE allows for being recogniced by the extraenterocytes..

62
Q

What happens if MTP synth isn’t there? (disease name and setup)

A

Can’t absorb fat. abetalipoproteinemia. Affects both liver and intestines. Patients lack VLDL and LDL, and have impaired fat soluble vitamin absorption. Develop steatorrhea soon after birth and suffer growth and cognitive impairment in infancy.

63
Q

fate of chylomicrons.

A

Lipoprotein lipase is activated by apoCII on cell membrane surface. the apoproteins are transferred from HDL. apoE mediated uptake by hepatocytes for destruction.

64
Q

What happens if chylomicrons can’t deliver their TAGs? (another disease)

A

hyperlipoprotenemia (Type 1a). patients present with VERY high serum TAGs. cholesterol is normal. deficiency in lipoprotein lipase OR apoCII. Associated with pancreatitis. Patients exhibit erupt xanthomas (aggressive pimples all over body.)

65
Q

What happens if one can’t absorb fat?

A

excessive fat in stool (steatorrhea…fluffy stool). Caused by BA deficiency or quantitative defect in enzym production from cystic fibrosis or chronic pancreatitis. also note that these patients can’t absorb ADEK.

Digestive Systems, Nutrition, and Metabolism (20 decks)

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