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Physiology > DIGESTION & ABSORPTION I > Flashcards

DIGESTION & ABSORPTION I Flashcards

1
Q

Mucosa of the small inestine

A
  • Epithelial layer: single layer of epithelial cells linked by tight junctions
    • Endocrine & mucous cells
  • Lamina propria: connective tissue containing blood vessels, nerve fibers, and lymphatic ducts
  • Muscularis mucosae: thin layer of circular & longitudinal smooth muscle
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2
Q

Submucosa of small inestine

A

Network of nerves, submucous plexus, blood, and lymphatic vessels

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3
Q

Muscularis Externa of small intestine

A

Circular layer

Myenteric nerve plexus

Longitudinal layer

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4
Q

Main layers of small intestine

A
  • Mucosa
    • epithelial layer
    • lamina propria
    • muscularis mucosae
  • Submucosa - nerves, submucous plexus, blood, vessels
  • Muscularis externa
    • circular layer
    • myenteric nerve plexus
    • longitudinal layer
  • Serosa
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5
Q

Big structural & functional differences between small and large intestine

A
  • SI
    • Longer
    • Has more folds because it has villi
    • Absorbs nutrients
  • LI
    • Haustra
    • K+ secretion
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6
Q

Paneth cells are seen in small or large intestine?

Absorptive cells? Goblet cells? Stem cells? Enteric endocrine cells?

A

Paneth - Small

Absorptive, Goblet, Stem, and Enteric endocrine - both

Way more absorptive in the small intestine; way more goblet in large

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7
Q

Gut-associated lymphoid tissue (GALT)

A

Organized aggregates of lymphoid tissue called Peyer patches that sample contents of intestinal lumen via endocytosis and secretes IgA

  • protects against pathogens
  • provides immunologic tolerance to dietary substances and the good bacteria in the large intestine
  • minor: mast cells can release histamine & cytokines
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8
Q

2 systems regulating GI function

A

Autonomic nervous system - Ach & norepinephrine

Enteric nervous system - sensory, interneurons, motor neurons; only in GI tract

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9
Q

2 methods of regulating GI function

A

Hormones from enteroendocrine cells (G cells, I cells, S cells) in stomach & SI

Paracrine transmitters released from sensor cells to adjacent cells without entering the blood. Ex) Somatostatin & histamine

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10
Q

Gastrin

Hormone family

Site of secretion

Stimuli for secretion

Actions

A
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11
Q

Cholecystokinin (CKK)

Hormone family

site of secretion

stimuli

actions

A
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12
Q

Secretin

Hormone family

Site of secretion

Stimuli for secretion

Actions

A

“fireman” alkalinizes the acidity

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13
Q

Glucose-dependent insulinotropic peptide (GLIP/GIP)

Hormone family

Site of secretion

Stimuli for secretion

Actions

A
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14
Q

Describe the structure and function of the components in a villus

A
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15
Q

Starch

A

A mixture of straight & branched-chain glucose polymers

  • Amylase: straight
  • Amylopectin: branched

Glycogen: branched animal starch

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16
Q

What kind of carbs can the SI absorb?

What kind of carbs can the LI absorb?

A

SI absorbs monosaccharides - nothing larger!

LI doesn’t absorb carbs

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17
Q

Insoluble fibers are carbohydrate polymers that can’ tbe digested and get excreted

Soluble fibers are partially degraded by

A

enzymes from colonic bacteria

18
Q

First phase of starch digestion

A

Luminal digestion of starch by a-amylase

Salivary amylase & *Pancreatic amylase*

  • hydrolyze internal a-1,4 bonds in amylose & amylopectin
  • CAN’T hydrolyze terminal bonds, a-1,6 bonds (branch points in amylopectin), or a-1,4 bonds immediately adjacent to a-1,6 bonds.

–> result: short oligomers of glucose

  • Maltose (dimer)
  • Malto triose (trimer)
  • a-limit dextrins (from amylopectin)
19
Q

Second phase of starch digestion

A

Digestion of oligosaccharides at brushborder

  • Straight-chain oligomers (maltose & maltotriose)
    • ​Digested by hydrolases:
      • glucoamylase
      • sucrase
      • isomaltase
  • ​​​a-limit dextrins
    • Digested by isomaltase: cleaves a-1,4 and a-1,6

Result: free glucose monomers

20
Q

Uptake of glucose & galactose across the enterocyte

A
  1. Sodium/glucose transporter 1 (SGLT1)
    1. Symporter that takes up glucose (and galactose) against its conc gradient by coupling to Na+
  2. In the cytosol, glucose & galactose can stay for metabolism or leave across the basolateral GLUT2
21
Q

Uptake of fructose across enterocytes

Why is it associated with lactose intolerance

A
  1. Apical GLUT5 takes in fructose
  2. Can stay or leave via basolateral GLUT2

Because it’s not coupled to Na+ like glucose & galactose, fructose uptake is inefficient and easily overwhelmed –> lactose-intolerance symptoms

22
Q

Monosaccarides & disaccarides are completely absorbed in the SI.

The tiny amount of starch that does escape is metabolized by _____ into ____ taken up by ____

A

Metabolized by colonic bacteria into short chain fatty acids (acetate, proprionate, butyrate) taken up by colonic cells

23
Q
A
24
Q

Lactose intolerance

A

Lactase deficiency -> undigested lactose goes to the colon, where bacteria metabolize it and release gas that causes dirrhea & discomfort

25
Q

Sucrase-Isomaltase deficiency

A

very low levels of sucrase and isomaltase activity in brush border

26
Q

Glucose-galactose malabsorption syndrome

A

Mutations in the SGLT1 gene that result in a faulty or unexpressed protein or, more commonly, failure of the protein to traffic appropriately to the apical membrane of enterocytes.

Gastrointestinal symptoms

Oligosaccaridases are normal & fructose is well-tolerated

27
Q

3 types of enzymes that digest proteins

A
  • Luminal enzymes from pancreas & stomach
    • Peptides -> oligopeptides & aa
  • Brush border enzymes
    • Peptides -> oligopeptides & aa
  • Cytosolic enzymes
    • Oligopeptides -> aa
28
Q

Name the luminal enzymes and what activates them

A
  • Gastric: Pepsinogen -> pepsin 15%)
    • Activated by low pH
  • Pancreas: Trypsinogen, chymotrypsinogen, proelastase, procarboxypepidase A & B -> Trypsin, Chymotrypsin, Elastase, Carboxypeptidases
    • Activated by trypsin
    • Trypsinogen also activated by enteropeptidase (enterokinase from jejunum)
29
Q

Differentiate between the endopeptidases and the exopeptidases

A
  • Endopeptidases
    • Trypsin, Chymotrypsin, Elastase
    • cuts peptide bonds adjacent to specific aa –> oligopeptidases
  • Exopeptidases
    • Carboxypeptidases A & B
    • cuts peptide bonds adjacent to the carboxyterminus -> aa
30
Q
  • Luminal proteases convert dietary proteins to __.
  • Brush border peptidases then convert those to
    • __ (70%)
      • Taken across brush border by ___
    • __ (about 30%).
      • Taken across brush border by ___
  • In the cytosol of the enterocyte, ___ are cleaved to single ___
A
  • Luminal proteases convert dietary proteins to oligopeptides.
  • Brush border peptidases then convert oligopeptides to
    • aa (70%)
      • Amino acid transporter
    • dipeptides and tripeptides (30%).
      • Small peptide transporter
  • In the cytosol of the enterocyte, dipeptides and tripeptides are cleaved to single amino acids
31
Q

Absorption of oligopeptides

A
  1. H+/oligopeptide cotransporter PepT1 (apical)
  2. Peptidases in the cytoplasm digest oligpeptides -> aa
  3. Na+/aa transporters (basolateral)

If free glycine is in the lumen, enterocyte absorbs it only via <strong>apical aa transporters; </strong>whereas, glycylglycine in the lumen wil also get absorbed by PepT1

32
Q

Absorption of aa across luminal membrane

A

AA enter the enterocyte via Na-dependent (B0,+) and Na-independent (b0,+) transporters on luminal membrane

33
Q

Peptidomimetic drugs (antibiotics, cancer chemotherapy) are taken up by

A

PepT1

34
Q

The movement of aa at the ___ membrane is bidirectional. Explain

A

basolateral membrane

  • aa exit the enterocyte via Na+-independent transporters
  • aa enter the enterocyte via Na+-dependent transporters
35
Q

Villous cells vs Crypt cell

How do they get their necessary aa?

A

Villous cell (absorptive): amino acid absorbed across the apical membrane are used for protein synthesis in the cell

Crypt cell (secretory):does not absorb amino acids across the luminal side and needs to derive them from the blood across the basolateral membrane

36
Q

10% of absorbed aa are used for protein synthesis in the ___ cell

A

villous cell

37
Q
  1. _x_ is the major apical transporter for neutral AA.
  2. It is exploited by another apical transporter, _y__ which brings in _z_ in exchange for neutral AA.
    1. Then z will exit the basolateral membrane via w .
  3. If y is defective, then ____ will compensate.
A
  1. Na-dependent B0 is the major apical transporter for neutral AA.
  2. It is exploited by another apical transporter, Na-independent b0,+which brings in dibasic aa & cystine in exchange for neutral AA.
    1. Then dibasic aa & cystine will exit the basolateral membrane via y+
  3. If b0,+ is defective, then PepT1 will compensate.
38
Q

Lysurinic protein intolerance is assocaited with mutations in

A

basolateral y+L transporter

39
Q

Hartnup disease

A

Defective renal & intestinal B transporters –> can’t absorb neutral aa

  • Not protein deficient because of normal absorption of oligopeptides
  • Suffer from lack of niacin (vitamin B3) which is synthesized from tryptophan, a neutral aa lost in urine.
40
Q

Cystinuria

A

Disease caused by a defect in renal & intestinal b0+ transporters.

Accumulation of cysteine in the urine causes kidney stones. No protein deficiency because of normal absorption of oligopeptides.

Physiology (42 decks)

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