Block 2 Lecture 17

Question 1

what are the two sides to a plasma membrane

Answer 1

basolateral and apical

Question 2

where is the basolateral side located

Answer 2

the blood side of the layer and is exposed to ISF

Question 3

where is the apical side located

Answer 3

outside of the body such as the lumen of intestines)

Question 4

what is an exception to the apical side of the membrane

Answer 4

choroid plexus where the apical side is exposed to the cerebrospinal fluid

Question 5

what are the principle barrier epithelia that line organs that have large surfaces exposed

Answer 5

GI tract, kidney nephron, and liver (other significant are choroid plexus, retina and exocrine glands)

Question 6

what are the 2 types of pathways for transport

Answer 6

paracellular = between adjacent cells
transcellular = across individual cells

Question 7

what happens in paracellular transport

Answer 7

molecules move through tight junctions

Question 8

what are tight junctions

Answer 8

a complex of proteins that attach cells to one another

Question 9

what are the functions of tight junctions

Answer 9

1. ) barrier that reduces free diffusive movement of molecules via the paracellular pathway.
2. ) provides a barrier to prevent mixing of molecules between basolateral and apical membranes

Question 10

where are tight junctions located in between cells

Answer 10

typically closer to the apical side

Question 11

what size molecules can pass through tight junctions

Answer 11

any molecule less than 4A (glucose is slightly larger)

Question 12

what do claudins do

Answer 12

establish the permselective character of tight junctions and form aqueous pores of about 4A

Question 13

claudins that form pores that contain a surplus of cationic residues in the postulated pore forming region have been found to favor the paracellular movement of what?

Answer 13

anions such as Cl-

Question 14

claudins that form pores that contain a surplus of anionic residues in the postulated pore forming region have been found to favor the paracellular movement of what?

Answer 14

cations such as Na+

Question 15

what are zymogens released by in the pancreas?

Answer 15

secretagogues

Question 16

what are the 4 types of secretagogues

Answer 16

CCK, ACh, VIP, and secretin (CCK and ACh most important)

Question 17

how do CCK and ACh work

Answer 17

by binding to the G protein coupled CCK receptor or the muscarinic ACh receptor which activates protein kinase C (PKC) producing an increase in intracellular Ca2+

Question 18

how does VIP and secretin work

Answer 18

they bind to different receptors that stimulate PKA to release zymogens

Question 19

what “help” zymogen granule release (SSC)

Answer 19

SNARE proteins, Rab proteins, and synaptotagmin

Question 20

what do SNARE proteins do

Answer 20

there are more than 60 SNARE proteins expressed in cells to get right vesicles to fuse in right place. there are v-SNARES and t-SNARES

Question 21

what do Rab proteins do

Answer 21

they are small G proteins the bind and hydrolyze GTP that help assemble SNARE fusion complex. there are about 70 Rabs which aid in targeting specificity

Question 22

what are synaptotagmins proteins and what do they do

Answer 22

they are integral membrane proteins in vesicle membranes. they bind Ca2+ and once activated can stimulate the fusion event in exocytosis. 15 synaptotagmins involved in vesicle fusion in different cell types

Question 23

why is SSC energetically unfavorable

Answer 23

because when vesicle membranes and plasma membrane get close to each other they repel because they are both negatively charged

Question 24

what happens in acinar cells of the exocrine pancreas

Answer 24

1. ) driven by Na,K ATPase in basolateral membrane
2. ) Na+ gradient drives accumulation of Cl- from blood into acinar cells via Na,K, 2Cl cotransporter
3. ) Cl- moves from acinar cells into lumen through apical Cl- channel because of inside negative membrane potential
4. ) trans epithelial secretion of Na+ relies on paracellular pathway. Cl- moving into lumen makes it negative providing electrical force to support paracellular diffusion of Na+ through tight junction
5. ) increase in NaCl in lumen supports osmotically driven flux of water via aquaporins

Question 25

what results in inside negative membrane potential

Answer 25

outwardly directed K+ gradient

Question 26

how is the Na, K, 2Cl con transporter electrically neutral

Answer 26

because there’s a flow of 2 cations (1 Na+ and 1K+) and 2 anions (2Cl-)

Question 27

what does the apical Cl- channel open in response to

Answer 27

activation of PKC by CCK or ACh

Question 28

what happens in duct cells in the exocrine pancreas

Answer 28

1. ) driven by Na,K, ATPase
2. ) basolateral HCO3- entry involves Na/HCO3- cotransporter
3. ) HCO3- is secreted into lumen via Cl-/HCO3- exchanger
4. ) the Cl- that enters the cell via Cl-/HCO3- exchanger exits the cell back into lumen via 2 Cl- channels: CTFR and ORCC
5. ) the cycling of Cl- keeps ductal lumen negative supporting paracellular diffusion of Na+
6. ) net flux of NaHCO3 results in osmotic flux of water into lumen. this flow pushes zymogens into small intestine