Epithelial transport

Question 1

Epithelial tissues are organized into _____ and are ________

Answer 1

sheets; polarized

Question 2

Apical (mucosal or lumenal) epithelial cell surface

Answer 2

• faces toward the body exterior or into the body cavity

- has special transport properties

Question 3

Basolateral (serial or peritubular) epithelial cell surface

Answer 3

• face is exposed to interstitial fluid

- typically has generic transport properties

Question 4

What does the balance of peri- vs. transcellular pathways depend upon?

Answer 4

The tightness of junctions between cells (glued to neighbors by proteins occluding and claudin). Less proteins -> junctions can be leaky

Question 5

Leakier junctions allow _____ transport, but cannot maintain as _____ of concentration gradients.

Answer 5

faster, large

epithelia transporting large quantities of material tend to be leakier (GI tract)

Question 6

How is NaCl and water absorbed?

Answer 6

• Na+ enters the cell through apical membrane (high Na+ permeability and low K+ permeability) and is pumped out of basolateral membrane (low Na+ permeability and high K+ permeability) by the Na/K pump
• Cl- follows the moving Na+ and water follows through pericellular pathway

Question 7

How is NaCl and water secreted?

Answer 7

• Na-K-2Cl cotransporter on basolateral membrane keeps cytoplasm enriched in Cl- (powered by energy from Na+ moving down voltage and concentration gradient into cell
• Cl- channel usually closed on apical surface
• to secrete water - Cl- channel opens and Cl- secreted into lumen and Na+ and water follows via pericellular pathway

Question 8

How is water secretion and absorption balanced?

Answer 8

• depends on the state of Cl- channels (typically closed in GI tract, so net is typically absorption)
• during digestion, chemical signaling causes Cl- channels to open

Question 9

What does cholera do to Cl- channels?

Answer 9

secrete a protein that triggers opening of apical Cl- channels on epithelial cells in small intestine - > serious diarrhea (mortality >50%)

Question 10

What does cystic fibrosis do to Cl- channels?

Answer 10

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) - Cl- channel type
- in CF patients, this channel is mutated, impairing water secretion -> thickened mucous secretions

Question 11

How are aminos and sugars transported into blood?

Answer 11

• Transported through GI epithelium down the concentration gradient with Na+ dependent glucose cotransporter to bring nutrients into the cell (apical) (needs the energy from Na+)
• facilitated diffusion brings nutrients into interstitial fluid (basolateral)

Question 12

How is ECF composition regulated?

Answer 12

By the kidneys (the GI tract absorbs everything it can)

Question 13

Do our bodies have water pumps?

Answer 13

No

Question 14

How does sweating work?

Answer 14

• the base secretes NaCl-water solution
• distal end has tight junctions are impermeable to water and absorb salt from distal end, leaving dilute solution
• NaCl reabsorption depends on CFTR Cl- channel (so CF patients have salty skin)

Question 15

What is the driving force for nearly all transport?

Answer 15

Na/K pump (always located in the basolateral membrane)

Question 16

How do the kidneys get water out?

Answer 16

• NaCl is pumped out of the tubule and water cannot follow (impermeable membrane). salt collects in interstitial fluid
• water then passes through permeable membrane and passes into the capillary, so the urine osmolarity matches plasma
• then passes through permeable membrane passing through high osmolarity region. Water diffuses out of tubule so osmolarity ends up > 300 mosM