Lecture 23: Cell Processes

Question 1

what do tight junctions act as?

Answer 1

a barrier: restrict movement of substances through intercellular space bt cells

a fence: prevent membrane proteins from diffusing in the plane of the lipid bilayer

Question 2

epithelial transport properties

Answer 2

apical membrane that faces the lumen of the organ or body cavity

basolateral: adheres to the adjacent basement membrane and interfaces with the blood, also lateral intercellular space.

transcellular transport: across the cell

paracellular transport: outside the cell

Question 3

paracellular transport

Answer 3

electrical resistance can be measured (voltage + current) = resistance.

more resistance = tight epithelium (transcellular transport>)

less resistance = leaky epithelium (paracellular transport>)

Question 4

proximal resistance

Answer 4

leaky epithelium
low electrical resistance
low number of strands
bulk transport (paracellular)
e.g. duodenum/proximal tube

Question 5

distal resistance

Answer 5

tight epithelium
high electrical resistance
high number of strands
hormonally controlled (transcellular)
e.g. colon, collect duct.

Question 6

membrane permeability to warer (Pw)

Answer 6

Pw = pd + pf,

pd through lipid bilar
= small, mercury insensitive, temperature dependent

pf through water channel
= large, mercury sensitive, temperature independent, mediated by aquaporins.

Question 7

glucose absorption in the small intestine

Answer 7

tight junctions divide cells into apical + basolateral membrane domains

2. na pump sets up ion gradients
3. sodium glucose symporter SGLT uses energy of the na gradient to actively accumulate glucose above its concentration gradient (secondary active transporter)
4. facilitate glucose transporter (glut) mediates glucose exit across the basolateral membrane via passive diffusion down its gradient
5. na taken up via the glucose exits via basolateral na-pump
6. na and glucose across epithelium induces paracellular cl and water fluxes.

Question 8

what is oral rehydration therapy?

Answer 8

ability of glucose to enhance absorption of na/cl and water

simple sugar solution when give nto dehydrated babies suffering from diarrhoea is responsible for saving millions of lives per year

Question 9

glucose galactose malabsorption

Answer 9

mutation to glucose symporter in small intestine (sugar retained in lumen)

complex sugars –> 100s of small molecules –> increase in osmolarity –> body fluids will move outside the cell –> diarrhea

Question 10

what is the treatment for glucose galactose malabsorption

Answer 10

use a different transporter (glut5) transports fructose instead of glucose
- fructose exits across the basolateral membrane through glut2.

**also sometimes transforms into glucose.

Question 11

glucose reabsorption in the kidney

Answer 11

glucose in the plasma is filtered and needs to be reabsorbed into the bloodstream or it will appear in the urine.
- will have more glucose in the lumen of the tubule than in the gut because its constantly there.

too much glucose = diabetes –> sweet urine. in diabetes, glucose symporter cannot absorb glucose fast enough, and glucose appears in the urine.

Question 12

transporter kinetics

Answer 12

if glucose absorption is impaired or the transporter is saturated, glucose will appear in the urine.

saturation level = renal threshold –> glucose appears in the urine.