Session 2- Membrane Permeability, Cell Volume And pH Regulation

Question 1

What is passive diffusion?

Answer 1

Diffusion across the membrane, depends on permeability and concentration gradient

Question 2

What is facilitated diffusion?

Answer 2

Diffusion incorporating a specific protein in the bilayer-however it is till a passive process

E.g. ping-pong transport

Question 3

What is active transport?

Answer 3

Transport against a concentration/diffusion gradient

Requires ATP- the amount of free energy used is proportional to the concentration gradient

Question 4

Give an example of a co-transporter:

Answer 4

Na/Glucose co-transporter
It is a symport
Uses Na gradient to transport glucose up its concentration gradient

Question 5

What is the physiological role of Na/K ATP-ase?

Answer 5

Plasma membrane pump

It forms Na and K gradients which are necessary for electrical excitability and it drives secondary active transport

Question 6

What is the physiological role of membrane K channels?

Answer 6

Na/K ATPase creates high intracellular K
This moves out of the cell
Responsible for resting membrane potential

Question 7

How are cellular calcium concentrations controlled?

Answer 7

Large conc. gradient
PMCA and SERCA = high affinity, low capacity so remove residual Ca

NCX is high capacity/low affinity, driven by Na gradient (secondary active transport)

Question 8

What is the importance of NCX in ischamia?

Answer 8

ATP depleted
So Na/K ATPase stops
Na accumulates in cell, so NCX reverses
Cell flooded with Ca which is toxic

Question 9

What is the mechanism of action of a loop diuretic?

Answer 9

Inhibits NKCC2
So less Na reuptake, so lower osmotic gradient
In thick ascending limb

Question 10

How does ion transport regulate cellular pH?

Answer 10

Acid extruders
NHE
NBC

Base extruders
AE

Question 11

How does ion transport regulate cell volume?

Answer 11

If cell is shrinking = influx ions (Na or Ca channels) = influx water

If cell is swelling = extrude ions (K or Cl channels) = lose water

Question 12

How is bicarbonate reabsorbed from the proximal tubule?

Answer 12

Na/K powers other channels
Na conc. is low in the cell, so NHE pumps H into the tubule
This combines with bicarbonate
Carbonic anhydride turns H2CO3 to water and CO2 which diffuse into cell
IC carbonic anhydride reverses this
AE pumps bicarbonate into blood

Question 13

Why does the proximal tubule usually reabsorb all bicarbonate?

Answer 13

To retain base for pH buffers

Question 14

How does cystic fibrosis alter ion transport?

Answer 14

Defective CFTR proteins
Cl accumulates in the cell
Water moves from mucus into cell
Creates sticky thick mucus

Question 15

What is the role of CFTR in diarrhoea?

Answer 15

Overly phosphorylated by PKA
Cl excessively transported into lumen
Water follows
Ew.

Question 16

What types of molecules may easily pass through cell membranes unaided?

Answer 16

Hydrophobic molecules
Small, uncharged polar molecules

E.g oxygen, nitrogen, water, urea, glycerol