Membrane Permeability -- 2.1 & 2.2

Question 1

How do small polar uncharged molecules pass though the bilayer?

Answer 1

down their concentration gradient by diffusion.

Question 2

How do hydrophobi molecules move across the bilayer?

Answer 2

Hydrophobic molecules (O2 and CO2) diffuse across easily.

Question 3

How do large, uncharged polar molecules pass through the bilayer?

Answer 3

Large, uncharged polar molecules and ions must pass through by transport proteins, again down a concentration gradient.

Question 4

The smaller the membrane coefficient …

Answer 4

… the larger the net rate of transport

Question 5

Define secondary transport

Answer 5

transport of a substance is linked to the concentration gradient and transport of another. Usually uses sodium gradient.

Question 6

What are symports?

Give an example of a symport

Answer 6

two molecules move from the same side to another e.g. glucose cotransporter in small intestine

Question 7

What are antiports and give an example

Answer 7

molecules move to opposite sides e.g. Na/Ca exchange.

Question 8

What is the purpose of the Na/K/ATPase?

Answer 8

It is necessary for setting up the membrane potential but does not produce the membrane potential
– sets up the Na conc gradient that drive the majority of particle transport

Question 9

Describe some characteristics of Na/K/ATPase

Answer 9

antiport, 3Na for 2K, uses ATP hydrolysis, primary AT = ATP hydrolysis, inhibited by digoxin and ouabain (cardiac glycosides)

Question 10

Describe PMCA characteristics and what its purpose is

Answer 10

antiport, high affinity, low capacity for Ca, Ca in H out, primary AT = ATP hydrolysis, maintains low intracellular calcium

Question 11

Describe SERCA characteristics and what its purpose is

Answer 11

antiport, high affinity low capacity for Ca, increase Ca in SR/ER in exchange for H, primary AT = ATP hydrolysis, maintains stores of Ca in SR/ER to prevent cell death

Question 12

What cellular response does NCX have in ischaemia?

Answer 12

Low ATP levels due to low O2.

Na pump inhibited, NCX reverses to lower Ns a conc, high Ca is toxic and activates potent enzymes causing cell death

Question 13

Describe the characteristics of NCX

Answer 13

secondary AT, 3Na in 1Ca out, current follows Na so not dependent on ATP hydrolysis, involved in ischaemia

Question 14

Describe the characteristics of NBC

Answer 14

Na and HCO3 into cell and Cl out, follows Na gradient

Question 15

Describe the characteristics of NHE

Answer 15

uses Na gradient to move H out of the cell, increasing the intracellular pH

Question 16

How is cell volume controlled?

Answer 16

Efflux or influx of ions. Water will follow.

Question 17

What happens in response to cell swelling?

Answer 17

• • cells extrude ions and water follows

- - usually efflux K, Na, Cl or solute molecules

Question 18

What happens in response to cell shrinkage?

Answer 18

• • cells influx ions and water follows

- - influx K, Na, Cl

Question 19

How is bicarbonate resorbed in the kidneys?

Answer 19

Starts as NaHCO3 in lumen of kidney.
Splits into Na+ and HCO3-
Na+ taken up by NHE (exchanges H+)
HCO3- + H+ –> H2CO3 (carbonic anhydrase)
H2CO3 –> H20 + CO2 which diffuse across
H20 + CO2 –> H2CO3 (carbonic anhydrase)
H2CO3 –> H+ + HCO3-
HCO3- moved into capillary by anion exchanger (exchange Cl-)
Na into capillary by Na pump (exchange K)

Question 20

How much bicarbonate is reabsorbed by the kidneys and why is it resorbed?

Answer 20

All of it in order to retain a base for the buffers

Question 21

Where does Na resorption take place in the kidneys?

Answer 21

Thick ascending limb, distal convoluted tubule and cortical collecting duct.

Question 22

What is the mechanism for Na resorption in the thick ascending limb?

Answer 22

Na moves out of kidney lumen via NKCC2 (Na,K,2Cl)
K is filtered back into lumen via ROMK
KClCT moves K & Cl into capillaries
ClC-Kb moves Cl into capillaries
Na/K/ATPase transports 3Na into capillaries and 2K in exchange

Question 23

What inhibits resorption at the thick ascending limb and what does it treat?

Answer 23

Loop diuretics – inhibit NKCC2

Limits Na resorption, more Na stays in lomerular filtrate reducing blood volume which treats hypertension

Question 24

What is the main cotransported used for Na resorption in the distal convoluted tubule and what is its inhibitor drug?

Answer 24

NCC2 is inhibited by thiazides

Question 25

What is the main cotransported used for Na resorption in the cortical collecting duct and what is its inhibitor drug?

Answer 25

ENaC and Aquaporins

Inhibited by Spironolactone