M&R Session 2 - Membrane Permeability

Question 1

Give two examples of substances unable to pass through the plasma membrane unaided.

Answer 1

Glucose
Sucrose
Ions (H+, Na+, Cl-)

Question 2

What are the roles of transport processes?

Answer 2

Maintenance of ionic composition
Maintenance of intracellular pH
Regulation of cell volume
Concentration of metabolic fuels/building blocks
Extrusion of waste
Generation of ion gradients necessary for nerves/muscles

Question 3

Define (simple) passive diffusion.

Answer 3

Movement of substances down their concentration gradient through a channel.

Question 4

Define facilitated diffusion.

Answer 4

Passive movement of substances down their concentration gradient with the help of a protein carrier.

Question 5

Define active transport.

Answer 5

Movement of substances up their concentration gradient, driven by the hydrolysis of ATP.

Question 6

What is the difference between primary and secondary active transport?

Answer 6

Primary - ATP is hydrolysed at site of membrane protein.

Secondary - ATP is hydrolysed to creat a gradient which then goes on to passively transport a different molecule out while itself re entering the cell via an antiport

Question 7

What is a uniport and give an example.

Answer 7

A channel through which one particle is transported in one direction

eg. PMCA

Question 8

What is a symport and give an example.

Answer 8

A channel through which two different particles are transported in one direction

e.g. Glucose-Na+ Symport (also a secondary active transporter)

Question 9

What is an antiport and give an example.

Answer 9

A channel through which two different particles are transported in opposite directions.

e.g. Na+/K+/ATPase

Question 10

What is the difference between intracellular and extracellular concentrations of Ca2+ ?

Answer 10

Intracellular - 50-100nM

Extracellular - ~2mM

(~20,000 fold difference)

Question 11

What are the four channels involved in resting Calcium ion concentration?

Answer 11

Plasma Membrane Ca2+ ATPase (PMCA) - High affinity, low capacity

Sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) - High affinity, low capacity

Na+/Ca2+ Exchange (NCX) - Low affinity, high capacity (secondary active transport)

Mitochondrial Ca2+ uniports - operate at high [Ca2+]i, act as buffer.

Question 12

Why does a high intracellular calcium pose a threat to the cell? Hint: what compound does it form?

Answer 12

High [Ca2+]i is toxic to cells.

Would form Calcium Phosphate.

Question 13

Why is the NCX electrogenic?

Answer 13

3Na+ substitute for one Ca2+ (3+ for 2+)

In depolarisation (extracellular membrane more -ve) Na+ effluxes out of cell. This reverses in polarisation.

Question 14

What happens with Na-K-ATPase and NCX in ischaemia?

Answer 14

In ischaemia, ATP is depleted

Na-K-ATPase stops pumping

Na accumulates intracellularly

NCX reverses to efflux Na and influx Ca.

Question 15

Give two examples of molecules able to pass through the plasma membrane unaided.

Answer 15

O2
CO2
N2
Benzene
H20
Urea
Glycerol
Alcohol

Question 16

At what point do plasma membrane transporters involved in cellular pH regulation activate?

Answer 16

When cellular buffering capacity is exceeded.

Question 17

Name an acid extruder and its mechanism for increasing pH.

Answer 17

Na/H Exchanger (NHE)

One Na+ in, one H+ out.

(Inhibited by Amiloride)

Question 18

Name Base extruder and give its mechanism of action.

Answer 18

Anion exchanger

One Cl- in, one HCO3 out.

Question 19

What is the most important transporter in maintenance of pH and why?

Answer 19

Na/K/ATPase

Creates Na+ gradient to provide energy for other pH regulatory transporters.

Question 20

What is the advantage of using more than one transporter when regulating cell volume?

Answer 20

Allows movement of osmotically-active molecules while remaining electrostatically neutral.

Question 21

Give an example of how a cell may move ions to resist swelling.

Answer 21

Efflux of Na+ and K+

H20 follows.

Question 22

Give an example of how a cell resists cell shrinking.

Answer 22

Influx of K+, influx of Cl-

H20 follows

Question 23

Give an example of a diuretic and describe its mechanism of action.

Answer 23

Loop Diuretics

• Block NKCC2 channels in Thick Ascending Limb
• Sodium remains in tubule
• Water remains in tubule and is excreted.

Thiazides

• Block NCCT channels in Distal Convoluted Tubule
• Sodium remains in tubule
• Water remains in tubule

Amiloride

• Blocks ENaC (epithelial Sodium channels) in collecting ducts
• Na remains in tubule
• Water remains in tubule