PSC2002/L02 Membrane Transport I

Question 1

Give 2 types of macrotransfer movement.

Answer 1

Endocytosis
Exocytosis

Question 2

What is microtransfer?

Answer 2

Smaller scale movement
Cell membrane transport of small molecules and ions

Question 3

What is Clathrin?

Answer 3

‘Bendy’ protein that mediates endo/exocytosis
Composed of 3 heavy chains & 3 light chains
Coats inner surface of membrane and assembles into basket-like structure
Once vesicle is formed, clathrin coat is shed and vesicle merges with other cellular components

Question 4

How permeable is a phospholipid membrane (with no protein) to the following molecules:
a) Hydrophobic
b) large polar
c) small uncharged polar
d) Ions?

Answer 4

a) Very highly
b) low
c) high
d) impermeable

Question 5

Resting membrane potential is mostly due to which 2 ions?

Answer 5

Na+ and K+

Question 6

What is the Nernst equation?

Answer 6

(Em) = (RT/zF) Ln ([X]o/[X]i)
Em = membrane potential
R = Gas constant (8.314)
T = temp (K)
z = no. charges on ion
F = Faraday’s no (96485.3 C.mol^-1)
[X]o/i = conc of ion out/inside

Question 7

What is predicted by the Nernst equation?

Answer 7

Equilibrium membrane potential based on concentration gradient of ion across membrane (Nernst potential)

Question 8

How is the Goldman-Hodgkin-Katz equation different to the Nernst equation?

Answer 8

GHK equation takes into account all ions and their respective permeabilities

Question 9

What is the permeability ratio of non-excitable cells?

Answer 9

2:1
K+:Na+

Question 10

What is the permeability ratio of nerve/muscle cells?

Answer 10

25:1
K+:Na+

Question 11

What is the most powerful electrophysiological method for basic research?

Answer 11

Voltage clamp

Question 12

What can be determined by a voltage clamp experiment?

Answer 12

Detailed measurement and analysis of electrical activity across a tissue, cell or artificial membrane mediated by specialised ion channels and electrogenic carriers

Question 13

When combined with molecular biology, what can be found from a voltage clamp?

Answer 13

Fundamental information on structure, function and regulation of transport proteins

Question 14

Describe the process of using a voltage clamp. (3)

Answer 14

Investigator sets holding voltage
Voltage clamp uses negative feedback to maintain cell at command potential
Apparatus compensates for changes in current to maintain constant membrane potential

Question 15

When and who was the voltage clamp first developed by?

Answer 15

Cole and Marmont in 1930-1940s
First used in giant squid axon

Question 16

What is a patch clamp?

Answer 16

Recording pipette physically isolates a patch of membrane on cell surface
Can electrically clamp potential across patch to measure current flow through single ion channel

Question 17

Describe the basis of the patch clamp.

Answer 17

Formation of high resistance seal between membrane and micropipette
Giga-seal means current flowing through ion channels in patch can be recorded with low noise

Question 18

What are co-transporters/symporters?

Answer 18

Movement of a solute coupled to movement of another down its concentration gradient

Question 19

What are counter-transporters/antiporters?

Answer 19

Coupled movement of two or more solutes in opposite directions

Question 20

What is the equation for flux (flow)?

Answer 20

J = P./\C
P = permeability
/\C = conc difference

Question 21

What is the equation for permeability?

Answer 21

P = Dk/x
D = diffusion constant
k = partition constant
x = distance

Question 22

What are the 3 kinds of transporters that facilitate diffusion?

Answer 22

Channels (non-gated)
Gated channels
Uniporters

Question 23

Which channels allow direct access to:
a) cells
b) bacteria, mitochondria, NPC?

Answer 23

a) Aquaporins
b) Porins

Question 24

What are the 3 functional components of gated channels?

Answer 24

Gate
Sensor
Selectivity filter

Question 25

What can gated channels be modulated by? (3)

Answer 25

Voltage
Mechanical stimuli
Ligand binding

Question 26

Describe the 6 steps of facilitated diffusion.

Answer 26

Carrier open
Solute binds and enters
Outer gate opens
Inner gate opens
Solute released
Inner gate closes

Question 27

Which carrier-mediated transporters are found:
a) in RBC
b) in intestines
c) in skeletal/cardiac muscle?

Answer 27

a) GLUT1
b) GLUT2
c) GLUT 4