L5: Overview of transport

Question 1

Relative permeability of lipid membrane

Answer 1

Gases…
- Permeable: Carbon dioxide, Nitrogen, Oxygen
Uncharged polar molecules…
- Permeable: ethanol
- Slightly permeable: urea, water
- Impermeable: Glucose, fructose
Ions…
- Impermeable
Charged polar mol.s…
- Impermeable: AAs, ATP etc.

Question 2

Fundamental roles/properties of TM transport (x4)

Answer 2

• Selective; catalysing selective transport of nutrients into cell/waste out of cell
• Bioenergetics; ATP and ‘energy-coupling’ mechanisms
• Signal transduction
• Compartmentalisation; maintaining specific conditions in different regions

Question 3

Nernst Eqtn

Answer 3

Calculating voltage across membrane…
E^{ion}= RT / zF ln( {conc. outside}/{conc. cytosol})
R = Gas constant
F = Faraday constant

The nernst potential is the TM voltage at which TM ion movement is at equilibrium for a given ion conc. grad.

Question 4

Typical membrane voltage in animal cells, culprit

Answer 4

-59 mV
As a result of ‘leaky’ potassium channels. Electrochemical gradient established - Na+ influx becomes passive whereas efflux would require energy i.e. active

Question 5

Features of primary pumps

Answer 5

• Use primary source of energy usually ATP) to pump ions AGAINST electrochemical grad
• ‘Primary active transport’
• Usually transports H+ or Na+
• Electrogenic (i.e. produce electrical activity)
• Establish electrochemical gradients for ‘driver’ ions

Question 6

Carriers overview + the 3 types

Answer 6

• Can be energised by ‘driver’ ion electrochemical gradients
• ‘Secondary active transport’
• electrogenic or electroneutral* (a and b)
  Either…
  a) Symporters or co-transporters
  b) antiporters or counter-transporters
  c) Facilitators (can’t be energised*, allow facilitated diffusion)

Question 7

Channels overview

Answer 7

• Always passive transport (down electrochemical gradient)
• usually highly regulated w/ defined open and shut kinetics (‘gating’)
• Usually selective for specific ions

Question 8

General transport strategy in different organisms

Answer 8

• Animal cell plasma membranes run a sodium economy
• Plant and fungal cell plasma membranes run a proton economy (much greater gradient than in animal cells)
• Bacterial plasma membranes run a proton economy (w/ slight sodium economy)

Question 9

Discussing the different transporters in endo-membranes: roles of particular types wrt host organism

Answer 9

• V-type pumps generate acidity (large H+ electrochemical gradient)
• Most carriers are antiporters - Vacuole of plants and fungi, putting waste and nutrients into lumen
• Specialised antiporters in animal cells (e.g. nerve cells: neurotransmitters)
• Channels usually involved in membrane voltage regulation

Question 10

Turnover rate and protein density of transporter types

Answer 10

Pumps: 10^2 ions/s, 10^4 molecule/microM
Carriers: 10^3 ion/s
Channels: 10^8 ion/s, 1 molecule/microM