Lecture 2

Question 1

What are primary pumps

Answer 1

• Primary source if energy
• Transport ions against electrochemical gradients
• Primary active transport
• Usually transport H+ or Na+

Question 2

Why are primary pumps electrogenic?

Answer 2

Establish electrochemical gradients for ‘driver’ ions

Question 3

What are carriers?

Answer 3

• Secondary active transport ion movement
• Energised by ‘driver ion’ electrochemical gradients

Question 4

Two classes of secondary active tranport

Answer 4

• Symporters or co-transporters
  -Antiporters or counter-transporters

Can be electrogenic or electroneutral

Question 5

What are facilitators?

Answer 5

• Type of carrier
• Can’t be energised
• ‘uniporters’
• Allow facilitated diffusion down electrochemical or chemical gradients

Question 6

What are channels?

Answer 6

• Always passive down electrochemical gradients
• Highly regulated with defined open and shut kinetics
• Specific ion selectivity

Question 7

Animal cell plasma membrane economy

Answer 7

• Na+ economy
• Primary pumps include 3Na+/2K+ - expel sodium ions, import potassium ions - Generates large sodium ion electrochemical gradient
• Carriers:
  Facilitators - GLUT1, glucose transport

Symporters - Amino acid/sugar uptake coupled to Na+ influx in specialised cells

Antiporters - Na+/Ca2+ antiporter expels Ca2+

Channels
Sodium and potassium ion channels involved in action potential generation in nerve cells
Chloride ion channels involved in osmoregulation
Calcium ion channels for cell signalling

Sodium ion electrochemical gradient:
DeltaE = -60mV
[Na+]I = 15 mM
[Na+]o = 150 mM
–11.7 kJ mol-1

Question 8

Plant and fungal cell plasma membranes

Answer 8

• H+ economy
  Pumps
• Responsible for pumping 1H+/ATP hydrolysed - generates large H+ electrochemical gradient for influx

Carriers
Symporters - Responsible for taking up essential nutrients including phosphates, sulfates, and potassium ions#
Antiporters - Na+/H+ antiporter removes sodium ions to maintain salinity

Channels
- K+ for membrane voltage and guard cell regualtion
- Ca2+ ion channels regulate cytosolic Ca2+ during calcium ion signalling
- Cl- ions involved in membrane voltage regulation

DeltaE = -150mV
pHI = 7.5
pHo = 5.5
–25 kJ mol-1

Question 9

Bacterial plasma membrane economy

Answer 9

• H+ economy with some Na+ economy

Pumps - H+ pumps expel H+ due to electron transport - Forms large electrochemical gradient

Carriers
Symporters - Responsible for taking up essential nutrients including phosphates, sulfates, and potassium ions
Antiporter - Na+/H+ antiporter expels Na+ and generates an electrochemical gradient favouring Na+ influx - some nutrient uptake

Channels
Only a few studies investigating ion channel activity performed. To date, non selective cation channels identified – involved in osmoregulation

Question 10

Endomembranes

Answer 10

Pumps
- V-type H+ pump expels H+ from cytosol into lumen of endomembrane (2H+/ATP).
- Pump generates a large electrochemical gradient for H+ influx into the cytosol!

Carriers
- Most carriers are antiporters which exhibit coupling to H+ influx.
- Vacuole of plants and fungi: Antiporters places both waste products and nutrients in to the lumen.
- In animal cells specialised antiporters. For example, in nerve cells, neurotransmitters are stored in endomembrane vesicles at synaptic gaps.

Channels
Several types characterised. Usually involved in membrane voltage regulation (K+, anions) and in case of Ca2+ channel, signalling.

DeltaE = -30 mV;
pHI = 7.5 and pHo = 5.5.
A value of –14 kJ mol-1

Question 11

What is the Nernst potential

Answer 11

The Nernst potential is the transmembrane voltage (or electrical potential) at which transmembrane ion movement is at equilibrium for a given ion concentration gradient