Lecture 4 – ELECTROCHEMICAL GRADIENTS

Question 1

Movement down concentration gradients:

Answer 1

• Simple diffusion
• Molecules move spontaneously from regions of high to low concentrations
• Concentration gradient dissipated
• Entropy disorder increased

Question 2

Situation more complex when there is a membrane:

Answer 2

• Ions cannot cross lipid bilayer
• Ionic concentrations can be massively different on either side
• Concentrations are different a membrane is established

Question 3

A gap in the membrane:

Answer 3

• Ions flow down gradient until the concentrations are equal on both sides
• Equilibrium is established
• Facilitated diffusion

Question 4

Why do ions spread out?

Answer 4

• Molecules in liquids are in constant motion due to thermal agitation
• Water molecules average centre to centre distances (r) about 2.8A
• Ions move at short distances (A) and not long (psecs) before bumping into each other

Question 5

How quickly do ions diffuse?

Answer 5

The number of molecules (N) moving across an interface is proportional to the area of the interface (A) and the concentration gradient.

Question 6

How quickly do ions diffuse?

Answer 6

• Diffusion is due to random walk of molecules
• Molecules diffuse further if they can go in 3D
• Because chances of bumping into each other are lower
• Catalysts provide a surface to allow molecules to bump into each other
• Signalling molecules in membranes have a higher chance of interacting due to GCPRs
• Signalling molecules have longer ranges if not bound to a membrane

Question 7

The influence of electrical gradients:

Answer 7

The movements of ions under the influence of an electric field are called electrophoretic movements which adds or subtracts from diffusion.

Total gradient (electrochem) = gradient cause by diffusion – gradient caused by electrophoretic movement

Question 8

Ions moving through ion channels:

Answer 8

1. Direction ions move determined by electrochemical gradient (in or out of cell)
2. The rate depends on:
   - Size of electrochemical gradient
   - Nature of ion
   - Number of open ion channels
   - Properties of ion channels

Question 9

Electrophysiology:

Answer 9

• The current is the flow of ions – ions are charged
• Can measure currents using electrical recording equipment
• Measurements can be made on neurones (and other cells)
• In B) can’t get ion flow (current) unless have a potential difference

Question 10

The Principle is OHM’s Law:

Answer 10

1. Current (i) = Volts (V) / Resistance (R)
2. Electrophysiologists use a re-arranged equation:
   Current (i) = Volts (V) X Conductance (R)

Question 11

Why are electrophysiological recording made?

Answer 11

• Extremely fast events
• Extremely sensitive as one channel can be detected
• Spatial resolution
• Dissect details of individual channels which leads to:
  1. Activation
  2. Inactivation
  3. Pore properties

Question 12

Properties of ion channels:


Answer 12

1. selectivity

2. permeability

Question 13

number of ion channels

Answer 13

greater number of channels, more ions pass through

Question 14

nature of an ion

Answer 14

1. charge

2. electrochemical gradient