Electrochemical Gradients

Question 1

Give features of simple diffusion

Answer 1

• Simple diffusion is movement down a concentration gradient
• Molecules move spontaneously from regions of high to low concentration
• Concentration gradient dissipated
• Entropy (disorder) increased: 2nd law of thermodynamics

Question 2

If ion concentrations are different on either side of the membrane what do we get?

Answer 2

A gradient

Question 3

If there is a hole in the membrane for ions what happens?

Answer 3

• Ions flow down gradient until concentrations equal on both sides
• Equilibrium set up

Question 4

What are molecules that punch holes in membranes said to do and what are they called?

Answer 4

• Molecules that punch holes in membranes are said to facilitate diffusion
   Pumps
   Carriers
   Ion Channels

Question 5

What did Adolf Fick show?

Answer 5

• Showed that number of molecules (N) moving across an interface is proportional to:
   The area of the interface (A)
   The concentration gradient
• Fick’s law describes the rate of diffusion of a molecule

Question 6

What did Einstein show?

Answer 6

that diffusion was due to random walk of molecules

Question 7

What are Einstein’s basic diffusion laws?

Answer 7

 1D diffusion: movement along DNA
 2D diffusion: movement in a membrane
 3D diffusion: movement in liquid (cytosol, extracellular fluid)
- Molecules diffuse further if they can go in three dimension
 Because chances of bumping into each-other are lower
 Molecules have a greater chance of bumping into each-other if they have to move in two rather than three dimensions

Question 8

What are the implications of Einstein’s diffusion laws?

Answer 8

• This is why catalysts work – they provide a surface to allow molecules to bump into each-other (2D)
• Signalling molecules in membranes have more chance of interacting e.g. GPCRs (2D)
• Signalling molecules have longer ranges if they are not bound to membranes (3D)

Question 9

What is the movement of a charged substance in an electric field called?

Answer 9

Electrophoresis

Question 10

What is ion movement under the influence of an electric field called?

Answer 10

electrophoretic movements/ electrophoresis

Question 11

What is the total gradient across a membrane and what is this called?

Answer 11

 Gradient caused by diffusion – gradient caused by electrophoretic movement

• electrochemical gradient
• electrophoretic movements can either add to or subtract from diffusion

Question 12

When will you have a strong electrochemical gradient and when will you have a weak one?

Answer 12

• If the diffusion gradient and the charge gradient work in the same direction you will have a strong electrochemical gradient
• if the diffusion gradient and charge gradients work in opposite directions you will have a weak electrochemical gradient

Question 13

What is the direction in which ions move through ion channels determined by?

Answer 13

An electrochemical gradient

Question 14

What is current?

Answer 14

• Current is the number of ions per second
• Big current (many ions flowing per second)
• Small current (few ions flowing per second)

Question 15

What is voltage/ potential difference?

Answer 15

• Gradient between the two compartments
• Big potential difference (high concentration on one side, low on the other side)
• Lower potential difference (small concentration on one side, none on the other)
• No potential difference (equal number of ions on both sides)

Question 16

What is resistance?

Answer 16

• Related to the width of the path between the compartments
• Low resistance – wide path
• Higher resistance – narrow path
  • When we have low resistance we have a high current and when we have high resistance we have a low current for a fixed voltage

Question 17

What is Ohm’s law?

Answer 17

• Current (I) = Volts (V)/ Resistance ®
• Electrophysiologists use a re-arranged more intuitive equation:
• Current (I) = volts (v) x conductance
   Conductance = 1/R

Question 18

What are the four key factors influencing the rate at which ions move through an ion channel?

Answer 18

• The size of the electrochemical gradient
• The nature of the ion
• Number of open ion channels
• The properties of the ion channel

Question 19

What is the effect of size of the electrochemical gradient?

Answer 19

• If strongly negative and high concentration gradient then the electrochemical gradient is high
• If the charge is weakly negative and there is a low concentration gradient then the electrochemical gradient will be low and the rate of flow of ions across the membrane will be reduced

Question 20

What is the effect of the nature of ions?

Answer 20

• Sodium ions – electrochemical gradient high
• Chloride ions (concentration gradient high but electrical gradient in wrong direction). Low electrochemical gradient
• Charge on ion (-ve or +ve)
• Number of charges e.g. + or 2+

Question 21

What is the effect of the number of open ion channels?

Answer 21

More ion channels open then there is a faster rate of movement of ions across the membrane

Question 22

What is the effect of properties of the ion channel (selectivity filters and permeability)

Answer 22

• Selectivity filters
   Sodium selective channel will allow sodium through but not potassium
• Permeability
   Lots of subtypes of each type of ion channel
   Different subtypes differ in the permeability (the rate at which they will allow the ion through the channel) – links to resistance