Electrochemical gradients

• DIFFUSION

  • molecules move spontaneously from regions of high to low concentrations
  • concentration gradient dissipates with time

  1. entropy increases (2nd law of thermodynamics)

  • gradients form when different concentrations are present on different sides of a membrane

  1. lons flow down a gradient until concentrations equal on both sides so an equilibrium is set up
  2. molecules that punch holes in membranes are said to facilitate difusion (proteins provide holes: pumps, carriers and ion channels)
• DIFFUSION cont.

  • molecules in liquids are in constant motion due ti thermal agitation

  1. average centre to centre distance for water molecules is 2.8 A

  • the number of molecules (N) moving across an interface is proportional to: the area of the interface (A)/ the concentration gradient
  • how quickly ions diffuse depend on if the molecule moves in 1, 2 or 3 dimensions

  1. 1 dimension: t = d^2/2D
  2. 2 dimensions: t=d^2/4D
  3. 3 dimensions: t=d^2/6D
• DIFFUSION cont3.

  • basic diffusion laws have big implications for biology:

  1. 1D diffusion: movement along DNA
  2. 2D diffusion: movement in membrane
  3. 3D diffusion: movement in liquid (cytosol, extracellular fluid)

  • molecules diffuse further if they travel 3 dimensions as there's less chance of collision
• CATALYTS

  • catalysts work by providing a surface ti 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)
• ELECTROPHORETIC MOVEMENT

  • Ion movement under the influence of an electric field = electrophori movements
  • electrophoretic movement adds to or substrates from diffusion

  1. electrochemical gradient= gradient caused by diffusion- gradient caused by electrophoretic movement

  • cells are negative inside, so this corresponds to a membrane as follows

  
• OHMS LAW

  • direction ions move is determined by electrochemical gradient
  • the rate of ion movement across the membrane depends on:

  1. the size of the electrochemical gradient
  2. the nature of the ion
  3. number of open ion channels
  4. the properties of the ion channel

  • current is the flow of ions
  • voltage is potential difference (ion flow cannot exist without a potential difference)

  $current (I) =$$volts (V)/ resistance (R)$$current (I)=$$volts (V)*$$conductance$

  • where conductance is 1/R
• INFLUENCES OF MOVEMENT ON IONS

  • effect of size of electrochemical gradient

  
• INFLUENCES OF MOVEMENT ON IONS

  • effect of nature of the ion

  
• INFLUENCES OF MOVEMENT ON IONS

  • effect of number of open ion channels

  
• INFLUENCES OF MOVEMENT ON IONS

  • effect of properties of ion channels