module 5- electrochemical gradients

what is simple diffusion

spontaneous movement of ions down a concentration gradient (high-low)

what happens to ion movement if you punch a hole in the membrane

• ions flow down gradient until equilibrium is reached

• molecules that punch holes in the membranes facilitate diffusion

3 types of protein that allow for facilitated diffusion

• pumps

• carriers/transporters

• ion channels

why do ions spread out

molecules in liquid are constantly moving because of thermal agitation

Fick

rate of diffusion/number of molecules moving across an interface (membrane) is proportional to the concentration gradient & area of membrane

Einstein

• diffusion is due to random walk of molecules

• how far a molecule diffuses from its starting point at a given time depends whether the molecule moves in 1,2 or 3 dimensions

1D diffusion

movement along DNA

2D diffusion

movement in a membrane

3D diffusion

movemnt in liquid (cytosol/extracellular fluid)

2 implications of 2D diffusion

• allows enzymes to be catalysts

• signalling molecules in membranes are more likely to interact

1 implication of 3D diffusion

signalling molecules have longer ranges if they’re not bound to membranes

diffusion of ions in the presence of an electric field

ions become unevenly distributed (opposite charges attract to electrodes)

electrophoretic movement/electrophoresis

• ion movement under the influence of an electric field

• either adds or subtracts from diffusion → depends on direction of gradient

how to calculate total gradient/electrochemical gradient

gradient caused by diffusion - gradient caused by electrophoretic movement (pulled towards electrode)

example of electrochemical gradient

1. high Na concentration outside of the cell

2. weak electrochemical gradient: negative electrode placed on outside of the cell so Na ions have less tendency to move across cell via facilitated diffusion

3. strong electrochemical gradient: electrochemical & diffusion gradients are going in the same direction → most cells

4 factors that influence rate ions move across the membrane

• size of electrochemical gradient

• nature of the ion (charge)

• number of open ion channels

• properties of the ion channel

how does size of electrochemical gradient affect rate of ion movement

increased electrochemical gradient = increased movement of ions

how does nature of the ion (charge) affect rate of ion movement

• sodium ions (positive charge) = high electrochemical gradient

• chloride ions (negative charge) = concentration gradient high but low electrochemical gradient because its in the wrong direction

• number of charges → +1, +2

how does the number of open ion channels affect rate of ion movement

more open channels = faster rate of movement across the membrane (directly proportional)

2 properties of ion channels

selectivity & permeability

how does selectivity affect rate of ion movement

ion channels will only allow specific ions to move through membrane despite there being a favourable electrochemical gradient of another ion

how does permeability affect rate of ion movement

more permeable ion channel = faster rate of ion movement (less resistance)

what is current

flow of ions (number of ions p/s)

what is voltage

gradient between 2 compartments (potential difference)

what is resistance

width of path between 2 compartments

relationship between resistance & current

low resistance = big current, high resistance = small current

ohm’s law

• current = volts/resistance

• current = volts x conductance (1/resistance)