4. Electrochemical gradients

Question 1

Describe movement down a conc gradient. 3

Answer 1

1. simple diffusion, molecules move spontaneously from regions of high to low conc
2. conc gradient dissipated
3. increased entropy (disorder) - 2nd law of thermodynamics

Question 2

How does diffusion work when there is a membrane present? 5

Answer 2

1. more complex, ions can’t cross lipid bilayers as they repel charged molecules
2. ionic concs can be very different on either side of membrane
3. if concentrations are different it causes a gradient
4. water can cross because it is special and there is a high conc of water in water so something must move
5. not always simple diffusion, aquapores increase speed of diffusion through cells

Question 3

what are membrane pores? 4

Answer 3

1. ions flow down gradient until conc. equal on both sides
2. equilibrium established
3. pores facilitate diffusion
4. ion channels and transport proteins provide pores

Question 4

Why do ions spread out? 4

Answer 4

1. thermal agitation leads to movement of molecules in liquid
2. h2o molecules have an average centre to centre (R) distance of 2.8A’
3. A’ = angstram 0.nm
4. only travel short distances (A’) for short time periods, 2ps, before colliding

Question 5

Who was Fick and what did he say about diffusion? 4

Answer 5

1. adolf fick studied movement of molecules across interfaces and derived law at 26
2. showed that no of moleules, n, maoving axross interface is proportional to area of interface, a, and conc gradient
3. • dn/dt = d.a.dc/dx
     where -dn/dt is the rate of transfer
     d is proportionality constant - diffucion conc
     dc/dx is the conc gradient
4. Diffusion coefficient is different for different substances

Question 6

What did Einstein say about diffusion? 5

Answer 6

1. showed diffusion was due to random walk of molecules aged 26
2. question - how far will a diffusing molecule be from start point after a given time
3. anser depends on number of dimensions molecule moves in and diffusion coefficient
4. one dimension = t=(d^2)2D
   two dimensions = t=(d^2)/4D
   three dimensions= t=(d^2)/6D
   where d=root mean square distance in cm
   t = time in s
   D = diffusion coefficient in cm2s-1
5. molecules move quicker in 3 dimensions

Question 7

1. what was the importance of einstein’s work on diffusion? 4

Answer 7

1. . more diffusion in 3d because molecules have less chance of colliding
2. increased chance of colliding if 2d eg. proteins and signalling molecules in membreane
3. catalysts work by providing a surface for molecules to bump into each other on
4. signalling molecules have longer ranges if not bound to membranes

Question 8

describe ions and electrical gradients. 6

Answer 8

1. diffusion changes in an electric field because they are charged
2. membrane charge influences ion movement
3. movements of ions under influence of an electriv field are called electrophoretic movements
4. electrophoretic movements adds to/subtracts from diffusion
5. total gradient = gradient caused by diffusion-gradient caused by electrophoretic movement
6. total gradient = electrochemical gradients

Question 9

What is the effect of electrochemical gradients? 6

Answer 9

1. negativity in cell would cause na+ to move into cell independent of conc. gradient
2. there is a bigger push for na+ to move across membrane
3. strong electrochemical gradients involve both electrical and chemical gradients in same directions
4. charges can cause ions to oppose chemical con. gradients
5. charge can also intensify effect of chemical con gradients
6. cells are negative on the inside, like us all. very strong electrochemical gradient for sodium

Question 10

what determines the rate of ion movement? 4

Answer 10

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

Question 11

what is electrophysiology?4

Answer 11

1. ion are charged so flow of ions is current
2. can measure currents using electrical recording equipment, which can be used as a measure of activity of ions
3. measurement can be made on neurons and other cells
4. so, electrophysiology is the study of flow of ion across membranes

Question 12

What is current? 3

Answer 12

1. flow of ions
2. many ions flowing per second means greater current
3. must be a potential difference between two areas

Question 13

What is voltage? 1

Answer 13

1. potential difference/tendency for things for move

Question 14

What is resistance? 3

Answer 14

1. the difficulty of moving ions from one area to another
2. interface between solutions causes resistance
3. for the same voltage, more resistance means less current

Question 15

What is ohm’s law? 3

Answer 15

1. current (I) = Volts (V)/resistance (R)
2. electrophysiologists arrange this to I=Vxconductance where conductance = 1/R
3. conductance is the tendency for things to move

Question 16

Why do we use electrophysiological recordings? 4

Answer 16

1. can measure extremely fast events directly, sub ms timescale
2. very sensitive, as little as one ion channel can be detected. not many techniques allow this
3. goos spatial resolution, can focus on specific parts of a cell
4. can dissect details of individual channels - activation, inactivation, pore properties

Question 17

What ion channel and ion properties affect rate of movement? 5

Answer 17

1. charge on ion
2. number of charges
3. nernst equation
4. selectivity
5. permeability