Lab 4: Membrane Potential

Question 1

What are the general transport mechanisms?

Answer 1

Question 2

Several channels that facilitate cell permiability

Answer 2

Question 3

Membrane Potetial is influenced by:

Answer 3

Question 4

Gibbs- Donan equilibrium

Answer 4

• The Gibbs-Donnan effect describes the unequal distribution of permeant charged ions on either side of a semipermeable membrane which occurs in the presence of impermeant charged ions.
• At Gibbs-Donnan equilibrium,
  
  • On each side of the membrane, each solution will be electrically neutral
  • The product of diffusible ions on one side of the membrane will be equal to the product of diffusible ions on the other side of the membrane
  • The electrochemical gradients produced by unequal distribution of charged ions produces a transmembrane potential difference which can be calculated using the Nernst equation
  • The presence of impermeant ions on one side of the membrane creates an osmotic diffusion gradident attracting water into that compartment.
• The mechanisms which maintain the resting membrane potential and the mechanisms of the Gibbs-Donnan effect are different phenomena:
  
  • The Donnan equlibrium is a completely passive process: i.e. no active transporters are involved in maintaining this equilibrium.
  • A Donnan equilibrium is an equilibrium, i.e. ion concentrations on either side of the barrier are static.
  • If the Donnan equilibrium were to become fully established, the increase in intracellular ions would cause cells to swell due to the osmotic influx of water.
  • At a Donnan equilibrium, the resting membrane potential would be only about -20 mV. This potential would exist even if the membrane permeability for all ions was the same.
  • The resting membrane potential, in contrast, requires different permeabilities for potassium and for sodium, and is maintained actively by constant Na+/K+ ATPase activity.
  • Because biological membranes (especially of exciteable tissues) are never at equilibrium, the Goldman-Hodgkin-Katz equation is usually a better choice for explaining their electrochemical behaviour.

Question 5

Ion values for a typical nerve cell:

Answer 5

• mEq/L = mN
• mEq/L = Milliequivalents of solute per litre of solvent
• mN = milliNormal
• e.g 1mmol of Na+= 1meq
• e.g 1mmol of Ca2+ = 2meq
• Note monovalents = same whereas divalents = twice the amount of equivalents
• Ca = greatest transmembrane concentration gradient (10,000 fold)
• Abnormal (50% or more than normal) increase in extracellular Ca2+,Ca will bind to VGNa+, affects VGNa protein allosteric structure, VGNa less sensitive to changes, more difficult for membrane to depolarise
• Ionic concentration can change by “sweating”

Question 6

Learn values for Mg2+, HCO3-,HPO4-,H2PO4- along with the ones shown above, common BLOCK TEST question

Answer 6

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Question 7

Classification of membrane potentials:

Question 8

RESTING MEMBRANE POTENTIAL (RMP): Deffinition

Answer 8

•Potential difference across a membrane without ANY stimulation

Question 9

Do all cells have a resting membrane potential?

Answer 9

NO!

• Excitable cells vs pacemaker cells
  
  • Cajal cells in GIT
  • SA node

Question 10

Why is the RMP close to the equilibrium potential of K+?

Answer 10

1. Na and K both have leaky channels just Na is in low number. Therefore, cells have membranes which are MORE permeable to K+ than Na+ to contribute to RMP.
2. Determined by the NET movement of ions

Question 11

MP IN DIFFERENT CELL:

• Skeletal Muscle Cells
• Smooth Muscle Cells
• Astroglia
• Neurons
• Neurons
• Photoreceptor Cells

Answer 11

Question 12

NA+ K+ ATPASE

Answer 12

Question 13

NERNST EQUATION

Answer 13