17B14: Exam Report

Explain the mechanisms responsible for the cell resting membrane potential (60% of marks) and describe the Gibbs Donnan Effect (40% of marks).

35% of candidates passed this question.

A good answer included a definition of the resting membrane potential and a clear description of the factors that determine it. Explanation of these factors should have included a detailed description of the selective permeability of the membrane, electrochemical gradients and active transport mechanisms. Answers should demonstrate awareness of the Nernst equation and the Goldman-Hodgkin-Katz equation. These were often confused, sometimes with the Gibbs-Donnan effect. Descriptions of the Gibbs-Donnan effect generally lacked detail and understanding. The better answers included a definition and discussed in detail the influence of non-diffusible ions (intracellular proteins) on the distribution of diffusible ions.

Eii / 17B14: Explain the mechanisms responsible for the cell resting membrane potential (60 marks) and describe the Gibbs Donnan effect (40 marks)


The RMP is the electrical potential across a plasma membrane when the cell is in a non-excited state

Equilibrium Potential

Nernst Equation

  • The Nernst Potential (for each ion) calculates the potential difference the ion would produce if the membrane was permeable to it
Nernst Equation



[Intracellular] mM


[Extraceullular] mM


Nernst Potential (mV)




[Intracellular] mM


[Extraceullular] mM


Nernst Potential (mV)




[Intracellular] mM


[Extraceullular] mM


Nernst Potential (mV)


  • At rest the Nernst potentials of Na & Cl most closely match Membrane Potential because these ions are freely diffusing, whereas Na is not

Goldman-Hodgkin-Katz Equation

  • A modification of the Nernst Eqn
  • Takes into account the permeability factors for each ion and constructs the total membrane resting potential
Goldman-Hodgkin-Katz Equation

Resting Membrane Potential

  • By definition the cell membrane must be semi-permeable (to create a potential)
  • The cell membrane encompasses sodium (chemical gradient) and is freely permeable to K & Cl (electrochemical gradient)
  • Ion channels and transport proteins regulate the movement of molecules across the membrane, allowing for an unequal distribution of charge through anions and cations
  • The resulting chemical and electrical forces comprise the RMP
  • The RMP varies depending on cell type
    • Skeletal Muscle -90mV
    • Myelinated Peripheral Nerve -70mV
    • Cardiac Muscle -90mV
    • Smooth Muscle -50mV

Generation of an Electro-Chemical Gradient


  • K ions cross the membrane through open channels
  • Internal [] 150mM
  • External [] 5mM
  • K moves down its concentration gradient until opposing electrical gradient prevents further movement
  • Results in an electrical potential


  • Sodium is not freely permeable
  • Uneven concentration created by Na-K-ATPase
  • Small electrochemical potential due to low membrane permeability


  • Chloride can pass freely
  • Creates an electrochemical gradient

Gibbs-Donnan Effect

  • Applies when one compartment contains non-diffusable ions
  • Proteins which lack permeability are also contributing to the membrane potential
  • The proteins inside the cell are adding a (small) negative charge to the overall membrane potential
  • This accounts for the different plasma and ISF concentrations of the ions

Function of Membrane Potential

  • Conduction of electrical signalling
  • Excitation-Contraction-Coupling
  • Electrochemical Gradients use energy to run cell machinery