Physiology 3140A Lecture Notes - Lecture 13: Potassium Acetate, Mass Diffusivity, Nernst Equation
2 May 2018
School
Department
Course
Professor
Cell Physiology 3140
October 16 2017
IONIC DISTRIBUTION AND NERNST EQUATION
- Define the driving forces for ions across cell membrane.
- Explain the Nernst equilibrium as a balance of the chemical (concentration gradient) and electrical
forces.
- Predict the effects of changing ion concentrations on the equilibrium potentials.
Summary of conventions you will encounter
- Separation of charges gives rise to a potential, or voltage gradient
- Similar charges repel one another; opposite charges are attracted
- Permeable and semi-permeable membranes.
- Diffusion coefficient, D, a parameter to describe how fast an ion (or atom) can defuse through a
membrane barrier
o How fast or how rapid certain ions pass through the membrane barrier
Forces on an ion
- Chemical force depends on the concentration gradient and absolute temperature (T)
o Absolute temperature measured in: Kelvin (K) Normal temp = 273K
- Electrical force depends on the charge and electrical gradient (potential)
o Electrical potential between the two compartments (if you can separate the charges)
o Drives BOTH ions towards the opposite charged side
o DEPENDS ON SIZE OF CHARGE + SIGN OF CHARGE
- Total force = electrical force + chemical force
- Chemical force: concentration gradient
o HIGH CONC LOW CONC
- Electrical force: charge
o TOWARDS OPP CHARGE
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- In the second membrane: there is BOTH a chemical force and electrical force:
o Charge separated
o Type of ions separated on each side of the membrane
Diffusion of Electrolytes:
- Example 1: membrane is equally permeable to K+ and Ac-
- There is a chemical gradient present:
o More solute in compartment 1 (10mM) vs. compartment 2 (1mM)
- Potassium acetate dissociates into ions in solution:
o Potassium ions (positive)
o Acetate ions (negative)
o Forms a chemical gradient!
Diffusion Potential
- Assume diffusion coefficient for K+ > Ac- (DK+>DAc-) potassium moves faster than acetate
o Potassium diffusion coefficient >> Acetate diffusion coefficient
o This is because potassium is smaller compared to acetate
o Potassium will be the first ion to move from compartment 1 to compartment 2
- As soon as potassium moves over, there is potential present between the two compartments
o The potential is working on BOTH ions (both will go towards opposite charge)
- Diffusion potential reduces the rate of potassium moving into compartment 2, and accelerates the
acetate moving (negatively charge)
- Since there is a diffusion potential, it is difficult for potassium to continue moving into compartment
2, but easier for acetate to move into compartment 2
- Faster K+ moving into compartment results in net positive charges in , giving a diffusion
potential in which is electrically positive relative to [ K+ leaving compartment 1 results in
excess Ac-, or net negative charges, in ]
- The positive potential in repels K+, and attracts Ac-
- Ac- gradually moves to compartment 2 to re-establish electrical neutrality
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At equilibrium: no diffusion potential
- Eventually the compartments will have no chemical gradients.
- There will be charge neutrality in both compartments (no electrical gradient)
- There will be NO potential between the two compartments
- Even though acetate was SLOWER getting into compartment 2, it still reaches the equilibrium
- Note: the membrane is permeable to BOTH potassium and acetate
Membrane impermeable to anion
- Impermeable to anions making it impermeable to acetate
o Acetate can no longer move across the membrane
- Potassium will move into compartment 2, but the acetate will not be able to move over
- No Ac- movement across membrane, and compartment 2 will remain positive in potential with the
accumulation of + charges that repels further K+ movement
- Once potassium moves into compartment 2, a potential is generated
o This potential reduces the tendency of potassium moving over
When will the diffusion of K+ stop?
- Diffusion of K+ will stop when the chemical force is balanced by an electrical force resulting in an
electrochemical equilibrium, BALANCED STATE
o Electrical gradient = concentration gradient
- Stop = the rate of moving in and rate of moving out is similar to one another
o THERE IS STILL MOVEMENT
o RATE IN = RATE OUT
- Enough potential is generated at the membrane that there are equal forces pushing potassium back
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find more resources at oneclass.com
Document Summary
Define the driving forces for ions across cell membrane. Explain the nernst equilibrium as a balance of the chemical (concentration gradient) and electrical forces. Predict the effects of changing ion concentrations on the equilibrium potentials. Separation of charges gives rise to a potential, or voltage gradient. Similar charges repel one another; opposite charges are attracted. Diffusion coefficient, d, a parameter to describe how fast an ion (or atom) can defuse through a membrane barrier: how fast or how rapid certain ions pass through the membrane barrier. Chemical force depends on the concentration gradient and absolute temperature (t: absolute temperature measured in: kelvin (k) normal temp = 273k. Total force = electrical force + chemical force. Chemical force: concentration gradient: high conc low conc. In the second membrane: there is both a chemical force and electrical force: charge separated, type of ions separated on each side of the membrane. Example 1: membrane is equally permeable to k+ and ac-
