Lecture 6- Ion Equilibrium and Resting Membrane Potential

Question 1

Movement of Electrolytes Across Semi-Permeable Membranes

Answer 1

• Electrolytes movement is due to concentration gradient and voltage gradient (electromotive force)
• Net movement stops when forces balance
• Voltage gradient stronger than concentration gradient

Question 2

Transmembrane Voltage (Vm)

Answer 2

• Electric potential is capacity to do work. Always refers to electric charge difference between 2 compartments
• Voltage Across Membrane =Transmembrane Voltage =Vm (in volts)
• Vm always refers to inside charge vs. outside charge (more positive charge on inside means Vm is positive)

Question 3

Nernst Equation

Answer 3

-Used to calculate equilibrium potential (No net movement)for an ion at 37 C
E= -61.5/zlog10[ion]inside/[ion]outside
-E=equilibrium potential for ion
-z=charge of ion

Question 4

Equilibrium

Answer 4

• Movement across membrane always in direction that moves Vm toward equilibrium potential
• Membrane must be permeable to ion in question for equilibrium to be achieved
• Degree of permeability determines how quickly equilibrium is achieved

Question 5

Ion Equilibrium potentials

Answer 5

Na+= +60mV
K+= -88mV
Ca++= +107mV 
Cl-= -63mV
- IC [Cl-} varies between cells, so equilibrium potential varies between cells

Question 6

Resting Membrane Potential

Answer 6

• Membrane potential resulting from the different permeability of ions (mostly from K, Na, Cl)
• Permeability differs for all 3, K=1, Cl=.45, Ma=.04
• K+ permeability usually high from gated K+ leak channels that are usually open
• Permeability change is fundamental aspect of cell function

Question 7

Resting Potential of Most Cells

Answer 7

~-70 mV

• Cells are permeable to the ions in varying degrees, -70mV is weighted mixture of equilibrium potentialof K+(-88), Cl-(-63), Na+(+60)
• Permeability highest for K+, so resting potential is closer to K+ equilibrium potential
• Very few ions needed to change membrane potential, so concentration gradient largely unaffected

Question 8

Measuring Membrane Potential Changes

Answer 8

Depolarization: membrane becomes more positve
Repolarization: Membrane moves from positive toward resting potential
Hyperpolarization: Membrane becomes more negative than resting

Question 9

Types of Ion Gated Channels

Answer 9

• Selective channels for specific ions (Na+, K+, etc)
• Less selective channels
• How readily ions flow through is channels conductance
• Highly permeable ions will change vM