Chapter Goals (Membrane Potentials)

1. Chapter Goals (Membrane Potentials) After studying this chapter, students should be able to 1. explain what is meant by active transport and describe how the Na+/K+ pumps work. 2. explain how an equilibrium potential is produced when only one ion is able to diffuse through a cell membrane. 3. explain why the resting membrane potential is slightly different from the potassium equilibrium potential, and describe the effect of the extracellular potassium concentration on the resting membrane potential. 4. explain the role of the Na+/K+ pumps in the maintenance of the resting membrane potential.

2. E. Membrane Potentials • Electrical Potentials • Production 3.25a

3. E. Membrane Potentials • Electrical Potentials • Production 3.25b

4. E. Membrane Potentials • Electrical Potentials • Production 3.25c

5. E. Membrane Potentials • Electrical Potentials • Production 3.25d

6. Electrical Potentials • Measurement

7. Membrane Potential • Equilibrium Potentials • Resting Potentials • a. Differential ion concentration produced via Na+ pump (Na+/K+-dependent ATPase) • b. Differential diffusion rate produced via selective membrane permeability.

8. 3.29

9. 3.26

10. 3.27

11. 3.28

12. Nernst Equation Ex= (-58/z )(log [Xi]/[Xo]) Where x = ion in question z = valence of the ion [Xi] = concentration on inside of cell [Xo] = concentration on outside of cell

13. Nernst Equation Ex= (-58/z )(log [Xi]/[Xo]) Where x = ion in question = K+ z = valence of the ion = 1 [Xi] = concentration on inside of cell = [143 mM] [Xo] = concentration on outside of cell = [3 mM] EK+ = (-58/1)(log 143/3) = (-58)(1.678) = -97.3 mV

14. Nernst Equation Ex= (-58/z )(log [Xi]/[Xo]) Where x = ion in question = Na+ z = valence of the ion = 1 [Xi] = concentration on inside of cell = [6 mM] [Xo] = concentration on outside of cell = [150 mM] ENa+ = (-58/1)(log 6/150) = (-58)(-1.4) = 81.2 mV

15. Nernst Tips • First Calculate Nernst • Questions an Ion has to ask. 1. Where am I? 2. Where do I want to be? 3. Is #2 more “+” or more “-” than #1? 4. In which direction must I move to get where I want to be? • If two or more ions are moving simultaneously, E is algebraic average of Nernsts

16. Nernst Example for Na+ • First Calculate Nernst: Assume +85 • Questions an Ion has to ask. 1. Where am I? Assume Em= -108 mV 2. Where do I want to be? Answer: +85 3. Is #2 more “+” or more “-” than #1? More Positive. 4. In which direction must I move to get where I want to be? Answer: In • Assume Na+ and Ca+2 are moving, and Nernsts are 85 and 55. Em = 70 mV

17. Nernst Continued

18. Chapter Summary I. The cytoplasm of the cell contains negatively charged organic ions (anions) that cannot leave the cell; they are "fixed" anions. A. These fixed anions attract K+, which is the inorganic ion that can pass through the cell membrane most easily. B. As a result of this electrical attraction, the concentration of K+ within the cell is greater than the concentration of K+ in the extracellular fluid. C. If K+ were the only diffusible ion, the concentration of K+ on the inside and outside of the cell would reach an equilibrium. 1. At this point, the rate of K+ entry (due to electrical attraction) would equal the rate of K+ exit (due to diffusion). 2. At this equilibrium, there would still be a higher concentration of negative charges within the cell (because of the fixed anions) than outside the cell. 3. At this equilibrium, the inside of the cell would be ninety millivolts negative (-90 mV) compared to the outside of the cell. This is called the K+ equilibrium potential (EK).

19. Chapter Summary D. The resting membrane potential is less than EK; it is usually -65 mV to -85 mV. This is because some Na+ can also enter the cell. 1. Na+ is more highly concentrated outside than inside the cell, and the inside of the cell is negative. These forces attract Na+ into the cell. 2. The rate of Na+ entry is generally slow because the membrane is usually not very permeable to Na+.

20. Chapter Summary II. The slow rate of Na+ entry is accompanied by a slow rate of K+ pump, which maintains constant concentrations and a constant resting membrane potential. A. The Na+/K+ pump counters this leakage, thus maintaining constant concentrations and a constant resting membrane potential. B. Most cells in the body contain numerous Na+/K+ pumps that require a constant expenditure of energy. C. The Na+/K+ pump itself contributes to the membrane potential because it pumps more Na+ out than it pumps K+ in (by a ratio of three to two).