Lecture 3

Question 1

why are the intracellular and extracellular environments in electrical and chemical disequilibrium

Answer 1

• many of the solutes in the body are ions
• they carry a net electrical charge
• ex. lactate, pyruvate, phosphate, HCO3-, K+, Cl-, Ca2+, Na+, proteins

Question 2

why are ions unevenly distributed across cell membranes

Answer 2

• due to the selective permeability of the cell membrane to different ion -> based on presence of ion channels and transporters in the plasma membrane
• this results in an unequal distribution of ion concentration and net charge
• net charge difference makes inside of cell more negative

Question 3

what are ECF and ICF millimolar concentrations for K+ , Na+ , Ca2+, Cl-, Pi, Protein

Answer 3

K+ - ICF = 140, ECF= 4
Na+ - ICF = 15, ECF =145
Ca2+ - ICF = <0.001, ECF =1.8
Cl- - ICF= 4, ECF = 115
Pi - ICF = 40, ECF = 2
Protein - ICF = 4, ECF = 0.2

Question 4

what is the distribution of major solutes across the cell surface membranes

Answer 4

K+ - higher intracellular (cytosol
Na+ - higher extracellular
Ca2+ - higher extracellular
PO4- - higher intracellular
Proteins - higher intracellular
Cl- - higher extracellular
2 K+ in, 3 Na+ out -> cell is negative b/c more positive leaving than going in

Question 5

what is resting membrane potential

Answer 5

• the net charge difference between the intracellular fluid and the extracellular fluid at rest
• a result of the unequal distribution of ions on each side of the membrane
• all living cells have a resting membrane potential
• RMP is usually negative (-70mV)

Question 6

how can we measure a membrane potential difference

Answer 6

• Vm or Em to denote membrane potential difference
• in the laboratory a cells membrane potential is measured by placing one electrode inside the cell and a second in the extracellular bath
• the voltmeter measure the difference in electrical charge between the inside of a cell and the surrounding solution

Question 7

what is equilibrium potential and an example of Ek for K+

Answer 7

• for an ion the equilibrium potential Ex is the potential at which the electrical gradient exactly opposes the concentration gradient and there is no net movement of the ion
• permeable ions will move down their electrochemical gradient until Vm=Ex and equilibrium is achieved
• ex. K+
  -> the K+ concentration gradient drives K+ out of the cell
  -> the excess negative charge inside cell pulls K+ back in
  -> Ek = -94 mV , Vm = -94mV

Question 8

what is the nernst equation

Answer 8

• used to calculate the equilibrium potential
• if the membrane was only permeable to one type of ion, the membrane potential would be equal to the nernst potential for that ion
  Eion = 61/z log [ion]out/[ion]in
  -oi nernst equation is hard

Question 9

explain how the electrochemical driving force for ionic flow depends on Vm and Ex for a positive ion

Answer 9

• when Vm-Ex is greater than 0, ion flow outward
• when Vm-Ex is less than 0, ion flows inward
• when Vm-Ex is equal to 0, ion flow stop

Question 10

what is ENa

Answer 10

• +60mV

Question 11

what is E Cl-

Answer 11

-89mV

Question 12

explain how the electrochemical driving force for ionic flow depends on Vm and Ex for a negative ion

Answer 12

• when Vm-Ex is greater than 0, ions flow inward
• when Vm-Ex is less than 0, ions flow outward
• when Vm-Ex is equal to 0, ion flow stops

Question 13

what is goldman-hodgkin-katz equation

Answer 13

• predicts membrane potential that results from the contribution of all ions that can cross the membrane
• based on the combined contributions of concentration gradients and relative membrane permeability for each ion

Question 14

resting membrane potential is typically close to ____

Answer 14

-Ek
- most cells are 40 times more permeable to K+ than Na+
- K+ is the primary contributor to resting membrane potential in all cells
- results in a RMP that is closer to Ek than to ENa

Question 15

how is the resting membrane potential set by the equilibrium potentials of the ions the membrane is permeable to

Answer 15

• a cells whose membrane is impermeable to ions is filled with K+ and large protein anions
• it is placed in a solution of Na+ and Cl-, both cell and solution are electrically neutral
• K+ leaks out of the cell because there is a K+ concentration gradient (channel inserted)
• the negative membrane potential attracts the K+ back into the cell
• when the electrical gradient opposes the K+ concentration gradient the resting membrane potential is the equilibrium potential for K+

Question 16

what happens if the cells permeability to an ion increases

Answer 16

• get an increase in flow of that ion
• change in Vm towards that ions Ex

Question 17

what is hyperpolarization

Answer 17

• getting more negative

Question 18

what is depolarization

Answer 18

• getting more positive

Question 19

what is repolarization

Answer 19

• back to resting membrane potential

Question 20

explain how the Na+ K+ ATPase helps maintain concentration gradients and the RMP

Answer 20

• most cells are 40x more permeable to K+ than to Na+ and the RMP is -70mV
• helps maintain by maintaining the Na+ and K+ concentration gradients in the face of ion movement through leak channels